Clinical assessment of two new contrast sensitivity charts
- 1Department of Optometry, Moorfields Eye Hospital NHS Foundation Trust, London, UK
- 2UCL Institute of Ophthalmology, London, UK
- Correspondence to: Dr M D Crossland UCL Institute of Ophthalmology, 11-43 Bath Street, London EC1V 9EL, UK;
- Accepted 8 December 2006
- Published Online First 13 December 2006
Background: Contrast sensitivity measurement in UK clinical practice is most commonly performed with the Pelli–Robson chart.
Aims: To compare the repeatability of two new contrast sensitivity charts and to measure their agreement with the Pelli–Robson charts.
Method: Contrast sensitivity was measured monocularly using two versions of the Mars letter contrast sensitivity chart, two presentations on the Test Chart 2000 and two versions of the Pelli–Robson chart. Bland–Altman techniques were used to assess repeatability and agreement.
Results: 53 subjects were recruited with visual acuity from 6/4 to 6/72. The coefficient of repeatability was 0.182 for the Pelli–Robson chart, 0.121 for the Mars chart and 0.238 for Test Chart 2000. Limits of agreement with the Pelli–Robson chart were −0.29 to +0.15 log units for the Mars letter contrast sensitivity chart and −0.32 to +0.78 log units for the Test Chart 2000. For patients with poor contrast sensitivity, the limits of agreement between the Test Chart 2000 and the Pelli–Robson chart improved from −0.33 to +0.15 log units.
Conclusion: In a population of hospital ophthalmology patients, the coefficient of repeatability is better for the Mars chart and worse for the Test Chart 2000 when compared with the Pelli–Robson chart. The electronic test chart does not agree well with the Pelli–Robson chart, although this might simply be due to the performance of liquid crystal display screens at low contrast levels. The Mars letter contrast sensitivity chart shows good validity and reasonable agreement with the Pelli–Robson chart.
↵i The Sloan letters are C, D, H, K, N, O, R, S, V and Z.
↵ii Mahalanobis distance is a measure of the correlation between individual variables that can be used to determine the presence of outliers. A point with high Mahalanobis distance has high leverage—that is, it has a greater influence on the coefficients of the regression equation than other points. To include these points would (1) violate the assumption of normality required for these analyses and (2) potentially introduce bias from one erroneous measurement.
Published Online First 13 December 2006
Competing interests: None declared.
Parts of this work were presented at the Association for Research in Vision and Ophthalmology (ARVO) Annual Conference, Fort Lauderdale, Florida, 2 May 2006.