@ERSpublicationsWhat have we learned from the COPD Assessment Test over its first 5 years? http://ow.ly/z8JCdIt is 5 years since the development and first validation of the COPD Assessment Test (CAT) [1], so the review of papers concerning the CAT by GUPTA et al. [2] in this issue of the European Respiratory Journal is very timely. The authors identified over 40 studies using a systematic approach to minimise biases, although they acknowledge that they introduced one bias by selecting papers published only in English, French and Spanish, so eight papers were excluded. That may not be too problematic, as the included papers report data on over 25 000 patients from 35 countries across five continents (none from Australasia or Antarctica).One of the strengths of a systematic review is that it can allow comparison of variability in outcomes across different studies. To this end, the data extracted for this review provide an indication of the reliability of the CAT in different settings, and it is generally very good. For example, the internal consistency measured using Cronbach's alpha was in the range 0.85-0.98 across nine studies. This observation is important because the CAT was developed in English but these studies included Arabic-speaking countries, Brazil, Greece, Japan, South Korea and Turkey. This observation supports the finding by KWON et al. [3], in a direct comparison, that the relationship between the CAT and the St George's Respiratory Questionnaire (SGRQ) was the same across four Asian countries. The review also shows that test-retest reliability is good across studies, with intraclass correlation coefficients ranging 0.80-0.98, the worst repeatability being in an English language study.Cross-sectional comparisons with comprehensive disease-specific health status measures such as the SGRQ and Clinical COPD Questionnaire showed the expected good cross-sectional correlations, in the range 0.68-0.92. Encouragingly, there were also good within-patient longitudinal correlations, with r-values of ,0.6 (longitudinal correlations are always weaker than cross-sectional ones, because the range of within-patient changes is smaller than differences between patients). In contrast, the correlation with modified Medical Research Council (mMRC) grade was much more variable (0.29-0.61), showing that the CAT and the mMRC measure constructs that are rather weakly related. The mean CAT score in mMRC grade 0 patients was 13.8 in the studies reviewed here, showing that the mMRC is relatively insensitive compared with the CAT, at least at the mild end of the spectrum. It also supports the conclusion that, while a mMRC cut point of o1 identifies a similar proportion of patients to that identified by a CAT cut point of o10, the make-up these groups is different [4]. This confirms that the implication (made by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) in 2011) that CAT o10 is equivalent to mMRC o2 was incorrect, and it supports the GOLD 2014 recommendation that a comprehensive assessment such as the CA...