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Calculation of Provocative Concentration Causing a 20% Fall in FEV1*: Comparison of Lowest vs Highest Post-Challenge FEV1

Beth E. Davis, BSc; Donald W. Cockcroft, MD, FCCP
Author and Funding Information

*From the Division of Respiratory Medicine, Department of Medicine, Royal University Hospital, Saskatoon, Canada.

Correspondence to: Donald W. Cockcroft, MD, FCCP, Division of Respiratory Medicine, Royal University Hospital, 103 Hospital Drive, Saskatoon, SK S7N0W8 Canada; e-mail: cockcroft@sask.usask.ca



Chest. 2000;117(3):881-883. doi:10.1378/chest.117.3.881
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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 challenges.

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 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 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 significance.

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