How to determine a positive result in an erythema (skin) test?

How to Determine a Positive Erythema (Skin) Test Result

Direct Answer

For allergy skin prick testing, a positive result is defined as a mean wheal diameter ≥3 mm, while for intradermal testing, a positive result requires a mean wheal diameter ≥5 mm, both measured at 15-20 minutes after allergen placement. 1, 2

Measurement Technique and Timing

When to Read the Test

• Read skin test reactions at 15-20 minutes after allergen placement for immediate hypersensitivity (IgE-mediated) reactions 1, 3, 2
• This timing captures the peak wheal and flare response from mast cell degranulation and histamine release 1, 3

What to Measure

• Measure both wheal (raised, palpable induration) and erythema (redness), but the wheal diameter is the primary determinant of positivity 1, 2
• Record measurements of both wheal and erythema for allergen, positive control (histamine), and negative control 1
• Measure the mean diameter of the wheal in millimeters 2
• Erythema alone without wheal formation does not constitute a positive result 1, 2

Interpretation Criteria by Test Type

Skin Prick/Puncture Testing

• Positive: Mean wheal diameter ≥3 mm 2
• This threshold provides sensitivity and specificity typically exceeding 80% 1, 3
• The wheal must be accompanied by surrounding erythema (flare) 1

Intradermal/Intracutaneous Testing

• Positive: Mean wheal diameter ≥5 mm 2
• Intradermal testing is more sensitive than prick testing and should be reserved for cases where prick testing is negative but clinical suspicion remains high 1, 3
• The higher threshold (5 mm vs 3 mm) accounts for the increased sensitivity and potential for false-positive reactions with intradermal technique 2

Essential Controls

Positive Control (Histamine)

• Must demonstrate adequate skin reactivity to validate the test 1
• If the histamine control fails to produce an adequate wheal, the test is invalid and may indicate antihistamine interference or anergy 1

Negative Control (Saline/Diluent)

• Should produce no wheal or minimal reaction (<3 mm) 1
• A positive negative control suggests dermatographism or nonspecific skin reactivity 1

Critical Pitfalls to Avoid

Medication Interference

• Antihistamines suppress skin test responses and must be discontinued before testing 1
• Document all medications taken within the past week, as tricyclic antidepressants and other medications can also suppress reactions 1
• If medications cannot be discontinued, consider serum-specific IgE testing instead 1, 3

Measuring Erythema Instead of Wheal

• Common error: measuring only the erythema (redness) rather than the raised wheal 1, 2
• The wheal represents the actual allergic response; erythema alone is nonspecific 2
• Erythema typically extends beyond the wheal and should not be the primary measurement 4

Clinical Correlation Required

• A positive skin test indicates sensitization, not necessarily clinical allergy 1
• The negative predictive value is high (>95%), but positive predictive value is only 40-60% 1
• Correlate positive results with clinical history of symptoms upon exposure to that specific allergen 1
• Positive tests without corresponding symptoms may represent clinically irrelevant sensitization 2

Special Considerations

Dermatographism

• Patients with prominent dermatographism may have false-positive results 1
• The negative control will help identify this issue 1
• Consider serum-specific IgE testing as an alternative 1, 3

Extensive Skin Disease

• Eczema or other widespread skin conditions may preclude accurate skin testing 1
• Blood testing for allergen-specific IgE is preferred in these patients 1, 3

Intradermal Testing Safety

• Never proceed directly to intradermal testing without performing prick testing first 1, 3
• Five of six historical fatalities from intradermal inhalant testing occurred in asthmatics who did not receive prick testing first 1, 3
• Intradermal testing carries higher risk of systemic reactions 1