Diagnosing cystic fibrosis--asthma and failure to thrive as indications for a sweat test.

Children with asthma or failure to thrive (FTT) are frequently referred for the quantitative pilocarpine iontophoresis sweat test (QPIT) for diagnosing cystic fibrosis (CF). The aim of our study was to compare the prevalences of asthma and FTT among patients with a confirmed diagnosis of CF. Two-hundred and fifty-five children, who were referred for QPIT between 1991-1996 in order to rule out CF were studied retrospectively. All sweat samples had been obtained and analyzed according to the National Committee For Clinical Laboratory Standards. Asthma was defined as 24 episodes of wheezing, treated successfully by inhaled bronchodilators. Failure to thrive was was defined by either a weight below the 5th precentile or crossing of 2 major precentiles in 1 year. The prevalence of asthma was significantly higher than that of FTT among the cohort study (28.6% vs. 7.1% as a single indication, and 36.5% vs. 7.9% when combined with other indications). The prevalence of CF among patients with FTT was higher than among asthmatic patients (5.2% vs. 3%, p = 0.06). Not one patient diagnosed as having CF was referred to a sweat test only because of asthma. As a single presentation, the diagnostic yield of asthma seems to be poor, compared to FTT. However, further, larger-scale studies should be conducted on this issue.