Calculation of provocative concentration causing a 20% fall in FEV(1): comparison of lowest vs highest post-challenge FEV(1).

BACKGROUND Considerable, unexamined controversy exists surrounding the use of the highest vs the lowest FEV(1) for calculating the provocative concentration causing a 20% fall in FEV(1) (PC(20)) during direct bronchoprovocation challenges. OBJECTIVE To compare the PC(20) calculated using the lowest FEV(1) post-diluent and post-histamine/methacholine vs the PC(20) calculated using the highest FEV(1) 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 PC(20) was calculated twice, first using the lowest post-diluent FEV(1) to the lowest post-histamine/methacholine FEV(1), 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, and 0.95, respectively. The PC(20) calculated using the lowest to lowest FEV(1) was slightly and significantly lower in all three groups (paired t test p < 0.0001). CONCLUSIONS The PC(20) values calculated using the highest FEV(1) are almost identical to the PC(20) values calculated using the lowest FEV(1). The difference, although clinically irrelevant, holds statistical significance.