Mastering Colour Vision Testing

Whether you’re still a student or preparing for Stage 1, Stage 2, or OSCEs, here are the main takeaways from her session.

The Genetics of Colour Vision Defects

• Inheritance pattern: Most congenital colour vision defects are X-linked recessive, meaning they are much more common in males. Around 8% of men and 0.5% of women are affected.
• Family history: Boys inherit their X chromosome from their mother. If she is a carrier, there’s a chance her son will be colour deficient. Girls would need to inherit two defective X chromosomes to be affected, making it rarer.
• Clinical relevance: Because of this prevalence, it’s important to check colour vision in children—particularly boys under 16—and anyone with a relevant family history.

Types of Colour Vision Defects

Congenital

• Trichromatic: All three cone pigments are present.
• Dichromatic: One cone pigment is missing (Protanopia – L-cone, Deuteranopia – M-cone, Tritanopia – S-cone).
• Anomalous Trichromacy: All cones are present, but one is abnormal in sensitivity, leading to varying levels of colour discrimination.

Acquired

• Develop after birth, often linked to ocular disease or systemic conditions.
• Clues include monocular differences, progression over time, reduced acuity, and associated visual field loss.
• Causes can include optic neuritis, macular disease, cataract, diabetes, AMD, and drugs (e.g. hydroxychloroquine, tamoxifen, digoxin, sildenafil).

Occupational Implications

Colour vision can impact career choices. Under the Disability Discrimination Act (1995), restrictions must be justifiable, but certain professions have strict standards. Examples include:

• Police
• Train drivers
• Pilots
• Firefighters
• Electrical engineers
• Armed forces (Army, Navy, RAF)

Optometrists may be asked to complete vision assessment forms for applicants, so familiarity with occupational standards is key.

Colour Vision Tests

Ishihara Plates

• Most widely used screening test for red-green congenital defects.
• Standard version: 38 plates, though a 24-plate version is common in practice.
• Test distance: 66–75 cm, with daylight or equivalent illumination.
• 3+ errors = fail.
• Useful for children (using tracing tasks on the “pathway plates”).
• Limitation: Only detects red-green defects, not tritan (blue-yellow).

City University Test

• Two-part test:
  • Part 1: Spot the odd one out in vertical columns.
  • Part 2: Match central colour to one of four surrounding colours.
• Can classify protan, deutan, and tritan defects, and grade severity.
• More versatile than Ishihara, especially for acquired cases.

Other Tests

• Nagel Anomaloscope: Gold standard for diagnosing red-green anomalies, distinguishing between dichromats and anomalous trichromats. Rarely available in general practice.
• D15 test: Used for occupational screening in specialist settings. Not routinely performed in high-street practice.

Supporting Patients

While there is no treatment for congenital colour vision defects, optometrists play an important role in identifying and advising patients.

• For children, teachers may need to adapt classroom tasks (e.g. labelling pencils, avoiding reliance on colour-only cues).
• For adults, referral may be appropriate for further classification if occupational restrictions are a concern.

Key Revision Pointers

• Always test children (especially boys) and those with family history.
• Know the difference between congenital and acquired defects.
• Revise the setup and interpretation of Ishihara and City tests.
• Be aware of career implications and how to advise patients.

Colour vision testing is a small part of the optometric exam, but it carries big implications for patients’ daily lives and career choices. By understanding the genetics, classifications, and testing methods, you’ll not only be prepared for your exams—but also for real-world clinical practice.