Screening for congenital colour vision defects

Abstract

A prospective comparison between the Ohkuma' and Ishihara2 pseudoisochromatic (PIC) plates was carried out in a group of 400 patients attending a general ophthalmology practice. The sensitivity of the Ohkuma test was compared to the lshihara test, and the specificity of both was determined by reference to anomaloscopy as a gold standard.Both tests correctly identified the same group of 24 patients as having a red/green confusion axis, and the Ohkuma test was equally as sensitive as the Ishihara. The grading plates in both tests are unreliable, but the Ohkuma test is quicker, easier to administer, gives less ambiguous responses and has a clearer cut-off score for abnormality. On the basis of this experience the Ohkuma test is recommended as more appropriate for routine colour vision screening than either the 24 or 38 plate lshihara tests. Key words:Colour vision defects, lshihara plates, Ohkuma plates, pseudoisochromatic plates.The Ishihara plates constitute one of the most widely used screening tests for colour vision defects in ophthalmic practice. Although the pigments were chosen empirically, the test has been so successful that it is recommended as a screening tool by the International Civil Aviation Organisation. The sensitivity and specificity depends on the population under test and the cut-off level chosen. Hill and Aspinal13 report the probability of correctly classifying normal colour vision is 99% in both males and females, but the probability of correctly diagnosing an abnormal observer from a failed Ishihara test is 82% in males and 19% in females if the recommended cut-off is used. A lower cutoff improves the sensitivity, while a higher cut-off improves the specificity. These data suggest that ifthe Ishihara test is used with a cut-off at one error, the sensitivity will be over 99% but all positive results require a second highly specific test, such as anomaloscopy, to remove the inevitable false positives.The underlying principle of anomaloscopy is that if an observer is asked to mix a standard red with a standard green in the proportions necessary to match a test light of standard yellow, then the proportions of red and green accepted by the observer as a match for the yellow are informative about that observer's relative sensitivity for the red and green stimuli. The relationship between this test result and the absorption spectra of the anomalous photoreceptor pigments is complex, and at this stage the relationship to the underlying molecular genetics is ~n c e r t a i n .~ Nevertheless, the anomaloscope remains the gold standard for colour vision testing, with 100% specificity by definition.The variation in the redlgreen ratio accepted as a match is known as the matching range, which covers the full dynamic range of the instrument in a true dichromat. Normal trichromats accept very small variations in the redlgreen ratio around a mean value known as the mid-match point. Anomalous trichromats have a widened matching range and a shift in the mid-match point which is diagnostic of the col...