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Welcome to question of the day #183

Eyetools question of the day #183

Throughout the World, the most common colour vision test is the Ishihara Pseudoisochromatic Plates Colour Vision Deficiency test. However, this test has some well-known flaws:

  • Some people with colour vision deficiency memorise the numbers to pass the test, usually to get a job that requires good colour vision.
  • It can be difficult for people who aren’t good at recognising numbers.
  • Some of the plates have numbers that can only be seen by people with colour vision problems, and they may say that they can’t see the numbers in order to try and pass the test.
  • Pass and fail criteria are not clear.
  • The classification plates used to differentiate between people with problems with red recognition from those with problems with green recognition are not reliable.
  • None of the plates is able to detect blue-yellow colour deficiency, tritan deficiencies.

There is an alternative option, the Hardy Rand Rittler (HRR) pseudoisochromatic test.

The most recent version has colours nearer to the dichromatic confusion lines. It has 24 plates each displaying one or two symbols, which can be a cross, a circle or a triangle. The symbols are constructed of coloured dots on a background of grey dots. The coloured dots have chromaticity coordinates that lie on or close to the protan, deutan or tritan dichromatic confusion loci that pass through the chromaticity coordinates of the grey background colours. This means that for people with colour vision problems the coloured symbols blend into the grey background and cannot be identified.

The patient is asked to name the shape of each symbol they see and indicate its location, which can be in one of four quadrants of each plate.

There is a reasonably clear cut off between pass and fail. A patient who makes three or more errors on the screening plates has abnormal colour vision; a patient who makes one or two errors probably has abnormal colour vision, but there is a small risk (2.5 in 100) that this diagnosis is incorrect and the patient may have normal colour vision. Patients who make only two errors should be retested or tested with another test to establish whether they have abnormal colour vision.

The HRR can be used confidently to detect red?green colour vision deficiency and is as good as the Ishihara in doing this. It will detect tritan deficiencies, which the Ishihara does not.

The HRR is not as well-known as the Ishihara, so the risk of persons learning the correct answers to enable them to pass the test is low. Also, the HRR can be presented upside down to confound those who have been learnt the correct answers.

A matching card is available to help those who aren’t good at recognising numbers complete the test.