INT J TUBERC LUNG DIS 16(8):1033–1039© 2012 The Unionhttp://dx.doi.org/10.5588/ijtld.12.0027E-published ahead of print 11 June 2012

[A version in French of this article is available from the Editorial Offi ce in Paris and from the Union website www.theunion.org]

Well-quantified tuberculosis exposure is a reliable surrogate measure of tuberculosis infection

A. M. Mandalakas,*†‡ H. L. Kirchner,§ C. Lombard,¶ G. Walzl,# H. M. S. Grewal,**,†† R. P. Gie,† A. C. Hesseling†

* Section on Retrovirology and Global Health, Department of Pediatrics, Baylor College of Medicine, Houston, Texas, † Center for Global Health, Texas Children’s Hospital, Houston, Texas, USA; ‡ Desmond Tutu TB Centre, Department of Pediatrics and Child Health, Stellenbosch University, Cape Town, South Africa; § Geisinger Clinic, Danville, Pennsylvania, USA; ¶ Medical Research Council, Cape Town, # Molecular Biology and Human Genetics, Faculty of Health Sciences, Stellenbosch University, Cape Town, South Africa; ** Section of Microbiology and Immunology, The Gade Institute, University of Bergen, Bergen, †† Department of Microbiology, Haukeland University Hospital, Bergen, Norway

Correspondence to: Anna Mandalakas, Baylor College of Medicine, Texas Children’s Hospital, 1102 Bates St, FC-630, Houston, Texas 77030, USA. Tel: (+1) 832 822 1038. Fax: (+1) 832 825 1281. e-mail: anna.mandalakas@bcm.eduArticle submitted 12 January 2012. Final version accepted 27 February 2012.

S E T T I N G : Cape Town, South Africa.O B J E C T I V E : To develop a standardized, reliable mea-sure of household tuberculosis (TB) exposure that con-siders child-specific risk factors. D E S I G N : We assessed TB exposure in 536 children. Children were considered Mycobacterium tuberculosis infected if two of three tests of infection were positive. Principal component analysis identified a discrete set of components that collectively described exposure and con-tributed to a composite contact score. Logistic regres-sion assessed the odds of having M. tuberculosis infec-tion given increasing contact score while controlling for age and past TB treatment.R E S U LT S : Four components described 68% of data vari-ance: 1) maternal TB and sleep proximity, 2) index case infectivity, 3) duration of exposure, and 4) exposure

to multiple index cases. Components were derived from 10 binary questions that contributed to a contact score (range 1–10, median 5, 25th–75th interquartile range [IQR] 4–7). Among children aged 3 months to 6 years with household exposure, the odds of being M. tuberculosis-infected increased by 74% (OR 1.74, 95%CI 1.42–2.12) with each 1-point increase in the contact score.C O N C L U S I O N S : Well-quantified TB exposure is a good surrogate measure of M. tuberculosis infection in child household contacts in a high-burden setting, and could guide targeted preventive treatment in children at high-est risk of M. tuberculosis infection. K E Y W O R D S : latent tuberculosis infection; pediatrics; children; contact tracing; preventive therapy

CHILDREN carry approximately 15% of the global tuberculosis (TB) disease burden, leading to an esti-mated 1 million cases of childhood TB annually.1 Young children are at high risk of primary disease pro-gression and developing disseminated disease in the absence of appropriate preventive treatment.2 Early and accurate identifi cation of Mycobacterium tuber-culosis infected children could guide targeted delivery of preventive therapy and prevent TB in the most vul-nerable children.

There is no gold standard to detect M. tubercu-losis infection. Although it is routinely used, the tu-berculin skin test (TST) has limited sensitivity and specifi city. The identifi cation of genes in the M. tuber-culosis genome that are absent from M. bovis bacille Calmette-Guérin (BCG)3 and most non-tuberculous mycobacteria4 supported the development of more specifi c assays quantifying the in vitro production of interferon-gamma by T-cells after stimulation with

M. tuberculosis-specifi c antigens.5–7 In children, the specifi city of commercially approved interferon-gamma release assays (IGRAs) is estimated to be 91% for the enzyme-linked immunosorbent assay (ELISA) based test (QuantiFERON [QFT] assays, Cellestis, Carne-gie, VIC, Australia) and 94% for the ELISpot-based test (T-SPOT.TB, Oxford Immunotec, Oxford, UK), compared to 88% for the TST.8

In many high-burden TB settings, the usefulness of tests of infection is limited, not only by suboptimal performance in children, but also by cost and lack of operational capacity within routine health services. A growing body of evidence suggests that M. tuberculo-sis exposure may be quantifi ed and serve as a surro-gate measure of M. tuberculosis infection.8–14 To date, approaches used to quantify TB exposure have been heterogeneous, making pooled comparison diffi cult, and have included data that are inconsistently avail-able in high-burden settings.8–15

S U M M A R Y

1034 The International Journal of Tuberculosis and Lung Disease

We aimed to develop a standardized, reliable mea-sure of household M. tuberculosis exposure in child contacts that considers child-specifi c risk factors, and is operationally feasible using routinely available data.

