Predictive value of sGaw, FEF(25-75), and FEV1 for development of asthma after a negative methacholine challenge test

A 20% change in forced expiratory volume in 1 second (FEV(1)) during methacholine challenge testing (MCT) is a reliable marker of asthma. When the FEV(1) decrease is < 20%, there is controversy whether other changes in flows and conductance may be useful. We conducted this study to determine whether changes in sGaw, FEF(25 - 75), and FEV(1) in a negative MCT could predict future occurrence of asthma over a 3-year period.

A total of 100 consecutive patients with clinical suspicion of asthma but who had a negative MCT per ATS FEV(1) criteria (< 20% FEV(1) decline at 16 mg/mL of methacholine) performed by the 5-breath dosimeter method were analyzed. Two pulmonary fellows, blinded to MCT results, reviewed the patients' medical records. Patients were classified into one of three categories: asthmatic, unclear, and not asthmatic. Decreases in sGaw, FEF(25 - 75), and FEV(1) in the five groups were then retrieved. Analysis of variance (ANOVA) was used for data analysis.

Of 100 patients, 23 were excluded owing to lack of a 3-year follow-up. After complete data review, the number of patients (n) in each group was as follows: asthmatic (n = 15), unclear (n = 7), and not asthmatic (n = 55). sGaw and FEF(25 - 75) decreases from the negative MCT could not predict asthma; however, decreases in FEV(1) were associated with future asthma occurrence (sGaw p = 0.21, FEF25-75 p = 0.07, FEV(1) p = 0.0009). Forty-three percent of the patients who had a 10% to 20% decline in FEV(1) eventually developed asthma.

Up to 20% of patients who have symptoms suggestive of asthma but a negative MCT can still develop asthma. Declines in sGaw and FEF(25 - 75) in a negative MCT appear to have no clinical significance. A decrease in FEV(1), especially 10% to 20%, is associated with the diagnosis of future asthma.