Agreement Among Spectral-Domain Optical Coherence Tomography Instruments for Assessing Retinal Nerve Fiber Layer Thickness

Agreement Among Spectral-Domain Optical CoherenceTomography Instruments for Assessing Retinal Nerve

Fiber Layer Thickness

MAURO T. LEITE, HARSHA L. RAO, ROBERT N. WEINREB, LINDA M. ZANGWILL, CHRISTOPHER BOWD,PAMELA A. SAMPLE, ALI TAFRESHI, AND FELIPE A. MEDEIROS

● PURPOSE: To assess the agreement of parapapillary

retinal nerve fiber layer (RNFL) thickness measurements

among 3 spectral-domain optical coherence tomography

(SD-OCT) instruments.● DESIGN: Observational, cross-sectional study.● METHODS: Three hundred thirty eyes (88 with glau-

coma, 206 glaucoma suspects, 36 healthy) from 208

individuals enrolled in the Diagnostic Innovations in

Glaucoma Study (DIGS) were imaged using RTVue,

Spectralis and Cirrus in a single visit. Agreement among

RNFL thickness measurements was assessed using

Bland-Altman plots. The influence of age, axial length,

disc size, race, spherical equivalent, and disease severity

on the pairwise agreements between different instru-

ments was assessed by regression analysis.● RESULTS: Although RNFL thickness measurements be-

tween different instruments were highly correlated, Bland-

Altman analyses indicated the presence of fixed and

proportional biases for most of the pairwise agreements. In

general, RTVue measurements tended to be thicker than

Spectralis and Cirrus measurements. The agreement in

average RNFL thickness measurements between RTVue

and Spectralis was affected by age (P � .001) and spherical

equivalent (P < .001), whereas the agreement between

Spectralis and Cirrus was affected by axial length (P �

.004) and spherical equivalent (P < .001). Disease severity

influenced the agreement between Spectralis and both RT-

Vue and Cirrus (P � .001). Disc area and race did not

influence the agreement among the devices.● CONCLUSIONS: RNFL thickness measurements ob-

tained by different SD-OCT instruments were not entirely

compatible and therefore they should not be used inter-

changeably. This may be attributable in part to differences

in RNFL detection algorithms. Comparisons with histologic

measurements could determine which technique is most

accurate. (Am J Ophthalmol 2011;151:85–92. © 2011

by Elsevier Inc. All rights reserved.)

ALTHOUGH THE ASSESSMENT OF THE PARAPAPIL-

lary retinal nerve fiber layer (RNFL) is essential in

diagnosis and management of glaucoma, its objec-

tive evaluation remains a challenge in clinical practice.1

Quantitative measurements of RNFL thickness have be-

come possible with the development of imaging technol-

ogies, such as optical coherence tomography (OCT).

Earlier versions of this technology, known as time-domain

OCT (TD-OCT), have demonstrated good reproducibility

and accuracy for detection of RNFL loss in glaucoma.2–4

Recently, the introduction of spectral-domain optical co-

herence technology (SD-OCT) has greatly enhanced the

resolution and decreased scan acquisition times compared

to TD-OCT,5,6 potentially improving the ability to diag-

nose and follow glaucoma.7–12

Three of the current commercially available SD-OCTs

are the RTVue (Optovue Inc, Fremont, California, USA),

the Cirrus SD-OCT (Carl Zeiss Meditec, Inc, Dublin,

California, USA), and the Spectralis OCT (Heidelberg

Engineering, Dossenheim, Germany). The principle in-

volved in image acquisition is similar for all these devices

and involves a scan with a diode laser that collects

information of RNFL thickness in a 3.4-mm-diameter

circle centered on the optic disc. Although the working

principles are similar among the SD-OCTs, the agreement

between them has not yet been reported. With an increas-

ing number of commercially available SD-OCTs, it is

likely that patients examined with 1 machine will have

subsequent examinations performed with another device.

Therefore, it is important to evaluate the agreement in

RNFL thickness measurements among those devices.

The purpose of the present study was to assess the

agreement of parapapillary RNFL thickness measurements

among the RTVue, Cirrus, and Spectralis, and to evaluate

the influence of age, race, spherical equivalent, axial

length, disease severity, and optic disc size on the agree-

ment among the devices.

METHODS

● SUBJECTS: This was an observational cross-sectional

study. Subjects included in this study were recruited from

the longitudinal Diagnostic Innovations in Glaucoma

Accepted for publication Jun 30, 2010.From the Hamilton Glaucoma Center, Department of Ophthalmology,

University of California San Diego, La Jolla, California (M.T.L., H.L.R.,R.N.W., L.M.Z., C.B., P.A.S., A.T., F.A.M.); and Universidade Federalde São Paulo, Department of Ophthalmology, São Paulo, Brazil(M.T.L.,F.A.M.).