STUDY POPULATION AND METHODS

Study setting and designAs part of an ongoing community-based study assess-ing the utility of IGRAs for the detection of M. tuber-culosis infection, we assessed M. tuberculosis exposure among child contacts of adults routinely diagnosed with TB. The study was set in three impoverished ur-ban communities in Cape Town, Western Cape Prov-ince, South Africa. Two of the study communities are geographically adjacent and of mixed race ethnic-ity. The third community is of predominantly Black ethnicity.

The overall TB notifi cation rate in the Western Cape Province was 994 per 100 000, and it was >1000/100 000 in the study communities; TB reported in children aged 0–13 years accounted for 17% of dis-ease in the province and led to an age-specifi c TB no-tifi cation rate of 620/100 000 in 2008 (unpublished data, Western Cape Department of Health, 2009). The estimated annual risk of infection was 4.1% in the fi rst two communities, where 30% of children aged 6–9 years were TST-positive.16 In the third com-munity, 26% of children aged 6–11 years were TST-positive in 2007.17 Routine BCG vaccination is given at birth (99% coverage).18

Eligibility Children aged 3 months–15 years with and without known TB exposure were eligible to participate. Chil-dren with known TB exposure were recruited within 3 months of the index case being identifi ed by the community-based TB clinic. Children from neighbor-ing households whose parents or care givers reported no known TB exposure were enrolled as community controls to provide a measure of background commu-nity TB exposure. Up to four children per household were enrolled, with preference given to the youngest children. Weight < 5 kg, recently documented ane-mia (hemoglobin < 9 mg/dl), and current treatment for TB disease were exclusion criteria. Enrollment was deferred if a TST had been placed in the preced-ing 12 weeks, live attenuated vaccination had been given within the preceding 6 weeks, or in the pres-ence of acute severe respiratory, diarrheal or neuro-logic illness. Human immunodefi ciency virus (HIV) infected children were excluded from the analysis presented in this manuscript.

Study measuresData on age, ethnicity, BCG vaccination and previ-ous TB exposure or disease were collected from the parents/primary care givers. Standard weight and

height measurements and clinical examination were completed.

Standard data collection tools were used to gather child-specifi c data regarding recent TB exposure from parents/primary care givers and TB index cases, and to extract routine data from the TB register. Data were collected regarding infectivity of the index case (sputum smear and culture status, cough duration, disease site), proximity of the child to the index case (familial and caregiving relationship, sleep proxim-ity), and duration of the child’s exposure to the in-dex case (hours of daily contact, months of extended household contact). The number of TB cases within the household was enumerated. Household charac-teristics and socio-economic indicators were ascer-tained through an asset-based questionnaire.

HIV testing was completed on all children with un-known HIV status (Abbott Determine HIV-1/2 rapid test, Abbott Diagnostics, Hoofddorp, The Nether-lands). A positive rapid test was confi rmed with ELISA (children aged >18 months) and DNA polymerase chain reaction (children aged ⩽18 months).

Children completed three independent tests to as-sess M. tuberculosis infection status at baseline and at 3 months follow-up. Mantoux TST employed two tu-berculin units RT-23 (Statens Serum Institute, Copen-hagen, Denmark) injected intradermally on the volar aspect of the left forearm. The largest transverse di-ameter of induration was read 48–72 h after admin-istration, using the ‘ball-point’ technique and calipers. TST was not completed in children with history of TST ulceration. IGRAs were performed following manufacturers’ guidelines.19,20

All children were screened for TB through stan-dard symptom screening,21 chest radiography and mycobacterial culture of gastric aspirates or sputum. Antero-posterior and lateral chest radiographs were read by two blinded independent experts, using a standard international pediatric TB radiologic classi-fi cation tool.22 Children with TB were not excluded from analysis as they met our defi nition of infection.

Children aged 0–5 years with a positive TST or known TB contact were referred for 6 months’ isoni-azid preventive therapy (IPT; 10 mg/kg/day); children with TB were referred for 6 months’ directly observed anti-tuberculosis treatment, consistent with South African National TB Control guidelines. For clinical management, children were considered to be M. tu-berculosis infected when the TST was ⩾10 mm.

Study approval was granted by the research ethics committees of Case Western Reserve and Stellenbosch Universities, and the local health authorities. Written informed consent was obtained from all parents/care givers; children aged >7 years completed assent.

Statistical analysis For analytic purposes, children were considered to be M. tuberculosis-infected if positive results were

Exposure is a surrogate for TB infection 1035

obtained for two or more of three tests of infection. Binary interpretations of individual tests were em-ployed. A TST ⩾ 10 mm was considered positive, and QFT and T-SPOT.TB followed manufacturers’ guide-lines.19,20 Indeterminate results were excluded from analysis. To capture children with TST or IGRA con-versions, infection status was also defi ned consider-ing test results at 3 months follow-up. For this alter-native defi nition, infection was considered present if two tests of infection were positive either at baseline or at 3 months.

