a negative bag dipstick. Because not all negative results havebeen verified by a catheter culture, we do not know the realdistribution of test results.

To give an indication of the possible bias: Assumingthat the distribution of true- and false-negative test resultswas comparable in the not-included children (prevalence of8% UTIs in children with a negative bag dipstick), a numberof 225 not-included children would decrease the sensitivity ofthe bag-dipstick from 85% to 70%, which is comparable tothe sensitivity of the catheter dipstick.

The real magnitude of the effect of this source of biascannot be estimated because we do not know how the char-acteristics of the finally included children compare with thecharacteristics of all eligible children, nor do we know thenumber of not-included, eligible children.

It might well be that the authors can provide us withadditional data. For practical reasons, the urinalysis by bag ispreferred above urinalysis by catheter in non–toilet-trainedchildren presenting in primary care. The data presented donot allow recommendation of this procedure.

Marjolein Y. Berger, MD, PhDMiriam Monteny, MD

Johannes C. van der Wouden, PhDDepartment of General Practice

Erasmus MC – University Medical CenterRotterdam, The Netherlands

10.1016/j.jpeds.2006.07.035

REFERENCE1. McGillivray D, Mok E, Mulrooney E, Kramer MS. A head-to-headcomparison: ‘clean-void’ bag versus catheter urinalysis in the diagnosis ofurinary tract infection in young children. J Pediatr 2005;147:451-6.

Reply

To the Editor:We would like to thank Berger et al for their interest in

our article, in which we reported that the bag dipstick wasmore sensitive than the catheter dipstick. Urine obtained byusing a bag is more likely to contain bacteria or leukocytespresent on perineal skin or in the urethra but not necessarilypresent in the bladder. The longer dwell time of urine ob-tained by using bag specimens versus catheter specimensmight also increase the number of bacteria and white bloodcells, which would increase the yield of positive urinalyses.

Berger et al further suggest that the higher reported sen-sitivity of the bag urine may have been biased because thephysicians were not blinded to the results of the urinalysis. Suchverification bias would potentially increase the proportion ofcatheter urine culture tests performed after a positive bag resultand reduce that proportion after a negative bag result.

Verification bias is indeed a theoretical possibility inchildren older than 90 days, but much less likely in the�90-day age category, because most urine tests in infantsyounger than 90 days were ordered as part of a “septic work-up” for fever without source. The policy in our emergency

department is to obtain catheter culture tests on all infantsyounger than 90 days who have a fever without source. Bagurine culture tests are usually not performed on infants youngerthan 90 days. Table I in our article shows that in infants youngerthan 90 days, the sensitivity of the bag urine was lower than thatin infants older than 90 days.1 However, the trend toward highersensitivity in bag versus catheter urine was preserved (0.69 [95%CI, 0.44-0.94] for bag urine versus 0.46 [95% CI, 0.19-0.73] forcatheter urine). We admit that the numbers are small and thedifference is not statistically significant. However, the results arein keeping with the overall (significant) results reported in chil-dren older than 90 days.

Berger et al argue that 225 negative urinalysis results addedto the study group would make the sensitivity of the bag urinal-ysis equal to that of the catheter urinalysis. That argumentassumes an 8% urinary tract infection rate in children with anegative bag urinalysis results (ie, the same urinary tract infectionrate as those selected for catheter specimens). However, thephysician usually decides whether to order a catheter specimendespite a negative bag urinalysis result after considering thepre-test probability of infection on the basis of age, sex, heightand duration of fever, circumcision status, and presence of aco-existent viral infection. This is confirmed in our study by the28% (69/249) positive culture rate in infants older than 90 days.This clinical selection combined with a negative urinalysis resultwould significantly lower the post-test probability of a positiveculture to �8% in those infants who did not go on to have acatheter urine culture obtained.

In summary, we do not believe that verification bias hada major effect on our study results. We acknowledge in ourarticle that the selective catheterization strategy could miss4% to 12% of urinary tract infections in children older than 90days. The practical aspects of doing a catheterized urineculture in many settings must also be considered when de-ciding how best to detect a urinary tract infection in thelow-risk child. Urinary tract infection is the most commonbacterial infection in pediatrics. If we insist on a catheterculture in all low-risk children, some centers may, for practicalreasons, omit testing completely and a larger proportion ofurinary tract infections will be missed than would be missedby the “selective catheterization” strategy. As in all goodhealth care, availability and quality of follow-up must alwaysbe part of the decision making process.

David McGillivray, MDMichael Kramer, MD

Montreal Children’s HospitalDivision of Pediatric Emergency Medicine

Department of PediatricsMcGill University

Montreal, Quebec, Canada10.1016/j.jpeds.2006.07.037

REFERENCE1. McGillivray D, Mok E, Mulrooney E, Kramer MS. A head-to-headcomparison: “Clean-void” bag versus catheter urinalysis in the diagnosis ofurinary tract infection in young children. J Pediatr 2005;147:451-6.

886 Letters The Journal of Pediatrics • December 2006