The effect of reduced contrast sensitivity on colour vision testing

Objective: To assess the effect of reduced contrast sensitivity on three commonly used colour vision tests in order to establish key discrepancies that may be relevant for clinical practice. Methods: A prospective non-interventional clinical study of colour vision testing using three commonly used devices: Ishihara and Hardy–Rand–Rittler (H-R-R) pseudoisocochromatic plate tests, and Farnsworth D-15 arrangement test performed under progressively reduced contrast sensitivity conditions achieved with a neutral density filter bar. Results: The Pelli–Robson contrast sensitivity (PRCS) at which 5% of the population should first experience a 10% reduction in colour vision testing from baseline was calculated for each of the three colour vision devices: Farnsworth D-15 test: 1.81 log contrast sensitivity (CS), H-R-R test: 1.69 log CS, and Ishihara test: 1.34 log CS. Single factor repeated measures analyses, conducted separately at each contrast sensitivity level, revealed no difference between the colour vision testing devices at PRCS ≥1.80 log CS (P ≥ 0.367). However, in all PRCS ≤1.65 log CS, the differences were statistically significant (all P ≤ 0.004), demonstrating a significantly lower percentage of errors in the Ishihara test compared with both the Farnsworth D-15 (P < 0.023) and H-R-R (P < 0.035) tests. Conclusions: At high contrast sensitivities, all colour vision tests function almost equally; however, at decreased levels of contrast sensitivity, H-R-R and Farnsworth D-15 are more greatly affected.