Univariate analysis was completed to measure the association between baseline M. tuberculosis infec-tion status and exposure variables. Comparisons were performed using modifi ed versions of the Pearson’s χ2 test and t-test to account for clustering of chil-dren within households. Principal component analy-sis (PCA), a commonly used variable reduction tech-nique, described the association between measures of TB exposure and infection status in children. PCA re-sults were used to develop an operationally practical measure of M. tuberculosis infection referred to as an M. tuberculosis contact score. In brief, the M. tuber-culosis contact score is a linear, composite measure of M. tuberculosis exposure derived from the summa-tion of binary responses to 10 questions. Binary re-sponses were valued at 0 (no) or 1 (yes), leading to a contact score potentially ranging from 0 to 10. A value of 0 therefore refl ects the absence of household exposure. Cronbach’s coeffi cient α was used to mea-sure the reliability and internal consistency of the con-tact score.23 PCA employed a conventional approach and rotated (i.e., linearly transformed) the factor so-lution to simplify interpretation of results. More data regarding PCA and development of the contact score are available in the Appendix.*

To test the applicability of the contact score, a ran-dom effects logistic regression model was used to es-timate the association between the contact score and M. tuberculosis infection to account for multiple chil-dren within a household and to control for clinically relevant covariates, including age and previous TB treatment. Logistic regression was completed in sub-groups dichotomized at 6 years of age, as younger children are more likely to have been infected as a re-sult of household TB exposure. Logistic regression was also completed using the alternate defi nition of infection considering test conversion at 3 months. Exploratory regression analysis was completed to ex-amine the applicability of the M. tuberculosis contact score in children with and without household TB ex-posure. Results are presented as odds ratios (ORs) and 95% confi dence intervals (95%CIs).

RESULTS

Between January 2008 and January 2011, 536 non-HIV-infected children were enrolled, including 350 children with known household TB exposure and 286 community controls without known TB expo-sure. Among the 350 children with household expo-sure, ages ranged from 3 months to 15.6 years (aver-age 59 months); 4.9% (17/350) reported previous treatment for TB (Table 1). These children came from 182 households, with an average of 1.9 children per household. Children defi ned as M. tuberculosis-i nfected were more likely to have been exposed to an index case with the exposure attributes of interest (Table 1). Of 308 children with results available for the TST and both IGRAs, 82.5% (254/308) were con-cordantly positive or negative by all three tests and rates of test positivity were similar (TST 56.9%, 175/ 308; QFT 52.4%,161/308 and T-SPOT.TB 57.1%, 176/308). Similarly, 53.9% (166/308) of these chil-dren were categorized as M. tuberculosis-infected, defi ned as testing positive by two or more tests of infection.

PCA identifi ed 10 questions that were categorized into four groups, each of which described a unique as-pect of TB exposure. The 10 questions collectively de-scribed 68% of the observed data variance (Table 2). The fi rst group of questions (questions 1–4) described sleep proximity and maternal TB and accounted for 28% of the data variance. The second group of ques-tions (numbered 5–7) described the infectivity of the index case and accounted for 16% of the data vari-ance. Questions 8 and 9 described the frequency of the child’s exposure to the index case, and jointly ex-plained 13% of the data variance. Question 10 quan-tifi ed the number of adults with TB in the household, and by itself explained 11% of the data variance. We retained the fi nal three questions as they were epide-miologically meaningful and easily measured in a high-burden setting (Table 2). Consistent with the multi-dimensional nature of our contact score, the Cronbach’s coeffi cient α was 0.58 for the composite contact score and respectively 0.81, 0.48 and 0.58 for components one, two and three.

While controlling for age and prior M. tuberculosis treatment, the contact score was signifi cantly related to M. tuberculosis infection. For each one unit increase in the contact score, the odds of infection increased by 54% (OR 1.54, 95%CI 1.30–1.82; Table 3). A child with a contact score of 10 was thus 49 times more likely to be infected than a child with a contact score of 1. Results were similar when estimating the association between the contact score and the alter-native defi nition of M. tuberculosis infection, which considered month 3 test conversions (adjusted OR [aOR] 1.54, 95%CI 1.30–1.82). When analysis was restricted to children aged <6 years (n = 251), there

* The Appendix is available in the online version of this article at http://www.ingentaconnect.com/content/iuatld/ijtld/2012/00000016/ 00000008/art00009

1036 The International Journal of Tuberculosis and Lung Disease

was a stronger association between the contact score and M. tuberculosis infection (OR 1.74, 95%CI 1.42–2.12), while the contact score distribution was un-changed (range 1–10, median 5, 25th–75th interquar-tile 4–7; Table 3). Regression analysis controlling for the interaction between age and the contact score found a signifi cant difference in performance of the

contact score in children aged < vs. > 6 years of age (P = 0.024).

Among children with known household exposure, the M. tuberculosis contact score ranged from a value of 1 to 10 (median 5, 25th–75th interquartile 4–7). Sensitivity and specifi city of the contact score for the identifi cation of children with M. tuberculosis infec-tion varied inversely, with sensitivity being highest at lower contact score values. In children aged <6 years, a contact score of ⩾4 was associated with 97% sensi-tivity and 21% specifi city. In contrast, a contact score of ⩾7 was associated with 48% sensitivity and 89% specifi city (Figure).

