Calculation of provocative concentration causing a 20% fall in FEV1 - Comparison of lowest vs highest post-challenge FEV1

Background: Considerable, unexamined controversy exists surrounding the use of the highest vs the lowest est FEV, for calculating the provocative conc entration causing a 20% fall in FEV1 (PC20) during direct bronchoprovocatio n challenges. Objective: To compare the PC20 calculated using the lowest FEV1 post-diluen t and post-histamine/methacholine vs the PC20 calculated using the highest FEV1 post-diluent and post-histamine/methacholine. Method: Retrospective analysis of 225 challenges: 75 research methacholine challenges, 75 research histamine challenges, and 75 clinical methacholine challenges. For each test, the PC20 was calculated twice, first using the l owest post-diluent FEV1 to the lowest post-histamine/methacholine FEV1, and then using the highest to the highest. Results: The intraclass correlation coefficients for methacholine research, histamine research, and methacholine clinic challenges were 0.99, 0.98, an d 0.95, respectively. The PC20 calculated using the lowest to lowest FEV1 w as slightly and significantly lower in all three groups (paired t test p < 0.0001). Conclusions: The PC20 values calculated using the highest FEV1 are almost i dentical to the PC20 values calculated using the lowest FEV1. The differenc e, although clinically irrelevant, holds statistical significance.