The 1999 ATS guidelines for methacholine challenge testing discussed sGaw (specific airways conductance) as a surrogate marker for the FEV1 in airways obstruction, and indicated that a drop in sGaw of 45% correlates to a drop in FEV1 of 20%. This is felt to be due to the lack of specificity of sGaw compared to FEV1. In the absence of significant decline in the FEV1, other studies have promoted sGaw as a more sensitive test for airways reactivity while others suggest that sGaw may be an indicator of upper airway disease.
We evaluated 138 consecutive patients for quality control purposes at a large referral center to examine the relationship of how the FEV1 is related to change in sGaw during methacholine challenge testing. sGaw and FEV1 measurements were obtained as recommended by ATS guidelines using SensorMedics pulmonary function testing systems with Spectra software.
A decrease of 20% in FEV1 correlated with a drop of 56% in sGaw (95% confidence interval 52% to 60%, r2=0.32, p=.0001). In individuals who had zero change in FEV1 at the highest concentration of methacholine (5 puffs of 16 mg/ml), there was a decline in the sGaw of 31%. (95% confidence interval 27% to 37%, r2=0.32, p=.0001).
This data demonstrates that a drop in FEV1 >= 20% is associated with a decline in sGaw of 56 ± 4%, which is a significantly greater decline than the previously reported value of 45%. This data indicates sGaw measurements are even less specific than previously reported. Even in individuals who demonstrated absolutely no change in the FEV1 at the highest concentration of methacholine, there was a 31% decline in the sGaw.
As such, we believe that the cutoff of 45% change in sGaw is too sensitive and should not be used to define a positive study. Further research using well-defined subjects with and without asthma should be done to better assess the test characteristics of sGaw.
M. Sescioreanu, None.