Color Vision Testing for the CPOA Exam

Visual AssessmentColor VisionCPOA

Color vision testing identifies deficiencies in the ability to discriminate colors. These deficiencies may be congenital (inherited) or acquired (from disease). As a CPOA, you will perform color vision testing as part of the preliminary examination and document the results for the physician's review.

Types of Color Vision Deficiency

Color vision depends on three types of cone photoreceptors (short, medium, and long wavelength). Deficiency of any type creates characteristic color vision problems:

Protan defects (protanopia/protanomaly): Deficiency or absence of red-sensitive (long wavelength) cones. Difficulty distinguishing red from green; red appears dark or black. X-linked inheritance.
Deutan defects (deuteranopia/deuteranomaly): Deficiency or absence of green-sensitive (medium wavelength) cones. Most common congenital color deficiency. Difficulty distinguishing red from green; green appears grayed. X-linked inheritance.
Tritan defects (tritanopia/tritanomaly): Deficiency of blue-sensitive (short wavelength) cones. Autosomal dominant. Difficulty distinguishing blue from yellow.

Because red-green deficiency is X-linked, it affects approximately 8% of males and only 0.5% of females. Blue-yellow deficiency (tritan) is much rarer congenitally but more commonly acquired from disease.

Congenital color deficiency is almost always red-green (protan or deutan), bilateral, symmetric, and affects the patient throughout their life. Acquired color deficiency often affects blue-yellow discrimination, may be asymmetric between eyes, and is associated with optic nerve or macular disease.

Ishihara Plates

The Ishihara pseudoisochromatic plates are the most widely used screening tool for congenital red-green color deficiency. Each plate consists of colored dots arranged to show a number or path visible to those with normal color vision but not to those with red-green deficiency, or vice versa.

Testing technique:

Use in bright, natural-like illumination (or a daylight-balanced lamp). Do not test under fluorescent or incandescent light.
Hold the plate approximately 30 inches (75 cm) from the patient.
Allow 3 to 5 seconds per plate for a response.
Test each eye separately for a complete assessment.
Record the number of plates passed out of the total (e.g., "12/14 plates correct" or "fails red-green screening").

Hardy-Rand-Rittler (HRR) Plates

The Hardy-Rand-Rittler (HRR) plates are a more comprehensive color vision test that screens for both red-green and blue-yellow deficiencies. Unlike Ishihara, HRR uses symbols (circles, triangles, crosses) rather than numbers, making them useful for patients who cannot read. HRR plates can also classify deficiency severity (mild, medium, strong).

Congenital vs. Acquired Color Deficiency

Feature	Congenital	Acquired
Type	Usually red-green	Often blue-yellow
Symmetry	Symmetric (both eyes equally)	May be asymmetric
Progression	Stable lifelong	May progress with underlying disease
Association	Family history, males predominant	Optic nerve disease, macular disease, medications

Acquired color deficiency is an important finding because it may indicate optic neuritis, glaucoma progression, toxic optic neuropathy, or macular disease. A patient who previously had normal color vision and now fails is more clinically significant than one with a lifelong congenital deficiency.

Key Takeaways

Congenital red-green color deficiency is X-linked, affecting 8% of males and 0.5% of females.
Ishihara plates screen for red-green deficiency; HRR plates screen for both red-green and blue-yellow.
Test in bright, natural-like illumination; allow 3 to 5 seconds per plate; test each eye separately.
Acquired color deficiency affecting blue-yellow discrimination suggests optic nerve or macular disease.
Document the number of plates passed, which eye was tested, and any relevant comparison to prior results.

Frequently Asked Questions

Why does red-green color deficiency affect males more than females?

Red-green color deficiency genes are located on the X chromosome. Males have one X and one Y chromosome; females have two X chromosomes. If a male inherits one defective X chromosome, he has no second X chromosome to compensate, so he manifests the color deficiency. A female who inherits one defective X chromosome still has a second normal X that provides working photopigment genes, so she is a carrier but usually has normal color vision. For a female to be color deficient, she must inherit defective genes on both X chromosomes, which is much less likely statistically. This is why red-green color deficiency affects approximately 8% of males but less than 0.5% of females.

What illumination is required for Ishihara plate testing and why?

Ishihara plates must be viewed in bright, natural daylight or under a daylight-balanced light source (approximately 6500 K color temperature). Standard fluorescent or incandescent lighting distorts color rendering, potentially causing false results: a patient with normal color vision might fail plates due to poor lighting, or a color-deficient patient might pass due to reflections or color shifts from the light source. Ishihara test kits often come with recommended illuminant specifications. If a daylight-equivalent lamp is not available, the test can be performed near a window in natural (not direct sun) daylight.

What is the significance of new blue-yellow color deficiency in a patient who had normal color vision previously?

New blue-yellow color deficiency, or any new acquired color deficiency, is a clinically significant finding. Tritan (blue-yellow) defects are rarely congenital; when acquired, they are associated with optic nerve disease (glaucoma, optic neuritis, toxic optic neuropathy), macular degeneration, diabetic retinopathy, or certain medications (hydroxychloroquine, ethambutol). Unlike congenital red-green deficiency, which is stable and present since birth, acquired color deficiency is a warning sign of ongoing disease. The finding should be documented and reported to the physician for follow-up evaluation.

Can a patient with color deficiency pass Ishihara plates by memorizing the answers?

Standard Ishihara plates use numbers, which a motivated patient could potentially memorize if tested repeatedly with the same plates. Randomizing the plate order between tests reduces this risk. HRR plates use geometric symbols (circle, triangle, cross) that are less predictable and harder to memorize since the position and which symbol appears vary. For patients who raise suspicion of feigning or malingering color deficiency (such as in occupational fitness evaluations), HRR or other tests like the Farnsworth D-15 or Farnsworth-Munsell 100-hue test provide more comprehensive and less memorizable assessment.

How should a CPOA document color vision test results?

Color vision results should document: the test used (Ishihara, HRR), which eye was tested (OD, OS, or OU), the number of plates correctly identified out of the total (e.g., 12/14 OD, 14/14 OS), the illumination used, and any abnormality noted (e.g., 'fails red-green screening OD'). If a previous result is available in the chart, note any change from baseline. For acquired deficiency monitoring (such as in hydroxychloroquine toxicity screening), documenting consistently with the same test, illumination, and protocol allows meaningful comparison over time.