RAST Test: Definition, Indications, and Interpretation
The RAST (RadioAllergoSorbent Test) is an obsolete term that should be abandoned—it has been replaced by modern fluorescence-labeled allergen-specific IgE (sIgE) assays, which are useful for identifying potential IgE-mediated allergic reactions but are not diagnostic of allergy on their own 1.
What RAST Was (and What Replaced It)
RAST was an older radioimmunological method developed in the 1960s-70s to detect allergen-specific IgE antibodies in serum. Modern testing now uses more sensitive fluorescence enzyme-labeled assays (such as ImmunoCAP, Turbo-MP, or Immulite systems), and the term "RAST" should no longer be used 1.
When Allergen-Specific IgE Testing Is Indicated
Allergen-specific IgE tests are recommended for identifying foods or allergens that potentially provoke IgE-mediated allergic reactions 1. Specific situations where serum testing is particularly useful include:
- When skin prick testing cannot be performed: Patients with extensive dermatitis, dermatographism, or those unable to discontinue antihistamines 1
- Food allergy evaluation: To identify potential food allergens, though oral food challenge remains the gold standard 1
- Drug allergy assessment: For penicillin allergy, though skin testing is more reliable than serum-specific IgE testing for IgE-mediated reactions 2
Important Caveat
For penicillin allergy specifically, skin testing is superior to serum-specific IgE testing (sensitivity 30.7% vs 19.3%), and serum testing should not replace skin testing when evaluating IgE-mediated penicillin reactions 2.
How to Interpret Results
Critical Interpretation Principles:
The presence of allergen-specific IgE indicates allergic sensitization, NOT necessarily clinical allergy 1. This is the most common pitfall in interpretation.
- Higher sIgE levels correlate with increased probability of clinical reaction upon allergen exposure, though predictive values vary between studies 1
- Specified "cutoff" levels (95% predictive values) may be more predictive than skin tests in certain populations 1
- Undetectable sIgE levels do NOT rule out IgE-mediated allergy—if clinical history is highly suggestive, proceed to physician-supervised oral food challenge 1
Laboratory-Specific Considerations:
Results from different laboratories or assay systems are NOT comparable and cannot be used interchangeably 1. A study comparing three different systems (ImmunoCAP, Turbo-MP, Immulite 2000) found significantly different sIgE measurements for identical serum samples. Therefore, predictive values established for one system cannot be applied to another 1.
What NOT to Do:
- Do not measure total serum IgE for food allergy diagnosis—it has insufficient sensitivity and specificity 1
- Do not use the ratio of sIgE to total IgE—it offers no advantage over sIgE alone 1
- Do not diagnose allergy based solely on positive sIgE—clinical correlation and often oral challenge are required 1
Clinical Algorithm for Use:
- Start with thorough clinical history to determine if reaction pattern is consistent with IgE-mediated allergy
- Perform skin prick testing first when feasible (more sensitive and immediate)
- Use allergen-specific IgE testing when:
- Skin testing is contraindicated or impractical
- Results need to supplement unclear skin test findings
- Patient cannot discontinue antihistamines
- Interpret positive results as indicating sensitization requiring clinical correlation
- Proceed to supervised oral challenge when history is suggestive but testing is negative, or when confirming clinical relevance of positive tests
Never use allergen-specific IgE testing as a standalone diagnostic tool—it must be integrated with clinical history and, when appropriate, confirmatory challenge testing 1.