Inquiries to Felipe A. Medeiros, Hamilton Glaucoma Center, Univer-sity of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0946; e-mail: [email protected]

© 2011 BY ELSEVIER INC. ALL RIGHTS RESERVED.0002-9394/$36.00 85doi:10.1016/j.ajo.2010.06.041

mailto:[email protected]

Study (DIGS) conducted at the Hamilton Glaucoma

Center (University of California, San Diego).

Each participant underwent a comprehensive ophthalmo-

logic examination including review of medical history, best-

corrected visual acuity, slit-lamp biomicroscopy, intraocular

pressure measurement, gonioscopy, dilated funduscopic ex-

amination with a 78-diopter (D) lens, stereoscopic optic

disc photography, and automated perimetry with the 24-2

Swedish Interactive Threshold Algorithm (SITA; Carl

Zeiss Meditec, Inc). Axial length was acquired with

IOLMaster (Carl Zeiss Meditec). Optic disc area was

calculated using the Heidelberg Retina Tomograph (HRT

II; software version 3.1.2.0, Heidelberg Engineering).

Three scans centered on the optic disc were automatically

obtained for each test eye, and a mean topography was

created. Trained technicians outlined the optic disc mar-

gin while they viewed simultaneous stereoscopic photo-

graphs of the optic disc. Only images with a standard

deviation of �50 �m were included.

To be included, subjects had to have best-corrected

visual acuity of 20/40 or better, spherical refraction

within �5.0 D, cylinder correction within �3.0 D, and

open angles on gonioscopy. Eyes with coexisting retinal

disease, uveitis, or nonglaucomatous optic neuropathy

were also excluded from the investigation.

To study the agreement among the 3 devices in a broad

cohort of patients, we included glaucomatous patients,

individuals suspected of having the disease, and normal

subjects. To be classified as glaucomatous, patients had to

have at least 2 consecutive and reliable standard auto-

mated perimetry (SAP) examinations with either a pattern

standard deviation (PSD) outside the 95% normal limits or

a glaucoma hemifield test (GHT) result outside the 99%

normal limits. Patients considered suspect for glaucoma

had either an IOP greater than 21 mm Hg or suspicious

appearance of the optic nerve head with at least 2 reliable

normal visual fields, defined as a PSD within 95% confi-

dence limits and a GHT result within normal limits.

Normal control subjects were recruited from the general

population and had IOP �22 mm Hg with no history of

elevated IOP and with at least 2 reliable normal visual fields,

defined as a PSD within 95% confidence limits and a GHT

result within normal limits. All visual fields were reviewed by

the VisFACT (Visual Field Assessment CenTer) visual field

reading center. VisFACT checked for the presence of arti-

facts such as lid and rim artifacts, fatigue effects, inattention,

or inappropriate fixation. To be considered reliable, all tests

had to have false-positive responses, fixation loss, and false-

negative responses �33%.

● INSTRUMENTATION: Measurements of the parapapil-

lary RNFL thickness were obtained in the same visit using

the RTVue (Optovue Inc), the Cirrus HD-OCT (Carl

Zeiss Meditec) and the Spectralis OCT (Heidelberg Engi-

neering). Patients were examined without correction. Para-

papillary RNFL thickness parameters that were automatically

calculated by the machines and investigated in this study

included average thickness (360-degree measure), temporal

quadrant thickness (316 degrees to 45 degrees), superior

quadrant thickness (46 degrees to 135 degrees), nasal quad-

rant thickness (136 degrees to 225 degrees), and inferior

quadrant thickness (226 degrees to 315 degrees). All

included images were checked for motion artifacts by

inspection of the continuity of the scanned images (align-

ment of blood vessels). In addition, all images that had

errors on RNFL segmentation were excluded from the

analysis.

The RTVue (software version 4.0.5.39) uses a scanning

laser diode with a wavelength of 840 nm to provide

high-resolution images and has an acquisition rate of 26

000 A-scans per second. The imaging protocol used in this

study was ONH (optic nerve head scan). This protocol

generates a polar RNFL thickness map that is measured

along a circle 3.45 mm in diameter centered on the optic

disc. The RNFL thickness parameters are measured by

assessing a total of 2325 data points between the anterior

and posterior RNFL borders. Only good-quality images, as

defined by a signal strength index of �30, were used for

analysis.

