Predictive Value of sGaw, FEF_{25 − 75}, and FEV₁ for Development of Asthma after a Negative Methacholine Challenge Test

Abstract

Background. A 20% change in forced expiratory volume in 1 second (FEV₁) during methacholine challenge testing (MCT) is a reliable marker of asthma. When the FEV₁ 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 FEV₁ in a negative MCT could predict future occurrence of asthma over a 3-year period. Methods. A total of 100 consecutive patients with clinical suspicion of asthma but who had a negative MCT per ATS FEV₁ criteria (< 20% FEV₁ 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 FEV₁ in the five groups were then retrieved. Analysis of variance (ANOVA) was used for data analysis. Results. 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 FEV₁ were associated with future asthma occurrence (sGaw p = 0.21, FEF25-75 p = 0.07, FEV₁ p = 0.0009). Forty-three percent of the patients who had a 10% to 20% decline in FEV₁ eventually developed asthma. Conclusion. 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 FEV₁, especially 10% to 20%, is associated with the diagnosis of future asthma.

Keywords:

• asthma
• methacholine challenge test
• specific conductance (sGaw)
• forced mid-expiratory flow (FEF_{25 − 75})
• forced expiratory volume in 1 seconds (FEV₁)