How do you interpret the results of an Ishihara color blindness test in a patient with a medical history of potential color vision affecting conditions, such as age-related macular degeneration, diabetic retinopathy, or multiple sclerosis?

How to Interpret the Ishihara Color Blindness Test

The Ishihara test is interpreted by counting errors on the first 13-15 transformation and vanishing plates, with 5 or more errors indicating inherited color vision deficiency requiring referral for additional diagnostic testing, while fewer than 5 errors represent incidental non-pathological errors in individuals with normal color vision. 1, 2

Standard Interpretation Protocol

Error Threshold for Screening

• Subjects making fewer than 5 errors on the first 13 plates make common incidental (non-typical) errors not related to color defective vision 1
• Five or more errors identifies some degree of inherited color defective vision and warrants subsequent referral for additional color vision diagnostics 1
• The combined sensitivity of transformation and vanishing plates reaches 95.5% at 8 errors, 97.5% at 6 errors, and 99.0% at 3 errors for detecting red-green color deficiency 2

Test Reliability and Administration

• Test-retest reliability for the Ishihara is high for both persons with inherited color defectiveness and those with normal color vision 1
• The 38-plate edition should be used as the standard screening instrument, with primary focus on the transformation and vanishing plates rather than hidden digit designs 2
• Hidden digit designs only identify approximately 50% of color-deficient subjects and should not be relied upon as primary screening plates 2

Classification of Color Vision Deficiency Type

Protan vs. Deutan Differentiation

• The protan/deutan classification plates are more effective for deutans than protans, with no classification obtained for 18% of protanopes and 3% of deuteranopes who see neither figure 2
• When subjects see both classification figures (40% of protanomalous and 37.5% of deuteranomalous trichromats), classification is based on the relative luminance (clarity) of these figures 2

Special Considerations for Acquired Color Vision Defects

Monitoring for Drug-Induced Toxicity

• For patients on ethambutol therapy, baseline visual acuity testing (Snellen chart) and Ishihara color discrimination testing must be performed before initiating treatment 3
• Monthly testing of visual acuity and color discrimination is recommended for patients taking ethambutol doses greater than 15-20 mg/kg, those receiving the drug for longer than 2 months, and any patient with renal insufficiency 3
• Patients should be questioned at each monthly visit regarding possible visual disturbances including blurred vision or scotomata, and instructed to contact their physician immediately if they experience any change in vision 3

Hydroxychloroquine/Chloroquine Monitoring

• Baseline Ishihara color discrimination testing should be performed within the first year of hydroxychloroquine or chloroquine use 3
• Annual screening with Ishihara tests should begin after 5 years of use, or sooner in the presence of major risk factors including daily dosage >5.0 mg/kg real weight, renal disease, concomitant tamoxifen use, or pre-existing macular disease 3

Critical Pitfalls to Avoid

Common Interpretation Errors

• Failure to utilize the recommended "pass-fail" criterion of 5 or more errors may unnecessarily lead to job discrimination and interfere negatively with career selection 1
• Do not rely solely on hidden digit plates, as they have poor sensitivity (approximately 50%) for detecting color deficiency 2
• Patients who fail initial screening must be allowed recourse to additional testing to establish type and degree of color defective vision before making employment or career decisions 1

Context-Dependent Interpretation

• In patients with age-related macular degeneration, diabetic retinopathy, or multiple sclerosis, abnormal Ishihara results may reflect acquired color vision defects rather than congenital deficiency 3
• Significant central photoreceptor loss from macular disease may cause test abnormalities that interfere with interpretation, making the test less reliable in these populations 3
• The presence of drusen, retinal pigment epithelial atrophy, or macular edema can produce false-positive results that do not represent true color vision deficiency 3

Drug-Specific Monitoring Requirements

• Ethambutol should be discontinued immediately and permanently if there are any signs of visual toxicity detected on Ishihara testing 3
• For hydroxychloroquine patients, Ishihara abnormalities should prompt immediate comprehensive ophthalmologic evaluation including spectral-domain OCT and visual field testing to assess for retinal toxicity 3