The Cirrus SD-OCT (software version 4.5; Carl Zeiss

Meditec) uses a superluminescent diode laser with a center

wavelength of 840 nm and an acquisition rate of 27 000

A-scans per second. The protocol used for RNFL thickness

evaluation was the optic disc cube. This protocol is based

on a tridimensional scan of a 6 � 6-mm2 area centered on

the optic disc where information from a 1024 (depth) �

200 � 200-point parallelepiped is collected. Then, a

3.46-mm-diameter circular scan is placed around the optic

disc and the information about parapapillary RNFL thick-

ness is obtained. To be included, images were reviewed for

noncentered scans and had to have a signal strength �7

and the absence of movement artifact.

Spectralis OCT (software version 3.1; Model Spectralis

HRA�OCT) uses a dual-beam SD-OCT and a confocal

laser scanning ophthalmoscope (CSLO) that uses a scan-

ning laser diode with a wavelength of 870 nm and an

infrared reference image simultaneously to provide images

of ocular microstructures. The instrument has an acquisi-

tion rate of 40 000 A-scans per second. Spectralis OCT

incorporates a real-time eye tracking system that couples

CSLO and SD-OCT scanners to adjust for eye movements

and to ensure that the same location of the retina is

scanned over time. The protocol used was the RNFL circle

scan, which consists of 1024 A-scan points from a

3.45-mm circle centered on the optic disc. All patients had

their corneal curvature inputted into the machine before

the examination. To be included, all images were reviewed

for noncentered scans and had to have a signal strength

�15 dB.

● STATISTICAL ANALYSIS: The agreement among RNFL

measurements obtained by the different instruments was

AMERICAN JOURNAL OF OPHTHALMOLOGY86 JANUARY 2011

investigated using Bland-Altman plots.13 The differences

between measurements for each parameter were plotted

against their mean. These plots allowed us to determine

the existence of any systematic differences between the

measurements (ie, a fixed bias). The mean difference

between RNFL measurements obtained by the instruments

reflects an estimation of the bias. In addition, we calcu-

lated the 95% limits of agreement for each comparison;

that is, an estimate of how much the measurements

differed in most individuals. Because both eyes per indi-

vidual were used for the analysis, the limits of agreement

were corrected for multiple measurements per individual,

using a method described by Bland and Altman.14 Bland-

Altman plots were also used to evaluate any possible

relationship of the difference between the measurements

and their average (ie, a proportional bias). The presence of

proportional bias indicates that the limits of agreement

will depend on the actual measurement. To formally

evaluate this relationship, the difference between methods

was regressed on their average. If the slope of the regression

line was statistically significant, we considered the exis-

tence of proportional bias. To assess correlation between

the 3 devices, we calculated the coefficients of determina-

tion (R2) for pairwise measurements.

The influence of covariates on the agreement among

instruments was assessed by regression analysis. The differ-

ences in RNFL thickness measurements between pairs of

instruments were included as dependent variables and the

covariates (age, axial length, spherical equivalent, race,

disc size, mean deviation [MD]) as independent variables.

Generalized estimating equations with robust standard

errors were used to adjust for potential correlations

between both eyes of the same individual. Our sample

size was sufficient to provide a narrow 95% confidence

interval of approximately �1.2 �m for the limits of

agreement for the parameter average thickness in all

pairwise comparisons.

All statistical analyses were performed with commer-

cially available software (Stata version 10; StataCorp,

College Station, Texas, USA, and SPSS ver 16.0; SPSS

Inc, Chicago, Illinois, USA). The alpha level (type I error)

was set at 0.05.

RESULTS

THE STUDY INCLUDED 330 EYES (88 WITH GLAUCOMA, 206

glaucoma suspects, 36 normals) from 208 individuals.

Table 1 shows clinical and demographic characteristics of

included subjects. Mean age was significantly different

among groups; glaucomatous patients were older, followed

by glaucoma suspects and normals (P � .001). Mean

disease severity as measured by the visual field MD was

�0.2 dB for normals, �0.66 dB for suspects, and �5.07 dB

for glaucomatous (P � .001). Mean axial length was

similar for all groups; normals had a mean of 24.02 mm,

TABLE 1. Clinical and Demographic Characteristics of the Studied Populationa

Normals n � 36 Suspects n � 206 Glaucomatous n � 88 P Value

Age (years) 58 � 12 64 � 12 69 � 10 �.001

Sex (% male) 29.4 39.8 53.4 .026

Race (% African-American) 50 74.2 57.9 .002

Axial length (mm) 24.02 � 1.12 23.99 � 1.1 24.07 � 1.27 .986

Disc area (mm2) 2.03 � 0.57 2.03 � 0.46 2.09 � 0.51 .802

Mean deviation (dB) �0.2 � 0.96 �0.66 � 1.45 �5.07 � 5.43 �.001

Spherical equivalent (diopters) �0.24 � 2.07 �0.53 � 2.21 �0.445 � 1.77 .738

Pseudophakia (%) 0 16.5 32.9 �.001

aMean � standard deviation values of age, axial length, disc area, spherical equivalent, and mean

deviation reported by diagnostic group.

