Colour vision testing
- Examination
Colour plates screen for inherited red-green deficiency. A new loss of colour vision in one eye points to the optic nerve.
See it in 3D: Retina · Macula · Optic nerve
What it is
Colour vision depends on three types of cone. Inherited deficiencies, mostly red-green and X-linked, are common in men and stable for life. Acquired deficiencies come from diseases of the optic nerve or macula and from drugs, and they change over time. Pseudoisochromatic plates (such as Ishihara) screen for red-green defects; arrangement tests (such as the Farnsworth D-15) classify the defect.
Normal values and scales
- Inherited red-green deficiency affects about 8% of men of European ancestry and under 1% of women.
- Inherited defects affect both eyes equally and do not change.
- Red desaturation in one eye is an early sign of optic neuropathy.
- Drugs such as ethambutol can cause acquired colour vision loss.
How it is done
- Test each eye separately, in good daylight-type illumination, with reading glasses if needed.
- Show the plates at about reading distance for a few seconds each and record the number of plates read correctly.
- For a quick bedside comparison, ask the patient to compare the redness of a red object seen with each eye (red desaturation).
- In children who cannot read numbers, use plates with shapes or paths.
How to interpret it
- In optic neuritis colour vision is reduced more than acuity would suggest.
- Ethambutol toxicity often starts with loss of red-green discrimination, so colour vision is part of its monitoring.
- Some occupations require normal colour vision; screening is done at school or work entry.
Next step / Initial management
- New colour vision loss in one or both eyes: ophthalmology for optic nerve and macular assessment.
- Patients starting ethambutol: baseline acuity and colour vision, and stop the drug and refer if they fall.
- Inherited deficiency found at screening: explain its nature; no treatment exists.
- Red desaturation with an afferent pupillary defect: follow the optic neuritis or compressive neuropathy pathway.
Test yourself
Think of the answer, then tap the question to see it.
What is the inheritance of the commonest colour vision deficiency?
X-linked recessive red-green deficiency, much more common in men.
What does red desaturation in one eye suggest?
An optic neuropathy, such as optic neuritis or compression.
Why is colour vision checked in patients taking ethambutol?
Because ethambutol optic neuropathy often begins with loss of red-green discrimination.
Connections
See it in 3D3
The structures of Durgical's 3D anatomy where it is found.
In Pharmacology1
Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.
Sources: Color Vision (StatPearls Publishing (NCBI Bookshelf), 2022); Optic Neuritis (StatPearls Publishing (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.