Value of a negative aeroallergen skin-prick test result in the diagnosis of asthma in young adults: correlative study with methacholine challenge testing.

BACKGROUND: None of the existing tests for the diagnosis of asthma are considered to be definitive. Certain circumstances require prompt diagnosis, and a test able to predict the absence of asthma would be very useful.

OBJECTIVE: To evaluate the contribution of a skin-prick test (SPT) to the diagnostic workup of subjects with suspected asthma.

PATIENTS AND METHODS: The study included three groups of subjects aged 18 to 24 years: group A, asthmatic patients (n = 175); group B, control subjects (n = 100); and group C, subjects with suspected asthma (n = 150) with normal spirometry findings and a negative exercise challenge test result. All underwent an SPT to a battery of common aeroallergens, and group C underwent a methacholine challenge test (MCT) in addition. The sensitivity, specificity, positive predictive value, and negative predictive values (NPV) of the SPT were calculated using provocative concentrations of methacholine causing a 20% fall in FEV(1) (PC(20)) of < 4 mg/mL and < 8 mg/mL as diagnostic cutoff values for asthma in the MCT. Bayes' formula was used to determine posttest probabilities of having asthma, both for positive and negative SPT results.

RESULTS: A positive SPT result to at least one allergen was found in 95.5%, 54%, and 69% of patients in the three groups, respectively. The sensitivity, specificity, and NPV of the SPT were 90.7%, 52.0%, and 84.8%, respectively, with a cutoff value of PC(20) < 8 mg/mL. The lower cutoff, PC(20) < 4 mg/mL, increased the sensitivity and NPV to 98.2% and 97.8%, respectively. A negative SPT result decreased the probability of having asthma by 10-fold to 20-fold in subjects whose pretest probability was low to moderate.

CONCLUSIONS: Incorporating an SPT into the workup of subjects with suspected asthma can reduce the cost of this process significantly. The SPT may be used as a simple, fast, safe, inexpensive, and reliable method to predict the absence of asthma in young adults.