Introduction

The recently published INPULSIS studies, two duplicate randomised trials evaluating nintedanib in the treatment of idiopathic pulmonary fibrosis (IPF) over 1 year, reported effectiveness in terms of declining lung function, but inconsistent results on exacerbations of IPF.1 ,2 Indeed, in the prespecified pooled analysis of the two trials, the risk of an investigator-reported acute exacerbation was lower by 36% with nintedanib compared with placebo but not statistically significant (HR, 0.64; 95% CI 0.39 to 1.05; p=0.08). On the other hand, another prespecified analysis found that, after adjudication of these same investigator-reported exacerbations, the risk of a confirmed acute exacerbation was lower by 68% and statistically significant (HR, 0.32; 95% CI 0.16 to 0.65; p=0.001). Such differences in results for the same outcome may puzzle readers and even possibly arouse suspicion of selective or incorrect reporting, especially when the findings straddle the magic 5% conventional level of statistical significance. We believe such differences have a statistical explanation which shows how one result is biased and the other valid.

In this paper, we describe the phenomenon of treatment effect dilution, which can lead to false non-significant effects, using the INPULSIS trials as an illustration. We also provide a contrasting example to this phenomenon using, as an illustration, the case of the risk of pneumonia in trials of inhaled corticosteroids in COPD.

Adjudication of exacerbations

An acute exacerbation of IPF in the INPULSIS trials needed to meet the following criteria: ‘unexplained worsening or development of dyspnoea within the previous 30 days; new diffuse pulmonary infiltrates visualised on chest radiography, HRCT or both, or the development of parenchymal abnormalities with no pneumothorax or pleural effusion (new ground-glass opacities) since the preceding visit and exclusion of any known causes of acute worsening, including infection, left heart failure, PE and any identifiable cause of acute lung injury, in accordance …