Abstract
BACKGROUND: The methacholine challenge requires a 20% fall in forced expiratory volume in one second (FEV(1)). The fall is measured as litre (L) change from the pre-challenge (baseline) value. A higher baseline FEV(1) requires a greater volume change to reach a 20% fall. The aim of this study was to evaluate change using percent predicted, which may remove dependence on the baseline value. METHODS: Challenge data from a cohort of 114 asthma patients was re-analysed. The dose causing an 20% fall from baseline (PD(20)) was compared to a 15% fall in predicted value (PD(15%)) for the classification of bronchial hyperresponsiveness. RESULTS: There was significant agreement between PD(20) and PD(15%) (r = 0.95, p < 0.0001), with an ICC of 0.97. PD(20) was significantly higher than PD(15)(%) (0.0055 mg, p < 0.0001). Greater decreases in FEV(1) were observed with PD(20) versus PD(15%) (21.4% pred vs 19.1% pred respectively, p = 0.0004), with 29% of patients requiring at least one additional dose of methacholine to achieve PD(20) compared to PD(15%). A higher baseline FEV(1) resulted in higher PD(20) values, whereas no relationship was found for PD(15%). Variability in FEV(1) between repeated visits (n = 15) was associated with the change in PD(20), but not the change in PD(15%). CONCLUSION: We suggest a PD criteria based on 15% predicted change should be used for bronchial challenge testing. This method is less influenced by baseline airflow obstruction, and is a more efficient and safer way of measuring airway hyperresponsiveness.