TABLE 2. Mean (95% Confidence Interval) Average and Quadrant-wise Retinal Nerve Fiber

Layer Thickness Measured by the 3 Devices

RTVue Cirrus Spectralis

Average thickness (�m) 92 (91.2–94.2) 83 (81.5–85.3) 85 (83.4–86.7)

Superior thickness (�m) 110 (108.4–112.3) 101 (98.7–102.9) 99 (97–101.7)

Temporal thickness (�m) 69 (67.8–70.6) 58 (56.4–59) 66 (64.2–67.2)

Inferior thickness (�m) 119 (116.5–121.2) 104 (102–106.9) 111 (107.8–113.2)

Nasal thickness (�m) 72 (70.9–73.8) 68 (67.2–69.6) 65 (62.9–66.3)

AGREEMENT AMONG SPECTRAL-DOMAIN OCTSVOL. 151, NO. 1 87

suspects had 23.99 mm, and glaucomatous had 24.07 mm

(P � .986). Disc area, as measured by the HRT, did not

significantly differ among groups: normals had a mean disc

area of 2.03 mm2, glaucoma suspects had a mean of 2.03

mm2, and glaucomatous had a mean of 2.09 mm2 (P �

.802). No difference was found in spherical equivalent

among groups (P � .738).

Table 2 shows mean values of average and quadrant

RNFL thickness parameters obtained by each instrument.

Table 3 shows the agreement between instruments. For

average thickness, RTVue measurements were significantly

thicker than Cirrus (difference � 9.77 �m; P � .001) and

Spectralis (difference � 7.65 �m; P � .001) measure-

ments. Statistically significantly thicker RTVue measure-

ments were also found for all quadrants (P � .001)

compared to Cirrus and Spectralis. Cirrus measurements

were significantly thinner than Spectralis for average

thickness (difference � 2.12 �m; P � .001), temporal

quadrant (difference � 8.01 �m; P � .001), and inferior

quadrant (difference � 5.99 �m; P � .001). Measurements

with Spectralis were thinner than Cirrus for the nasal

quadrant (difference � 3.81 �m; P � .001). No significant

difference was found for the superior quadrant between

Cirrus and Spectralis (P � .073).

FIGURE 1. Bland-Altman plot for the agreement between

RTVue and Cirrus for the average retinal nerve fiber layer

thickness parameter. The regression line of the difference

between the measurements on their average is represented by

the solid line. Dashed lines show the 95% limits of agreement.


RTVue and Spectralis for the average retinal nerve fiber layer




TABLE 3. Bland-Altman Regression-based 95% Limits of Agreement for RTVue, Cirrus, and Spectralis for the Parapapillary

Retinal Nerve Fiber Layer Thickness

Parameter Agreement

Mean

Difference P Value Fixed Bias R2P Value

Proportional

Bias

95% Limits of

Agreementa

Average thickness (�m) RTVue-Cirrus 9.77 �.001 YES 0.029 .002 YES �0.02 to 19.57

RTVue-Spectralis 7.65 �.001 YES 0.04 .005 YES �3.68 to 18.98

Cirrus-Spectralis �2.12 �.001 YES 0.129 �.001 YES �13.13 to 8.89

Superior thickness (�m) RTVue-Cirrus 9.61 �.001 YES 0.014 .06 NO �7.32 to 26.53

RTVue-Spectralis 11.01 �.001 YES 0.096 �.001 YES �10.71 to 32.73

Cirrus-Spectralis 1.4 .073 NO 0.063 �.001 YES �17.23 to 20.03

Temporal thickness (�m) RTVue-Cirrus 11.52 �.001 YES 0.013 .291 NO �4.56 to 27.60

RTVue-Spectralis 3.51 �.001 YES 0.01 .296 NO �14.77 to 21.79


Inferior thickness (�m) RTVue-Cirrus 14.33 �.001 YES 0.018 .048 YES �3.47 to 32.12

RTVue-Spectralis 8.34 �.001 YES 0.107 �.001 YES �12.5 to 29.17


Nasal thickness (�m) RTVue-Cirrus 3.96 �.001 YES 0.056 .003 YES �17.66 to 25.57

RTVue-Spectralis 7.77 �.001 YES 0.042 .001 YES �14.73 to 30.27

Cirrus-Spectralis 3.81 �.001 YES 0.142 �.001 YES �22.38 to 29.99

aCorrected for multiple measurements per individual according to Bland and Altman.