Community controls were older (median age 60 months, 25th–75th quartile: 25–107) and less likely to be QFT- and T-SPOT.TB-positive compared to con-tacts; 34% of community controls met our defi nition of M. tuberculosis-infected. There was no difference in rates of TST positivity or prior TB treatment between contacts and community controls. Exploratory anal-ysis examined contact score performance when the analytic sample was enlarged to also include child community controls. In this model (n = 536), which controlled for age and prior TB treatment, the contact score ranged from 0 to 10 (median 4, 25th–75th inter-quartile 1–6) and the association between the contact score and M. tuberculosis infection was weaker (aOR 1.27, 95%CI 1.17–1.38). The association between previous TB treatment and M. tuberculosis infection became stronger (aOR 3.84, 95%CI 1.24–11.93), while the association with age remained unchanged.

Table 2 Ten-item questionnaire from which the M. tuberculosis contact score was derived (n = 304)*

Question group†

Individual questionOne Two Three Four

X 1 Is the index case the child’s mother?X 2 Is the index case the child’s primary

care giver?X 3 Does the index case sleep in the

same bed as the child?X 4 Does the index case sleep in the

same room as the child?X 5 Is the index case coughing?X 6 Does the index case have reported

pulmonary TB?X 7 Does the index case have smear-

positive sputum?X 8 Does the index case live in the same

household as the child?X 9 Does the index case see the child

every day?X 10 Is there more than one adult TB case

in the child’s household?

* 46 children were excluded from principal component analysis due to miss-ing data.† Principal component analysis categorized questions into four groups, each of which described a unique aspect of TB exposure in the study participants.TB = tuberculosis.

Table 1 Characteristics of children with known household TB exposure (n = 350)

All children (n = 350)n/N (%)

Non-M. tuberculosis-infected children*

(n = 185)n/N (%)

M. tuberculosis-infected children*

(n = 158)n/N (%)

Age, months, median [25th–75th quartile]† 49 [25–81] 40 [20–62] 62 [38–107]

Female sex 188/350 (54) 97/185 (52) 89/158 (56)

Past TB treatment† 17/350 (4.9) 5/185 (2.7) 12/158 (7.6)

Children with positive results for individual tests of M. tuberculosis infection‡

TST† 151/350 (43) 7/185 (4) 140/158 (89) QFT-GIT† 158/332 (48) 7/176 (4) 151/155 (97) T-SPOT.TB† 139/324 (43) 4/172 (2) 134/148 (91)

Children exposed to index cases with the following attributes Mother† 92/350 (26) 41/185 (22) 50/158 (32) Primary care giver† 89/345 (26) 34/182 (19) 54/156 (35) Sleeps in same bed as child† 94/348 (27) 36/184 (20) 57/157 (36) Sleeps in same room as child† 166/348 (48) 68/184 (37) 96/157 (61) Lives in same household as child 334/350 (95) 174/185 (94) 153/158 (97) In daily contact with child 331/348 (95) 172/184 (94) 152/157 (97) Cough present† 240/343 (70) 115/180 (64) 121/156 (78) Pulmonary TB present† 306/319 (96) 153/165 (93) 146/147 (99) Sputum smear-positive† 193/328 (59) 88/169 (52) 99/152 (65) More than one contact in household† 46/350 (13) 15/185 (8) 29/158 (18)

* Baseline measure of M. tuberculosis infection: infection defined if two of three tests of infection (TST, QFT-GIT, T-SPOT.TB) were positive. M. tuberculosis infection status could not be defined in seven children.† Significant difference (P < 0.05) in values among M. tuberculosis-infected compared to non-M. tuberculosis-infected.‡ Indeterminate IGRA results have been excluded from analysis, including 18 QFT-GIT and 26 T-SPOT.TB indeterminate results.TB = tuberculosis; TST = tuberculin skin test; QFT-GIT = QuantiFERON®-TB Gold In-Tube; IGRA = interferon-gamma release assay.

Exposure is a surrogate for TB infection 1037

DISCUSSION

Preventive treatment substantially reduces TB pro-gression, morbidity and mortality among close con-tacts of TB cases.24,25 As tests for M. tuberculosis in-fection are inconsistently available in high-burden settings, the World Health Organization recommends IPT in young and HIV-infected children in close con-tact with an infectious TB case, regardless of proof of M. tuberculosis infection.26 Nevertheless, targeting a smaller group of children more likely to be infected and hence benefi t from IPT could focus limited re-sources to improve IPT uptake and adherence, which is characteristically poor to non-existent in high-burden settings.27–30 Our fi ndings illustrate that a child’s degree of M. tuberculosis exposure may be re-liably quantifi ed using routinely available data, and is associated with risk of infection. Well-quantifi ed ex-posure could therefore provide a practical tool guid-ing targeted delivery of preventive therapy in settings where limited resources preclude testing for infection or treatment of all household-exposed children.

We described TB exposure in child household contacts by measuring the association between expo-

sure variables and M. tuberculosis infection status. As there is no gold standard measure, we used the three most common current tests of M. tuberculosis infection to determine the infection status in children. By limiting analyses to healthy, non-HIV-infected children in whom test performance is likely to be more reliable, we were able to achieve a high level of test concordance (83%) and maximize diagnostic accuracy.

