Different Allergy Tests: Different Results

When you go to your doctor for a blood test to see if you have allergies, the test the doctor picks may or may not give you the right answer.

That's the depressing news from a study published in the May issue of the Journal of Allergy and Clinical Immunology.
"Correlation of serum allergy (IgE) tests performed by different assay systems," by Julie Wang, James H. Godbold, and Hugh A. Sampson, Volume 121, Issue 5, Pages 1219-1224 (May 2008)

Methods
Fifty patients from the Mount Sinai Pediatric Allergy practice were prospectively enrolled. For each deidentified sample, specific IgE levels were measured to egg, milk, peanut, cat, birch, and Dermatophagoides farinae at different laboratories, each using a different assay system (Phadia ImmunoCAP, Agilent Turbo-MP, and Siemens Immulite 2000). Results were analyzed to determine whether IgE measurements were equivalent. Food allergen–specific IgE levels were correlated with clinical data and around empirically determined thresholds that predict probability of clinical disease in 50% or 95% of subjects.

Results
Variable degrees of agreement existed among the 3 assays. Immulite 2000 overestimated all specific IgE levels compared with ImmunoCAP. Turbo-MP overestimated for egg but underestimated for birch and D farinae. Differences for milk, peanut, and cat were observed, without a trend toward overestimation or underestimation. Furthermore, several values for the food allergens were discrepant around the 50% and 95% positive predictive values for clinical reactivity.

Conclusion
Discrepancies in specific IgE values from 3 different assays can potentially lead to altered management and treatment. The predictive values for clinical reactivity associated with food-specific IgE levels determined by ImmunoCAP should not be applied to results from other assays.

This is a fairly specialized result, aimed at practicing doctors rather than patients. While it implies that current results are not always as accurate as they could be - hardly news to many people with allergies judging from the complaints I hear about testing - being able to tell in the future which test works best for different allergies can help to improve the accuracy of diagnoses, so if these results hold up they would be good news for allergy sufferers.