Background: Contrast sensitivity measurement in UK clinical practice is most commonly performed with the Pelli-Robson chart. Two new clinical tests of contrast sensitivity that have recently become commercially available are the hand-held Mars Letter Contrast Sensitivity chart and the computer-based Test Chart 2000. This study compares the variability of these new contrast sensitivity charts and their agreement with the Pelli-Robson charts, for a cohort of hospital patients with and without visual impairment.
Method: 53 subjects were recruited from the low vision and refraction clinics at Moorfields Eye Hospital. Contrast sensitivity was measured in the right eye only with the optimal refractive correction using two versions of the Mars chart, two presentations on the Test Chart 2000 and two versions of the Pelli- Robson chart. Corrections were made to allow for the test working distance. Tests were presented in a counterbalanced order. Bland-Altman techniques were used to assess repeatability and agreement.
Results: 53 subjects were recruited with visual acuity from 20/12 to 20/240. 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 Test Chart 2000 and Pelli-Robson improved to -0.33 to +0.15 log units.
Conclusion: In a population of hospital ophthalmology patients, coefficient of repeatability is better for the Mars chart and worse for the Test Chart 2000 when compared to 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 LCD screens at low contrast levels. The Mars letter contrast sensitivity chart shows good validity and reasonable agreement with the Pelli-Robson chart.
- Contrast sensitivity
- Visual impairment