We used well-established analytic techniques to develop a simple tool that quantifi es TB exposure in children and predicts risk of M. tuberculosis infection based on routinely available data. This practical mea-sure of exposure should therefore be operationally feasible even in resource-constrained settings. When applied in household contacts aged <6 years in whom IPT is recommended,26 a child with a contact score of 10 was 162 times more likely to be M. tuberculosis-infected than a child with a contact score of 1. In set-tings with limited resources and no access to reliable tests of infection, the largest impact may be achieved by using available resources to target children at highest risk of TB infection. Among child household contacts aged <6 years in our setting, a contact score cut-off of ⩾6 would target IPT in 57% of infected children while reducing over-treatment of non-infected children by 76%. The appropriate cut-off for any set-ting should pragmatically consider available health system resources, and should ideally consider the cost of under- and over-treatment to the health care sys-tem and society.

Our analysis illustrates the signifi cant risk of in-fection among children exposed to TB in the mother or primary care giver and links this to sleep proxim-ity. This fi nding is consistent with previous studies, il-lustrating that 50% of young South African children hospitalized for TB had a known index or source case; the majority of these index cases were parents, usu-ally the mother.31 Our data emphasize the importance of screening in children exposed to maternal TB.

The risk of M. tuberculosis infection in children was associated with the infectivity of the index case. Al-though we collected data on multiple index case char-acteristics that may serve as markers of infectivity,8

Table 3 Association between baseline M. tuberculosis infection status* and M. tuberculosis contact score, controlling for age and past TB treatment

Covariates

Age group analyzed

3 months–15 years (n = 350)

3 months–5 years (n = 254)

6–15 years (n = 96)

Unadjusted OR (95%CI)

Adjusted OR (95%CI)

Unadjusted OR (95%CI)

Adjusted OR (95%CI)

Unadjusted OR (95%CI)

Adjusted OR (95%CI)

M. tuberculosis contact score 1.50 (1.28–1.76) 1.54 (1.30–1.81) 1.73 (1.43–2.10) 1.74 (1.42–2.12) 1.20 (0.90–1.61) 1.21 (0.90–1.62)Age, months 1.01 (1.01–1.02) 1.02 (1.00–1.04) 1.00 (0.98–1.02)Prior TB treatment 1.52 (0.42–5.49) 2.18 (0.40–12.03) 1.07 (0.14–8.00)

* Baseline measure of M. tuberculosis infection: infected defined if two of three tests of infection (TST, QFT-GIT, T-SPOT.TB) were positive.TB = tuberculosis; TST = tuberculin skin test; QFT-GIT = QuantiFERON®-TB Gold In-Tube.

Figure M. tuberculosis contact score distribution stratified by infection status and test performance at sequential cut-off points. Distribution of the M. tuberculosis contact score among children aged <6 years with and without M. tuberculosis infection is il-lustrated in the frequency distribution graph. The sensitivity and specificity of the contact score at sequential contact score cut-off points are represented by the black and grey lines, respectively.

1038 The International Journal of Tuberculosis and Lung Disease

the usefulness of several observations was limited by poor reliability (e.g., duration of cough) and incon-sistent availability (e.g., sputum smear grading). We reliably quantifi ed index case infectivity using simple, routinely available, inter-related risk factor data, in-cluding sputum smear status (positive/negative), pulmonary TB reported per TB register (yes/no) and presence of current cough (present/absent).

Our M. tuberculosis contact score provides a prac-tical method to systematically quantify the degree of household TB exposure in children. The score per-forms reliably in children up to 15 years of age, and performs comparably when M. tuberculosis infection status is defi ned both at baseline and longitudinally. These data suggest that our measure could be a use-ful research tool to reliably quantify variable degrees of TB exposure in children and support standardized comparisons across studies.

The utility of our contact score may vary in other epidemiologic contexts. In HIV-affected households, transmission dynamics may vary, as HIV-infected adults may be less infectious and TB re-exposure may be more frequent.32 Our contact score may also be less useful in households where sleeping patterns or household composition differ, and may be more use-ful in settings with lower TB burden, as transmission is more likely to occur in the household. However, in this high-burden setting, our contact score describes 80% of a child’s risk of infection, suggesting that up to 20% of transmission occurs outside the home. This hypothesis is further supported by our results demonstrating that 34% of children with no known TB exposure were M. tuberculosis-infected. Future studies are needed to validate this contact score in varying settings and populations.

CONCLUSIONS

Our fi ndings illustrate that children’s risk of M. tu-berculosis infection may be easily quantifi ed using data routinely available in settings with varying re-sources and high rates of M. tuberculosis transmis-sion. In settings where limited resources preclude pre-ventive treatment of all young TB household contacts, our fi ndings can help to systematically guide use of preventive tuberculosis in children at greatest risk of M. tuberculosis infection and reduce the considerable TB disease burden in children by focusing resources on improving IPT uptake and adherence.

