Bronchial challenge testing in occupational epidemiology: is the diluent step really necessary?

There is no unanimous agreement on the use of a diluent step to preface methacholine challenge testing. The "pros" for this step are that it allows a better training of the patient; the "cons" are that it requires additional time and effort.

We examined if there were any systematic effects when either the baseline or postdiluent (saline) forced expiratory volume in 1 second (FEV1) was used to define the reactivity status.

All methacholine challenge tests performed during a 1-year period by a group of occupationally exposed workers (n = 183) were examined.

The mean percentage change in FEV1 from baseline to postsaline was -1.44 (+/- 3.47)% and the mean absolute change was -0.043 (+/- 0.11) L (P < 0.0001 for both comparisons). The maximum decrease and increase in FEV1 from baseline were -12.7% and +10.9%, respectively. Three subjects had a fall in FEV1 after saline of 10% or more and were not given methacholine. From the remaining 180 subjects, 172 were equally classified as reactors (n = 67) or nonreactors (n = 105), both by baseline FEV1 and postsaline FEV1. Eight subjects were classified as reactors by baseline FEV1 but as nonreactors by saline FEV1. In these subjects, the average FEV1 dropped 4.9% from baseline to saline and 17.3% from saline to end-test; thus, the total FEV1 drop (22.5%) exceeded the 20% required for the test to be positive. Among reactors, no relationship was found between the response to saline and the subsequent response to methacholine (r = 0.13).

Our data did not provide evidence to support the compulsory use of a diluent step when measuring bronchial responsiveness in populations. In general, the diluent step added time and expense to the test and, on occasion, forced a greater absolute drop in FEV1 than is needed to demonstrate bronchial hyperresponsiveness.