Table 3 also reports the evaluation of the presence of

proportional bias. Proportional bias was present if the differ-

ence and average of measurements by 2 instruments were

significantly correlated. We found proportional biases for all

pairwise measurements except for the superior RNFL thick-

ness agreement between RTVue and Cirrus, the temporal

RNFL thickness agreement between RTVue and Cirrus, and

the temporal RNFL agreement between RTVue and Spect-

ralis. The agreement among the devices and the 95% limits of

agreement for the average RNFL thickness parameter are

represented graphically in Figures 1 (RTvue vs Cirrus), 2

(RTVue vs Spectralis), and 3 (Cirrus vs Spectralis).

Table 4 shows the coefficients of determination (R2) for

all pairwise comparisons. Correlations ranged from R2�

0.30 (nasal quadrant, Cirrus-Spectralis) to R2� 0.87

(average thickness, RTVue-Cirrus). All correlations were

statistically significant (P � .001).

Table 5 shows the effects of covariates on the agreement

between the devices for the parameter average RNFL

thickness in univariable regression analysis. Agreement

between RTVue and Spectralis was affected by age (P �

.001) and spherical equivalent (P � .001); agreement

between Spectralis and Cirrus was affected by axial length

(P � .004) and spherical equivalent (P � .001); agreement

between Spectralis and both RTVue and Cirrus was

affected by disease severity (P � .001). For more severe

disease, the difference between RTVue and Spectralis and

between Cirrus and Spectralis increased, with Spectralis

providing thinner RNFL thickness measurements with

more advanced disease. Disc area and race did not affect

the agreement among the devices.

Incorporating all covariates (age, MD, axial length, race,

spherical equivalent, and disc size) in multivariable models

resulted in a significant influence of spherical equivalent

(P � .001) and MD (P � .001) on the agreement between

RTVue and Spectralis and a significant influence of MD

(P � .001) and spherical equivalent (P � .04) on the

agreement between Cirrus and Spectralis. No significant

effect of the covariates was found on the agreement

between RTVue and Cirrus.

DISCUSSION

THE PRESENT STUDY EVALUATED THE AGREEMENT IN RNFL

thickness measurements obtained by 3 different SD-OCTs.

Although the technology used by these instruments is

similar, important differences in measurements obtained

by these devices were found, indicating that the measure-

ments should not be used interchangeably.

The narrowest 95% confidence interval of limits of

agreement was obtained for the parameter average RNFL

thickness measurements in all pairwise comparisons. Pre-

vious studies assessing the agreement between TD-OCT

(Stratus OCT, Carl Zeiss Meditec) and RTVue also found

a better agreement for average thickness.7,12 Although all

images were checked for centering of the scans around the

optic disc, the better agreement for the average thick-

ness parameter compared to quadrant measures may

represent the effect of small misplacements of the scan

around the optic disc. It has been shown that noncen-

tered scans induce a greater error in quadrant-wise

measurements obtained with OCT.15 Despite showing

better agreement, the RNFL average thickness measures

were still significantly different among the devices;

therefore, it is unlikely that this parameter would be

robust enough to be used in the follow-up of patients

imaged with different instruments.


Cirrus and Spectralis for the average retinal nerve fiber layer




TABLE 4. Correlation Among RTVue, Cirrus, and

Spectralis for Average and Quadrant-wise Parapapillary

Retinal Nerve Fiber Layer Thickness

Parameter Agreement

Coefficient of

Determination (R2) P Value

Average thickness

(�m)

RTVue-Cirrus 0.87 �.001

RTVue-Spectralis 0.85 �.001

Cirrus-Spectralis 0.86 �.001

Superior thickness

(�m)




Temporal

thickness (�m)




Inferior thickness

(�m)




Nasal thickness

(�m)





When evaluating whether the agreement between in-

struments is acceptable or not, one must consider the

within-subject repeatability of the measurements. If an

instrument has low repeatability, it is likely that compar-

isons with other instruments are going to result in poor

agreement. Previous studies have investigated the repeat-

ability of measurements using the Cirrus and the RTVue.