AcknowledgementsThe authors thank the children and families who graciously partic-ipated in the study. They also thank the SUN Immunology Group at Stellenbosch University for tireless laboratory support and com-pletion of interferon-gamma release assays. This work was sup-ported by the National Institute of Allergy and Infectious Disease at the National Institutes of Health (R01A076199) and the Nor-wegian Cooperation for Higher Education (NUFU: NUFUPRO-2007/10183). AM received salary support from the United States

Department of State to serve as a Senior Fulbright Scholar to South Africa during the completion of this work. The cost of purchasing QuantiFERON tests was supported through the Foundation for Innovative New Diagnostics, Geneva, Switzerland. T.SPOT.TB tests and kits were purchased at reduced rates from the manufac-turer. Funding sources played no role in project implementation, analysis or reporting.

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22 Marais B J, Gie R P, Schaaf H S, et al. A proposed radiological classifi cation of childhood intra-thoracic tuberculosis. Pediatr Radiol 2004; 34: 886–894.

23 Cronbach L J. Coeffi cient alpha and the internal structure of tests. Psychometrika 1951; 16: 297–334.

24 Woldehanna S, Volmink J. Treatment of latent tuberculosis in-fection in HIV infected persons. Cochrane Databases Syst Rev 2004; (1): CD000171.

25 Smieja M, Marchetti C, Cook D, Smaill F. Isoniazid for preven-ting tuberculosis in non-HIV-infected persons. Cochrane Data-base Syst Rev 2000; (2): CD001363.

26 World Health Organization. Guidance for national tuberculo-sis programmes on the management of tuberculosis in children. WHO/HTM/TB/2006.371. Geneva, Switzerland: WHO, 2006.

27 Marais B J, van Zyl S, Schaaf H S, van Aardt M, Gie R P, Beyers N. Adherence to isoniazid preventive chemotherapy: a pro-spective community based study. Arch Dis Child 2006; 91: 762–765.

28 van Zyl S, Marais B J, Hesseling A C, Gie R P, Beyers N, Schaaf H S. Adherence to anti-tuberculosis chemoprophylaxis and treatment in children. Int J Tuberc Lung Dis 2006; 10: 13–18.

29 Guwatudde D, Nakakeeto M, Jones-Lopez E C, et al. Tubercu-losis in household contacts of infectious cases in Kampala, Uganda. Am J Epidemiol 2003; 158: 887–898.

30 Alperstein G, Morgan K R, Mills K, Daniels L. Compliance with anti-tuberculosis preventive therapy among 6-year-old children. Aust NZ J Public Health 1998; 22: 210–213.

31 Schaaf H S, Marais B J, Whitelaw A, et al. Culture-confi rmed childhood tuberculosis in Cape Town, South Africa: a review of 596 cases. BMC Infect Dis 2007; 7: 140.

32 Cotton M F, Schaaf H S, Lottering G, Weber H L, Coetzee J, Nachman S. Tuberculosis exposure in HIV-exposed infants in a high-prevalence setting. Int J Tuberc Lung Dis 2008; 12: 225–227.

Exposure is a surrogate for TB infection i

APPENDIX

There is no gold standard for the diagnosis of Myco-bacterium tuberculosis infection. Emerging evidence suggests that well-quantifi ed tuberculosis (TB) expo-sure may be a robust surrogate measure of M. tuber-culosis infection. Among South African child con-tacts, we aimed to develop a standardized, reliable measure of household M. tuberculosis exposure that considers child-specifi c risk factors and is operation-ally feasible using routinely available data.

MethodsWe assessed TB exposure in 536 South African chil-dren, including 350 with recent household exposure. A child was considered M. tuberculosis-infected if two of three tests of infection were positive. Princi-pal component analysis identifi ed a discrete set of components that collectively described exposure and contributed to a composite contact score. Logistic regression assessed the odds of having M. tubercu-losis infection, given increasing contact score while controlling for age and history of anti-tuberculosis treatment.

Expanded description of principal component analysis To identify household exposure domains among chil-dren aged 3 months to 15 years, principal component analysis (PCA) was conducted using ones as prior communality estimates.1 The PCA initially consid-ered all variables that were associated with baseline M. tuberculosis infection (P > 0.20) in univariate analysis. The principal axis method was used to ex-tract the components, and this was followed by a varimax (orthogonal) rotation. Variables were elimi-nated from analysis if they were associated with mul-tiple components or only loaded independently onto components. The fi nal model retained variables that contributed to components displaying eigen values >1.00. For interpretation of the rotated factor pat-tern, a variable was considered to load on a given component if the factor loading was >0.40 for that component, and <0.40 for others. To develop an op-erationally practical measure of M. tuberculosis in-fection, a factor-based score was derived by adding responses of the variables that demonstrated mean-ingful loadings for the retained components. The re-sulting linear, composite measure of M. tuberculosis exposure, the M. tuberculosis contact score, was de-rived from the summation of binary responses to 10 questions. Binary responses were valued at 0 (no) or 1 (yes), leading to a contact score potentially ranging from 0 to 10. A value of 0 therefore refl ects the ab-sence of household exposure. Cronbach’s coeffi cient α was used to measure the reliability and internal consistency of the contact score.2

PCA resultsBetween January 2008 and January 2011, 536 non-human immunodefi ciency virus infected children with and without TB exposure were enrolled into the par-ent study, including 350 children with known house-hold TB exposure (Table 1).

