The use of the mannitol test as an outcome measure in asthma intervention studies: a review and practical recommendations

Table 3 Worked examples for sample size calculations—based on choice of study design, existing literature on MCID and within- and between-subject SD using PD₁₅ to inhaled mannitol as outcome measure

	Crossover study design—single PD₁₅ measure	Crossover study design –pre/post PD₁₅ measures	Parallel group design—pre/post PD₁₅ measures
Recommended if	Single dose of drug	Short duration of intervention	Longer duration of intervention
Outcome measure	PD₁₅ post-treatment	Change in PD₁₅ pre/post-treatment	Change in PD₁₅ pre/post-treatment
Within-subject SD*	0.8	–	–
SD* of within-subject difference	–	1.13	–
Between-subject SD* of differences	–	–	1.4
MCID	1 on DD scale (i.e., doubling in PD₁₅)	1 on DD scale (i.e., doubling in PD₁₅)	1 on DD scale (i.e., doubling in PD₁₅)
Total Sample size needed^†	13	23	64

DD dose doubling, MCID minimum clinically important difference, PD₁₅ provoking dose at 15% drop in FEV₁; SD standard deviation
*All SDs are on DD scale i.e., log₂ transformed data; ^†With a probability of a type-I error of 2-sided 5% and power of 80%

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