Leung and associates10 reported an intra-visit repeatability

using the Cirrus of 5.12 �m for the average thickness

parameter in normal and glaucomatous patients. Gonzalez-

Garcia and associates7 found a repeatability coefficient

using the RTVue of approximately 4.34 �m for normal

participants and 4.68 �m for glaucomatous patients using

the average thickness parameter. Considering the intra-

test variability of approximately 5 �m for each instrument,

even if the instruments had perfect agreement, an error of

approximately 10 �m (� 5�m) in the agreement could be

expected because of the variability of measurements. How-

ever, if this difference is larger than 10 �m, other factors

besides repeatability are probably affecting the agreement.

For the average thickness parameter, we obtained 95%

limits of agreement of around 20 �m for the differences

between instrument pairwise comparisons, nearly twice the

expected error from within-instrument variability. It is

likely that other factors, such as differences in hardware

and software, may be affecting the agreement among these

devices.

A consistent difference (fixed bias) between measure-

ments was observed in almost all pairwise comparisons.

RTVue measurements were consistently thicker than Cir-

rus and Spectralis for average and quadrant-wise RNFL

thickness. Interestingly, the differences between measure-

ments using the Cirrus and the Spectralis were not in the

same direction for all parameters. For average thickness,

temporal quadrant, and inferior quadrant thicknesses, the

Spectralis provided thicker readings, while the Cirrus gave

thicker readings for nasal quadrant. This discrepancy may

be explained by the angle of incidence of the laser beam,

which can affect RNFL measurements,16 especially in the

nasal quadrant where the scan light is generally dimmer.17

These results are in concordance with the literature.

Previous studies evaluating the agreement between Stratus

OCT and SD-OCTs showed that average RNFL thickness

as measured by the Stratus was thicker than the Cirrus10

and thinner than the RTVue.7 Therefore, it seems that the

RTVue tends to report thicker RNFL thickness measure-

ments compared to the other devices.

In addition to fixed bias, we investigated the presence of

proportional bias. The existence of proportional bias indicates

that the difference between RNFL thickness measurements

obtained by the devices varies according to the actual

measurement. A study by Vizzeri and associates12 showed a

proportional bias for the agreement between Stratus and the

SD-OCTs. We found proportional biases for almost all

pairwise comparisons. Interestingly, proportional bias was

more pronounced for comparisons with Spectralis. For exam-

ple, the RTVue obtained systematically thicker measure-

ments than the Spectralis; however, this difference was

greater for thinner RNFL. A similar effect was observed for

the agreement between Cirrus and Spectralis. The obvious

TABLE 5. Effect of Covariates on the Agreement Among RTVue, Cirrus, and Spectralis for

the Average Retinal Nerve Fiber Layer Thickness Measurements

Agreement Coefficient RSE P Value

Age (years) RTVue-Cirrus �0.041 0.027 .120

RTVue-Spectralis �0.082 0.026 .001

Cirrus-Spectralis �0.06 0.028 .124

Disc area (mm2) RTVue-Cirrus 0.50 0.56 .379

RTVue-Spectralis 0.58 0.76 .441

Cirrus-Spectralis 0.10 0.70 .888

Axial length (mm) RTVue-Cirrus �0.415 0.278 .135

RTVue-Spectralis 0.678 0.39 .083

Cirrus-Spectralis 1.084 0.377 .004

Mean deviation (dB) RTVue-Cirrus �0.009 0.064 .862



Spherical equivalent (D) RTVue-Cirrus �0.085 0.143 .553

RTVue-Spectralis �0.707 0.15 �.001

Cirrus-Spectralis �0.627 0.157 �.001

Racea RTVue-Cirrus �0.335 0.699 .633



D � diopters; dB � decibels; RSE � robust standard error.aAfrican-Americans � 0; Caucasians � 1.


importance of these results is that the agreement among

instruments is not the same in all ranges of RNFL thicknesses

and, therefore, it is not likely to be the same throughout the

range of glaucoma severity. Although all instruments were

periodically calibrated according to the manufactures’ recom-

mendations and only good-quality images were included in

this study, differences in laser output power may have affected

the agreement in RNFL thickness measurements among

devices. In addition, it is possible that a floor effect is present,

ie, the ability to detect differences in thin RNFL, and is not

equal for all instruments. However, no conclusion can be

drawn regarding accuracy of the measurements without a

direct comparison to histologic measurements.

As expected, a high correlation between the 3 devices

for most of the studied sectors was found. The average

thickness, inferior, and superior quadrant thicknesses, were

strongly correlated for all pairwise comparisons. However,

a poor correlation was found for the nasal quadrant.