PCA identifi ed four components that described 68% of the variance of the data and had eigen values >1.09 (Table 2); the next highest ranked component had an eigen value of 0.84. The fi rst component de-scribed sleep proximity and maternal disease, and was defi ned by four items that accounted for 28% of the variance. The second component was infectivity of the index case, which was defi ned by three items and accounted for 16% of the variance. The fi nal three items loaded on two components that explained re-spectively 13% and 11% of the variance. We retained these fi nal two components as they were epidemio-logically meaningful and easily measured in our study setting (Table 2). Cronbach’s coeffi cient α was 0.58 for the composite contact score and respectively 0.81, 0.48 and 0.58 for components one, two and three.

Expanded discussion of PCAOur fi ndings illustrate that a child’s degree of M. tu-berculosis exposure may be reliably quantifi ed and can accurately determine infection. Well-quantifi ed exposure could therefore provide a practical tool guiding targeted isoniazid preventive therapy (IPT) delivery, potentially eliminating reliance on tests of infection. Our measure may also be a useful research tool for studies of diagnostic accuracy requiring that TB exposure be quantifi ed and controlled for in multi-variate analysis.

We used PCA, a variable reduction technique, to describe TB exposure in children in household con-tact with adults recently diagnosed with TB at the routine TB clinic. PCA identifi ed four components that collectively explained a large portion of the vari-ance in the data. Our multi-dimensional measure had an appropriately modest reliability coeffi cient (Cron-bach’s alpha = 0.58), refl ecting the inclusion of di-verse constructs of exposure. The four components were derived from 10 easily measured binary obser-vations that rely upon routinely available data. This practical measure should therefore be operationally feasible even in resource-constrained settings.

The component describing the greatest amount of variance in the data was derived from four observa-tions that capture maternal disease and sleep proxim-ity. Our analysis quantitatively proves that among South African children, a signifi cant portion of M. tu-berculosis transmission occurs in the room where children sleep and is associated with TB in the mother or primary caretaker. This fi nding is consistent with previous studies illustrating that 50% of young South African children hospitalized for TB had a known

ii The International Journal of Tuberculosis and Lung Disease

i ndex case; the majority of these index cases were parents, usually the mother.3 Our fi ndings also support the previous use of sleep proximity as a surrogate measure of infection.4 This growing body of evidence highlights the relevance of maternal TB and empha-sizes the importance of TB screening and IPT provi-sion to children whose mothers have been diagnosed with TB. Use of these simple risk factors to target IPT delivery could have a signifi cant impact on disease prevention and improve IPT cost-effectiveness in high-burden settings.

The next signifi cant component contributing to our proxy measure of M. tuberculosis infection describes the infectivity of the index case. We collected data on multiple index case characteristics as markers of in-fectivity.5 The usefulness of several observations in our sample was, however, limited by poor reliability (e.g., duration of cough) and inconsistent availabil-ity (e.g., sputum smear grading). The component that described index case infectivity was ultimately com-posed of three simple and reliable observations that are routinely available: sputum smear status (binary), pulmonary TB reported per TB register and presence of cough. Health care workers in most settings should be able to operationalize this component of our con-tact score to target IPT delivery.

The remaining components contributing to the M. tuberculosis contact score quantifi ed the number of index cases in the household and described the child’s duration of household exposure. Although these measures were relevant in a setting with a high annual rate of tuberculous infection, these observa-tions may be less useful for the prediction of M. tu-berculosis infection in settings with a lower TB bur-den. It is therefore important that our M. tuberculosis contact score be validated in children living in set-tings with varying burden of TB burden and with di-verse household composition.

Our M. tuberculosis contact score was designed to provide a practical method to systematically quan-tify children’s degree of exposure to M. tuberculosis within the household. When analysis was limited to children aged <6 years, there was a stronger associa-tion between the M. tuberculosis contact score and

infection status. Hence, our measure performs best in younger children, where the majority of TB exposure is thought to occur in the household. Among children aged <6 years, a child with a contact score of 10 was 162 times more likely to be M. tuberculosis-infected than a child with a contact score of one. In compari-son, when applied in children up to 15 years of age, a child with a contact score of 10 was 48 times more likely to be M. tuberculosis-infected than a child with a contact score of 1. The performance of our contact score was comparable when M. tuberculosis infec-tion status was defi ned at baseline only compared to an alternative longitudinal defi nition of M. tubercu-losis infection considering test conversion. These data suggest that our practical contact score is a reliable measure of M. tuberculosis infection among South African children with household TB exposure across a wide age range.

Accurate identifi cation of children with M. tuber-culosis infection could be a powerful tool to guide targeted IPT. Currently available tests of M. tubercu-losis infection have limited sensitivity and specifi city in children,5 and are cost-prohibitive in many high-burden settings. Our fi ndings illustrate that children’s risk of M. tuberculosis infection may be easily and re-liably quantifi ed via a practical measure of exposure amenable to use in settings with varying resources and high rates of M. tuberculosis transmission.

