Sample sizes for clinical trials using sputum eosinophils as a primary outcome

Clinical trials do not report sputum eosinophil data in a consistent method and this makes it difficult to compare across studies and to evaluate the sample sizes estimated in these studies. The objectives of the paper are: 1) to systematically review reporting of effect size and sample calculations in randomised controlled trials using sputum eosinophil count as a primary outcome and 2) to illustrate sample size estimation under different methods of data representation using data from an effective anti-eosinophil treatment strategy (mepolizumab). Randomised controlled trials in adults (excluding allergen provocation models) of treatment of asthma and chronic obstructive pulmonary disease for the past 10 years were searched in Ovid MEDLINE and 20 studies were identified that met all the inclusion criteria. Only nine studies discussed sample size calculation. Change from baseline was used as an outcome in 11 studies and was expressed as change in absolute percentage count, percentage change from baseline or as fold changes. Assuming a minimal clinically important reduction of 15% in absolute terms, 18 subjects in each arm will be required to achieve 80% power using an ANCOVA analysis, which we recommend, to detect significance with an alpha error of 0.05.