The “C test” for tritan discrimination

The aim of this study was to develop and evaluate a clinical standard capable of generating ordinal measures of monocular tritan discrimination. A novel pseudoisochromatic plate test was developed, called the “C test.” It contains 10 progressively desaturated plates arranged in perceptually distinct steps that provide a standard for analyzing the threshold for tritan discrimination. The most difficult plate that can be detected is the “C score.” Relevant diagnostic findings including the C score, Logmar acuity, Pelli‐Robson score, and LOCS III lens grading were prospectively recorded in 568 eyes. A total of 355 normal eyes were selected for statistical analysis. The correlations between C score, Logmar acuity, and Pelli‐Robson score with LOCS III grading were analyzed with Spearman rank analysis. The strongest correlations were between the Logmar acuity with posterior subcapsular cataract (PSC) (0.68), the C score with nuclear colour (NC) (–0.58), and the Logmar acuity with NC (0.57). The lower normal limit for the C score was 9 in pseudophakes and phakic eyes with NC < 3, 7 for eyes with NC = 3, but undetermined for eyes with NC > 3. A comparison of C score distribution between three distinct age ranges of pseudophakic eyes ( n  = 136) showed no significant variation ( P  = .486). The Spearman rank correlation between C score and Logmar acuity was 0.028, and between C score and Pelli‐Robson score was 0.012. The normal limit for the C score used under 800 Lux is 9 when NC < 3 and 7 when NC = 3 in all age groups.