My practice is in a town with a strong military tradition. Lots of the residents want to join the military and many come to me to make sure their visual function meets the entrance standards required by the military. These are mainly vision, visual acuity and colour vision. I use a 24-plate Ishihara pseudoisochromatic test to assess colour vision. I want to make sure I am using the test properly as I do not want to make a mistake and miss a patient that has poor colour vision. Do you have any advice?

The 24-plate Ishihara test is commonly used for employment-related colour vision testing including testing prior to joining the military to screen for red-green colour perception problems and has been shown to be the most efficient test for this purpose. A very general indication of red and green defects is given in this test, but it is not, as such, a diagnostic test. The test is easy to use; special training is not necessary. However, interpretation of results is not always easy if only a few plates are failed.

The test plate can be handheld or placed on a table at ‘arm’s length’, approximately 66 cm from the eye. Ideally, daylight illumination or a suitable alternative should be at 45° to the plate surface, i.e. not directly above. The examiner instructs the subject to ‘Tell me the numbers that you can see as I turn the pages.’ The examiner turns the pages, keeping control of the viewing time. About four seconds are allowed for each plate. Undue hesitation can be a sign of slight colour perception problems.

People are often desperate to pass the colour vision test in order to gain employment. The Ishihara plates are available online, and occasionally people try to learn the correct responses in advance of the test because they suspect (or know) that they have a colour vision problem and are prepared to cheat in order to pass the test. Although the test consists of 24 plates there are only 15 plates which should be used to test an adult. People can remember a sequence of 15 single and double-digit numbers.

My strong advice is for all patients who need to pass the test for employment reasons that the plates should be shown in a different order and not the order they appear in the book. This will prevent anyone who has memorised the numbers to pass the test.

Most people with colour perception problems make 12 or more errors with 99% of people with colour perception problems identified with a minimum of six errors. This pass/fail criterion is adequate as a screening formula to determine the presence of colour perception problems.

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