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Measurement airway hyperresponsiveness

Airway hyperresponsiveness can be measured and this measurement is expressed as the PC20, i.e., the concentration of the provocative agent (such as histamine or methacholine) which causes a 20% fall in FEV1. The less required, the lower the PC20  and the more responsive the airways.

Exposure to an initiating agent can cause asthma without associated reduction in PC20. During the pollen season the PC20 falls. It rises as the pollen season ends, but takes considerable time to return to normal.

References

Sterk PJ, Bel EH. Bronchial hyperresponsiveness: the need for a distinction between hypersensitivity and excessive airway narrowing. Eur Respir J 1989;2:267-274.