Background

The gold standard calculation of methacholine PD20 uses the fraction of methacholine particles <5µm contained in the output of the nebuliser. In the absence of this information, the comparable fraction with saline must be used. We have previously reported that this results in differing PD20 values between nebulisers. Albuterol fraction is more comparable to that of methacholine and is more commonly characterised for available nebulisers.

Aim

To determine if methacholine PD20 calculated using Albuterol particle fraction improves accuracy.

Methods

15 subjects performed a tidal dosimetric challenge (ProvoX system) and a tidal breathing challenge (AeroEclipse BAN II nebuliser), at least 3 days apart. The tidal dosimetric PD20 was calculated using particle fraction previously reported for both saline (PD20_S) and albuterol (PD20_A), whereas the tidal breathing PD20 was reported using methacholine particle fraction (PD20_M). The former utilised doubling output time to increase dose, with fewer methacholine concentrations, and the latter utilised doubling concentrations to increase dose. P values <0.05 was considered significant, after adjusting for multiplicity.

Results

Median (IQR) PD20 values were: PD20_S = 46 (35-77) µg; PD20_A = 14 (11-24) µg; PD20_M = 21 (13-49) µg. Median differences were: PD20_S ? PD20_A = 31.6 µg (p<0.0001); PD20_S ? PD20_M = 30.6 µg (p=0.0105); PD20_A ? PD20_M = -3.6 µg (p=0.60).

Conclusion

Methacholine dose characterised by Albuterol resulted in lower PD20 values compared with those from saline, and were similar to the PD20 values derived from the gold standard calculation (using methacholine fraction). Saline particle fraction should not be used to characterise methacholine dose.