Optimal skin prick wheal size for diagnosis of cat allergy.
BACKGROUND
The skin prick test is the diagnostic procedure of choice for determination of immediate hypersensitivity. A wheal diameter of 3 mm or larger is generally accepted as the cutoff for a positive test result, although the validity of this assumption has not been rigorously demonstrated.


OBJECTIVE
To determine the skin prick wheal size that best identifies clinical allergy to cat.


METHODS
Forty-five patients referred for evaluation of rhinoconjunctivitis underwent determination of atopic status by skin testing using the Greer Dermapik device and a combination of other modalities, including history, in vitro determination of specific IgE level, and nasal challenge with standardized cat pelt extract. Parameters evaluated before and after nasal challenge included symptom score and nasal lavage tryptase and prostaglandin D (PGD2) levels.


RESULTS
The widely accepted 3-mm wheal for a positive skin test result to cat is highly sensitive but only moderately specific for diagnosis of cat allergy as evaluated by history, specific IgE level, postchallenge symptom score, and tryptase and PGD2 levels. Optimal cutoffs for a positive skin test result to cat based on receiver operating characteristic analysis and 95% positive predictive value were 5.5 mm or greater for each of these parameters. When a true-positive result for cat allergy was defined as a combination of positive history, specific IgE level, postchallenge symptom score, and tryptase and PGD2 levels and a true-negative result as all of these parameters being negative, a 6-mm cutoff was able to distinguish cat allergic from cat nonallergic individuals.


CONCLUSIONS
In a potentially allergic population undergoing skin prick testing with the Greer Dermapik using standardized extracts, a 3-mm skin prick wheal will overestimate the presence of cat allergy. A 6-mm wheal appears to distinguish those individuals who are cat allergic from those who are not.
Zarei
Maryam
M
Department of Allergy, Center for Medical Education, Kaiser Permanente Medical Center, Los Angeles, California, USA.
Remer
Candace F
CF
Kaplan
Michael S
MS
Staveren
Anne M
AM
Lin
Ching-Kow E
CK
Razo
Elma
E
Goldberg
Bruce
B
D000328:Adult; D000368:Aged; D000818:Animals; D002415:Cats / Q000276:immunology*; D006801:Humans; D006967:Hypersensitivity / Q000097:blood / Q000175:diagnosis*; D007073:Immunoglobulin E / Q000097:blood; D008875:Middle Aged; D017817:Nasal Lavage Fluid / Q000737:chemistry / Q000276:immunology; D012016:Reference Values; D012882:Skin Tests / Q000592:standards*
D007073:Immunoglobulin E