The cambridge colour test: Reliability of discrimination trivectors in colour space

Background: The reliable assessment of colour discrimination has become increasingly important as some ophthalmic diseases and systemic conditions manifest themselves via acquired impairments of colour vision. The Cambridge Colour Test (CCT) is a computerised procedure, developed using the basic principles of traditional pseudoisochromatic plates to evaluate colour function or discrimination. Aim: The study investigated reliability of the CCT, with emphasis on the Trivector subtest, which measures thresholds along the three protan, deutan and tritan confusion lines to probe the sensitivity of the long, medium and short wavelength cones. Setting: The study used a prospective observational and quantitative design and took place in a research and clinical environment within the Department of Optometry at the University of Johannesburg, South Africa. Participants were 20 young adults of both genders aged 19-24 years with normal colour vision. Methods: Two measurements (test and retest) of the Trivector test were measured monocularly (right eye) for all participants to assess the reliability of repeated measurements. Reliability was assessed using coefficients of repeatability, coefficients of variation and Bland-Altman plots with limits of agreement (LoA). Results: Outcomes for the Bland-Altman LoA showed good intra-individual agreement and coefficients of repeatability and reliability revealed generally high test-retest repeatability. Multivariate stereo-pair scatter plot comparisons of means and differences of test and retest measurements show that points cluster tightly within the same region in the three-dimensional colour space, and the centroids of the ellipsoids representing the mean differences are close to the origin at zero. Conclusion: Although departure from normality and outlying values can have important influences, essentially the CCT provides very similar test and retest results for the Trivector test.