Previous studies comparing Stratus OCT and SD-OCT

also showed a weaker correlation and repeatability for the

nasal quadrant.7,9,12 One explanation is that measure-

ments of the nasal quadrant may be influenced by the

incidence angle of the laser beam, leading to a dimmer

light in that quadrant, influencing the RNFL thickness

measurements. It is important to note that the overall good

correlation between measurements obtained by the differ-

ent devices does not represent good agreement, as indi-

cated by our study and discussed in the literature.13

The effect of covariates on the agreement between the

devices was also investigated. Disc area and race had no

influence on the agreement among the devices. Disease

severity influenced the agreement between RTVue and Spec-

tralis and between Cirrus and Spectralis. For more advanced

visual field loss, the Spectralis gave proportionally thinner

readings than RTVue and Cirrus. Because visual loss and

RNFL thickness are closely related, the influence of visual

field loss may be explained by the existence of proportional

bias related to RNFL thickness, as discussed above. We also

found that spherical equivalent affected the agreement be-

tween RTVue and Spectralis and between Cirrus and Spec-

tralis. As the spherical equivalent became more negative (ie,

toward myopia), the Spectralis gave relatively thinner read-

ings than RTVue and Cirrus. Axial length influenced the

agreement between Cirrus and Spectralis; for greater axial

lengths, Spectralis gave thinner measurements compared to

Cirrus, but this effect was not observed when we adjusted for

spherical equivalent. A study by Nagai-Kusuhara and associ-

ates18 reported that eyes with greater axial length tend to

have thinner RNFL, indicating that our findings may be

explained by thinner RNFL thickness. Alternately, it is

possible that magnification errors might be affecting the

measurements obtained from each device and, thus, their

agreement. It is difficult to separate the effect of actual RNFL

thickness from spherical equivalent, axial length, and disease

severity on the agreement among the devices. However, it is

clear that these covariates should be considered when eval-

uating patients with different devices.

In conclusion, RNFL thickness measurements between

several SD-OCT instruments are not entirely compatible,

and therefore measurements from these instruments should

not be used interchangeably. Comparisons with histologic

measurements could determine which technique is most

accurate.

PUBLICATION OF THIS ARTICLE WAS SUPPORTED IN PART BY NATIONAL EYE INSTITUTE, BETHESDA, MARYLAND (GRANTSNEI EY08208 [P.A.S.] and NEI EY11008 [L.M.Z.]); CAPES Ministry of Education of Brazil (Grant BEX1327/09-7 [M.T.L.] and participant retentionincentive grants in the form of glaucoma medication at no cost (Alcon Laboratories Inc, Allergan, Pfizer Inc, SANTEN Inc). Robert N. Weinreb hasreceived financial support from, Optovue, Topcon, Heidelberg Engineering, and Carl Zeiss Meditec, Inc. Linda M. Zangwill has received financialsupport from, Carl Zeiss Meditec, Inc, Heidelberg Engineering, Optovue, Inc, and Topcon Medical Systems, Inc. Pamela A. Sample has receivedfinancial support from, Carl Zeiss, Meditec, Inc, and Felipe A. Medeiros has received financial support from Carl Zeiss Meditec, Inc, HeidelbergEngineering, Reichert, Inc. Involved in design of the study (M.T.L., F.A.M.); analysis and interpretation (M.T.L., H.L.R., F.A.M.); writing the article(M.T.L., H.L.R., F.A.M.); critical revision of the article and final approval (M.T.L., H.L.R., R.N.W., L.M.Z., C.B., P.A.S., A.T., F.A.M.); data collection(M.T.L., H.L.R., A.T.); provision of materials, patients, or resources (R.N.W., L.M.Z., P.A.S., C.B., F.A.M.); statistical expertise (M.T.L., F.A.M.);obtaining funding (R.N.W., P.A.S., L.M.Z., F.A.M.); literature search (M.T.L., H.L.R.); and administrative, technical, or logistical support (R.N.W.,L.M.Z., C.B., P.A.S., F.A.M.). This study was approved by the University of California San Diego Human Subjects Committee and adhered to theDeclaration of Helsinki. Informed consent was obtained from all participants.

This is an original submission and has not been considered elsewhere.

REFERENCES

1. Herrmann J, Funk J. [Diagnostic value of nerve fibre layer

photography in glaucoma]. Ophthalmologe 2005;102(8):

778–782.

2. Jaffe GJ, Caprioli J. Optical coherence tomography to detect

and manage retinal disease and glaucoma. Am J Ophthalmol

2004;137(1):156–169.