References1 Kolenikov S, Angeles G. Socio-economic status measurement

with discrete proxy variables: is principal component analysis a reliable answer? Rev Income Wealth 2009; 55: 128–165.

2 Cronbach L J. Coeffi cient alpha and the internal structure of tests. Psychometrika 1951; 16: 297–334.

3 Schaaf H S, Marais B J, Whitelaw A, et al. Culture-confi rmed childhood tuberculosis in Cape Town, South Africa: a review of 596 cases. BMC Infect Dis 2007; 7: 140.

4 Hill P C, Brookes R H, Adetifa I M, et al. Comparison of e nzyme-linked immunospot assay and tuberculin skin test in healthy children exposed to Mycobacterium tuberculosis. P ediatrics 2006; 117: 1542–1548.

5 Mandalakas A M, Detjen A K, Hesseling A C, Benedetti A, Men-zies D. Interferon-gamma release assays and childhood tubercu-losis: systematic review and meta-analysis. Int J Tuberc Lung Dis 2011; 15: 1018–1032.

Exposure is a surrogate for TB infection iii

C O N T E X T E : Cape Town, Afrique du Sud.O B J E C T I F : Elaborer une mesure standardisée et fiable de l’exposition à la tuberculose (TB) dans les ménages qui prennent en compte les facteurs de risque spécifiques aux enfants.S C H É M A : Nous avons évalué l’exposition de 536 en-fants à la TB. Les enfants ont été considérés comme in-fectés par Mycobacterium tuberculosis si deux des trois tests d’infection étaient positifs. L’analyse des compo-santes principales a identifié un petit groupe de com-posantes qui décrivent collectivement les expositions et contribuent à un score composite de contact. La régres-sion logistique a permis d’évaluer les odds d’infection M. tuberculosis en rapport avec un score croissant de contact après contrôle pour l’âge et pour un traitement antérieur de la TB. R É S U LTAT S : Les quatre composantes suivantes ont con-stitué 68% de la variance des données : 1) TB mater-

nelle et proximité lors du sommeil, 2) degré d’infectiosité du cas-index, 3) durée de l’exposition, et 4) exposition à de multiples cas-index. Les composantes ont été dérivées à partir de 10 questions binaires qui ont contribué au score de contact (limites 1–10, médiane 5, limites inter-quartiles 25–75 4–7). Parmi les enfants âgés de 3 mois à 6 ans, subissant une exposition au sein du ménage, les odds d’infection par M. tuberculosis ont augmenté de 74% (OR 1,74 ; IC95% 1,42–2,12) par point d’augmen-tation du score de contact.C O N C L U S I O N S : Une exposition bien quantifiée à la TB est une bonne mesure de suppléance de l’infection par M. tuberculosis chez les contacts infantiles dans les ménages dans un contexte à fardeau élevé ; elle pourrait guider un traitement préventif ciblé sur les en-fants comportant le risque le plus élevé d’infection par M. tuberculosis.

M A R C O D E R E F E R E N C I A : La Ciudad del Cabo en Sudáfrica. O B J E T I V O : Establecer una medición normalizada y fi-able del grado de exposición domiciliaria a la tuberculo-sis (TB), que tenga en cuenta los factores de riesgo es-pecíficos de los niños. M É T O D O : Se evaluó la exposición de 536 niños a la TB. Se consideró que los niños presentaban infección por Mycobacterium tuberculosis cuando dos de tres prue-bas de infección daban resultados positivos. Mediante el análisis de componentes principales se determinó una serie discreta de elementos que describía en forma colec-tiva la exposición y contribuía a generar una escala compuesta de contacto. Con el análisis de regresión lo-gística se evaluó la probabilidad de adquirir la infección tuberculosa en función de una puntuación progresiva de contacto, al corregir con respecto a la edad y el ante-cedente de tratamiento antituberculoso. R E S U LTA D O S : Los siguientes componentes describieron

68% de la varianza de los datos: 1) la TB materna y la proximidad al dormir, 2) la contagiosidad del caso ini-cial, 3) la duración de la exposición, y 4) la exposición a múltiples casos iniciales. Los componentes se derivaron de 10 preguntas binarias que contribuyeron a establecer la puntuación de contacto (intervalo 1 a 10; mediana 5; intervalo intercuartil del 25% al 75% 4 a 7). En los niños de 3 meses a 6 años de edad con exposición domi-ciliaria, cada progresión de un punto en la escala de contacto correspondió a un aumento de la probabilidad de adquirir la infección tuberculosa de 74% (OR 1,74; IC95% 1,42–2,12).C O N C L U S I Ó N : Una cuantificación adecuada de la expo-sición a la TB constituye una buena medida indirecta de la infección tuberculosa en los niños que son contactos domiciliaros en los entornos con alta carga de morbili-dad por TB; esta medida podría orientar la administra-ción del tratamiento preventivo dirigido a los niños con el mayor riesgo de adquirir la infección tuberculosa.

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