Allergy blood testing measures specific IgE antibodies to particular allergens. The allergy blood test (often called RAST or specific IgE testing, with newer assays like ImmunoCAP) provides an alternative to skin testing for identifying allergies. The test has specific advantages and limitations. This guide covers what specific IgE testing measures and how to interpret results.
How specific IgE testing works
The body produces IgE antibodies against specific allergens. Higher specific IgE levels suggest allergic sensitization. Testing measures the amount of IgE specifically targeting individual allergens — pollens, foods, dust mites, animal dander, etc.
Reference ranges
Specific IgE results are typically classified by class (0-VI) or by quantitative kU/L:
- Class 0 (under 0.35 kU/L): negative
- Class I (0.35-0.7): low positive
- Class II-III (0.7-17.5): moderate positive
- Class IV-VI (above 17.5): high positive
Sensitization vs allergy
Important distinction: positive specific IgE indicates sensitization (the body has produced antibodies) but doesn’t necessarily mean clinical allergy (symptoms when exposed). Many people have positive IgE without symptoms.
Clinical allergy diagnosis requires:
- Positive IgE (sensitization)
- Plus symptoms on exposure
This means specific IgE testing complements rather than replaces clinical history. Random screening for allergens you’ve never been exposed to has limited value.
Skin testing vs blood testing
Skin prick testing:
- Faster results (15-20 minutes)
- More sensitive than blood testing for many allergens
- Cheaper
- Requires stopping antihistamines
- Small risk of severe reactions (rare)
Blood testing:
- No allergen exposure required (safer for severe allergies)
- Antihistamines don’t interfere
- Quantitative results
- More expensive
- Slower results
Skin testing is preferred for environmental allergens in most cases. Blood testing is appropriate for patients with severe atopic dermatitis, those who can’t stop antihistamines, infants, or those with high-severity reaction risk.
Food allergy considerations
Food allergy diagnosis is particularly complex. Many people have positive food IgE without clinical allergy. Component testing (testing IgE to specific allergen proteins rather than whole foods) improves specificity.
Definitive food allergy diagnosis often requires oral food challenge under medical supervision. Avoidance of foods based solely on positive IgE without history of reactions is generally not recommended.
Frequently Asked Questions
Does positive IgE mean I’m allergic?
Not necessarily. Sensitization without symptoms is common. Clinical allergy requires positive IgE plus reaction on exposure.
Should I get allergy tested?
For clinical symptoms suggesting allergies — testing helps identify specific triggers. For asymptomatic curiosity — limited value.
Is the blood test as good as skin testing?
For environmental allergies, skin testing is generally more sensitive. Blood testing is preferred in specific situations.
Can children get blood allergy testing?
Yes. Often used in young children where skin testing is difficult.
Should I avoid foods that test positive?
Only if you’ve had clinical reactions. Avoidance based solely on positive IgE without symptoms isn’t recommended.
The bottom line on the allergy blood test
Specific IgE testing identifies sensitization to specific allergens. Clinical interpretation requires correlating with symptoms. Skin testing is often preferred for environmental allergies; blood testing has specific advantages in certain situations. Discuss results with an allergist for proper evaluation.