TY - JOUR T1 - Number needed to treat: enigmatic results for exacerbations in COPD JF - European Respiratory Journal JO - Eur Respir J SP - 353 LP - 354 DO - 10.1183/13993003.01149-2015 VL - 47 IS - 1 AU - Sally Lettis AU - Oliver Keene Y1 - 2016/01/01 UR - http://erj.ersjournals.com/content/47/1/353.abstract N2 - Suissa [1] provides the event-based number needed to treat (NNT) for a reduction in exacerbations with fluticasone/salmeterol compared with placebo for the TORCH (Towards a Revolution in COPD Health) trial by period of follow-up (0–1 years after treatment, 1–2 years after treatment and 2–3 years after treatment) [2]. These NNT data are misleading as they fail to recognise that in chronic obstructive pulmonary disease (COPD) trials, patients who exacerbate are more likely to withdraw from the trial than patients with no exacerbations. Keene et al. [3] show that for the TRISTAN (TRial of Inhaled STeroids ANd long-acting β2 agonists) trial, the exacerbation rate is more than three per year among placebo patients withdrawing prior to 1 year compared with an exacerbation rate of one per year for patients completing a year of placebo treatment. In the TORCH trial, 25% of the placebo group withdrew compared with 15% of the fluticasone/salmeterol arm during the first year of follow-up; therefore, patients entering the second year of the trial on placebo and on fluticasone/salmeterol were no longer directly comparable. This is not accounted for in the calculation of exacerbation rates or NNT during years 2 and 3, as presented in table 2 of the study by Suissa [1].NNT calculations during years 2 and 3 are misleading as they don–t account for differential withdrawal in year 1 http://ow.ly/UeS61 ER -