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A comparison of skin prick tests, intradermal skin tests, and RASTs in the diagnosis of cat allergy

Robert A. Wood, MD; Wanda Phipatanakul, MD; Robert G. Hamilton, PhD; and Peyton A. Eggleston, MD

Journal of Allergy and Clinical Immunology, Vol. 103, No. 9, Part 1, May 1999

The Study
One-hundred twenty patients were challenged with a cat exposure model after first evaluating by history, skin prick tests (SPTs), intradermal skin tests (IDSTs) if SPT results were negative, and in vitro allergy testing using Pharmacia & Upjohn [Pharmacia Diagnostics AB] technology. Purpose of the study was to determine the predictive value of SPTs, IDSTs, and RASTs.

Conclusion
"Although both SPT and RAST values exhibited excellent efficiency in the diagnosis of cat allergy, IDST scores added little to the diagnostic evaluation." (pp. 773-779)

Editorial Highlights

SPTs and RASTs exhibited high levels of sensitivity, specificity, positive predictive values (PPVs), negative predictive values (NPVs), and overall efficiency. (p. 777)
The poor sensitivity, poor specificity, and positive predictive efficiency of IDSTs suggest they be used with caution and never be used alone as the basis for major therapeutic decisions.
For respiratory allergy, no gold standard for testing exists, although the authors suggest well-standardized and highly potent SPT as the most sensitive.
It is noted that potency of allergen extracts can result in variations in SPT results. Further, test methods used in the application of SPTs can vary, and it is imperative that clinicians select a method that has been adequately tested and standardized. (p. 779)
RAST displayed excellent sensitivity, specificity, PPVs, and NPVs.
RAST scores were indicated as somewhat less sensitive than SPT, although it was noted that sensitivity would have been improved had ImmunoCAP been used throughout the study.
While the authors acknowledge this study was based on cat allergen only, the expectation is that results would be similar across most, if not all, allergens. (p. 779)

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