Background: Considerable, unexamined controversy exists
surrounding the use of the highest vs the lowest FEV1 for
calculating the provocative concentration causing a 20% fall in
FEV1 (PC20) during direct bronchoprovocation
Objective: To compare the PC20
calculated using the lowest FEV1 post-diluent and
post-histamine/methacholine vs the PC20 calculated using
the highest FEV1 post-diluent and
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 lowest 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, and 0.95,
respectively. The PC20 calculated using the lowest to
lowest FEV1 was 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 identical to the
PC20 values calculated using the lowest FEV1.
The difference, although clinically irrelevant, holds statistical