3. Medeiros FA, Zangwill LM, Bowd C, Vessani RM,

Susanna R Jr, Weinreb RN. Evaluation of retinal nerve

fiber layer, optic nerve head, and macular thickness

measurements for glaucoma detection using optical

coherence tomography. Am J Ophthalmol 2005;139(1):

44 –55.

4. Wollstein G, Ishikawa H, Wang J, Beaton SA, Schuman

JS. Comparison of three optical coherence tomography

scanning areas for detection of glaucomatous damage.

Am J Ophthalmol 2005;139(1):39 – 43.

5. Nassif N, Cense B, Park B, et al. In vivo high-resolution

video-rate spectral-domain optical coherence tomography of

the human retina and optic nerve. Opt Express 2004;

12(3):367–376.


6. Wojtkowski M, Srinivasan V, Fujimoto JG, et al. Three-

dimensional retinal imaging with high-speed ultrahigh-reso-

lution optical coherence tomography. Ophthalmology 2005;

112(10):1734–1746.

7. Gonzalez-Garcia AO, Vizzeri G, Bowd C, Medeiros FA, Zang-

will LM, Weinreb RN. Reproducibility of RTVue retinal nerve

fiber layer thickness and optic disc measurements and agree-

ment with Stratus optical coherence tomography measure-

ments. Am J Ophthalmol 2009;147(6):1067–1074.e1.

8. Johnson DE, El-Defrawy SR, Almeida DR, Campbell RJ.

Comparison of retinal nerve fibre layer measurements from time

domain and spectral domain optical coherence tomography

systems. Can J Ophthalmol 2009;44(5):562–566.

9. Knight OJ, Chang RT, Feuer WJ, Budenz DL. Comparison of

retinal nerve fiber layer measurements using time domain

and spectral domain optical coherent tomography. Ophthal-

mology 2009;116(7):1271–1277.

10. Leung CK, Cheung CY, Weinreb RN, et al. Retinal nerve

fiber layer imaging with spectral-domain optical coherence

tomography: a variability and diagnostic performance study.

Ophthalmology 2009;116(7):1257–1263, 1263.e1–2.

11. Sung KR, Kim DY, Park SB, Kook MS. Comparison of

retinal nerve fiber layer thickness measured by Cirrus HD

and Stratus optical coherence tomography. Ophthalmology

2009;116(7):1264–1270.e1.

12. Vizzeri G, Weinreb RN, Gonzalez-Garcia AO, et al. Agreement

between spectral-domain and time-domain OCT for measuring

RNFL thickness. Br J Ophthalmol 2009;93(6):775–781.

13. Bland JM, Altman DG. Statistical methods for assessing

agreement between two methods of clinical measurement.

Lancet 1986;1(8476):307–310.

14. Bland JM, Altman DG. Agreement between methods of

measurement with multiple observations per individual.

J Biopharm Stat 2007;17(4):571–582.

15. Vizzeri G, Bowd C, Medeiros FA, Weinreb RN, Zangwill

LM. Effect of improper scan alignment on retinal nerve fiber

layer thickness measurements using Stratus optical coher-

ence tomograph. J Glaucoma 2008;17(5):341–349.

16. Hong S, Kim CY, Seong GJ. Adjusted peripapillary retinal

nerve fiber layer thickness measurements based on the optic

nerve head scan angle. Invest Ophthalmol Vis Sci. 2010;51(8):

4067–4074.

17. Knighton RW, Qian C. An optical model of the human

retinal nerve fiber layer: implications of directional reflec-

tance for variability of clinical measurements. J Glaucoma

2000;9(1):56–62.

18. Nagai-Kusuhara A, Nakamura M, Fujioka M, Tatsumi Y,

Negi A. Association of retinal nerve fibre layer thickness

measured by confocal scanning laser ophthalmoscopy and

optical coherence tomography with disc size and axial

length. Br J Ophthalmol 2008;92(2):186–190.


Biosketch

Mauro T. Leite, MD, is a glaucoma specialist and a post-doctoral fellow at the Hamilton Glaucoma Center, University

of California, La Jolla, California. Dr. Leite has received his medical degree and completed his residency in ophthalmology

at the Federal University of São Paulo, Brazil.

AGREEMENT AMONG SPECTRAL-DOMAIN OCTSVOL. 151, NO. 1 92.e1

Documents

Agreement Among Spectral-Domain Optical Coherence Tomography Instruments for Assessing Retinal Nerve Fiber Layer Thickness