The Advantages of FEV₁ Percent Predicted Change During Bronchial Challenge Testing

The methacholine challenge requires a 20% fall in forced expiratory volume in one second (FEV₁). The fall is measured as litre (L) change from the pre-challenge (baseline) value. A higher baseline FEV₁ 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.

Challenge data from a cohort of 114 asthma patients was re-analysed. The dose causing an 20% fall from baseline (PD₂₀) was compared to a 15% fall in predicted value (PD_15%) for the classification of bronchial hyperresponsiveness.

There was significant agreement between PD₂₀ and PD_15% (r = 0.95, p < 0.0001), with an ICC of 0.97. PD₂₀ was significantly higher than PD_15% (0.0055 mg, p < 0.0001). Greater decreases in FEV₁ were observed with PD₂₀ 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 PD₂₀ compared to PD_15%. A higher baseline FEV₁ resulted in higher PD₂₀ values, whereas no relationship was found for PD_15%. Variability in FEV₁ between repeated visits (n = 15) was associated with the change in PD₂₀, but not the change in PD_15%.

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.