title: Minimum clinically important difference for the COPD Assessment Test: a prospective analysis.
abstract: BACKGROUND
  The COPD Assessment Test (CAT) is responsive to change in patients with chronic obstructive pulmonary disease (COPD). However, the minimum clinically important difference (MCID) has not been established. We aimed to identify the MCID for the CAT using anchor-based and distribution-based methods.
  
  
  METHODS
  We did three studies at two centres in London (UK) between April 1, 2010, and Dec 31, 2012. Study 1 assessed CAT score before and after 8 weeks of outpatient pulmonary rehabilitation in patients with COPD who were able to walk 5 m, and had no contraindication to exercise. Study 2 assessed change in CAT score at discharge and after 3 months in patients admitted to hospital for more than 24 h for acute exacerbation of COPD. Study 3 assessed change in CAT score at baseline and at 12 months in stable outpatients with COPD. We focused on identifying the minimum clinically important improvement in CAT score. The St George's Respiratory Questionnaire (SGRQ) and Chronic Respiratory Questionnaire (CRQ) were measured concurrently as anchors. We used receiver operating characteristic curves, linear regression, and distribution-based methods (half SD, SE of measurement) to estimate the MCID for the CAT; we included only patients with paired CAT scores in the analysis.
  
  
  RESULTS
  In Study 1, 565 of 675 (84%) patients had paired CAT scores. The mean change in CAT score with pulmonary rehabilitation was -2·5 (95% CI -3·0 to -1·9), which correlated significantly with change in SGRQ score (r=0·32; p<0·0001) and CRQ score (r=-0·46; p<0·0001). In Study 2, of 200 patients recruited, 147 (74%) had paired CAT scores. Mean change in CAT score from hospital discharge to 3 months after discharge was -3·0 (95% CI -4·4 to -1·6), which correlated with change in SGRQ score (r=0·47; p<0·0001). In Study 3, of 200 patients recruited, 164 (82%) had paired CAT scores. Although no significant change in CAT score was identified after 12 months (mean 0·6, 95% CI -0·4 to 1·5), change in CAT score correlated significantly with change in SGRQ score (r=0·36; p<0·0001). Linear regression estimated the minimum clinically important improvement for the CAT to range between -1·2 and -2·8 with receiver operating characteristic curves consistently identifying -2 as the MCID. Distribution-based estimates for the MCID ranged from -3·3 to -3·8.
  
  
  CONCLUSIONS
  The most reliable estimate of the minimum important difference of the CAT is 2 points. This estimate could be useful in the clinical interpretation of CAT data, particularly in response to intervention studies.
  
  
  BACKGROUND
  Medical Research Council and UK National Institute of Health Research.
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Imperial College, London, UK. Electronic address: s.kon@rbht.nhs.uk.
authors.forename: Samantha S C
authors.initials: SS
authors.lastname: Kon
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Imperial College, London, UK.
authors.forename: Jane L
authors.initials: JL
authors.lastname: Canavan
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Imperial College, London, UK.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Jones
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Harefield Pulmonary Rehabilitation Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Imperial College, London, UK.
authors.forename: Claire M
authors.initials: CM
authors.lastname: Nolan
authors.affiliation: Harefield Pulmonary Rehabilitation Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK.
authors.forename: Amy L
authors.initials: AL
authors.lastname: Clark
authors.affiliation: The Hillingdon Hospital NHS Foundation Trust, Uxbridge, UK.
authors.forename: Mandy J
authors.initials: MJ
authors.lastname: Dickson
authors.affiliation: The Hillingdon Hospital NHS Foundation Trust, Uxbridge, UK.
authors.forename: Brigitte M
authors.initials: BM
authors.lastname: Haselden
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Imperial College, London, UK.
authors.forename: Michael I
authors.initials: MI
authors.lastname: Polkey
authors.affiliation: NIHR Respiratory Biomedical Research Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Harefield Pulmonary Rehabilitation Unit, Royal Brompton & Harefield NHS Foundation Trust, London, UK; Imperial College, London, UK.
authors.forename: William D-C
authors.initials: WD
authors.lastname: Man
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006305:Health Status Indicators; D006760:Hospitalization; D006801:Humans; D008297:Male; D008875:Middle Aged; D017063:Outcome Assessment, Health Care; D011446:Prospective Studies; D029424:Pulmonary Disease, Chronic Obstructive / Q000150:complications / Q000175:diagnosis* / Q000628:therapy; D012372:ROC Curve; D015203:Reproducibility of Results; D011795:Surveys and Questionnaires