Abstract
BACKGROUND:
American Thoracic Society (ATS) guidelines for methacholine challenge testing (MCT) discuss specific airways conductance (sGaw) as a surrogate marker for forced expiratory volume in the first second (FEV1) to diagnose airways obstruction. The guidelines suggest a cutoff value of 45% drop in sGaw to diagnose a positive MCT. However, there is no available evidence that supports this cutoff value of 45%. We conducted this study to examine the relationship between FEV1 and sGaw during MCT.
METHODS:
One-hundred thirty-eight patients who had both sGaw and FEV1 measured during MCT between April 2003 and March 2004 were retrospectively evaluated. The tests were done according to the ATS guidelines. Data were first analyzed using linear regression modeling, comparing the change in FEV1 to changes in sGaw. Then the sensitivity and specificity were generated for different cut points, using receiver operating characteristic analysis.
RESULTS:
Thirty-eight patients had a positive MCT based on ATS FEV1 criteria. A decrease of 20% in FEV1 correlated with a drop of 56% in sGaw (95% confidence interval 52% to 60%, r2 0.35, P < .001). Using 20% decline from baseline in FEV1 at different PC20 (provocational concentration that produced a ≥ 20% FEV1 decrease) values (4 mg/mL, 8 mg/mL, and 16 mg/mL), we then analyzed the sensitivity, specificity, positive predictive value, and negative predictive value of the 45% decline in sGaw and compared it with a 56% decline in sGaw. Using receiver operating characteristic analysis, we were able to find that a cutoff of 51–52% performed better than either of the 2 values.
CONCLUSIONS:
Our study suggests that the ATS suggested cutoff value of 45% decline in sGaw to diagnose a positive MCT may be rather generous, and a decline of 51% from baseline may provide a more accurate measure of airway hyper-responsiveness. Further studies using well defined subjects with and without asthma should be done to better assess the test characteristics of sGaw.
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