Differences in classification of COPD group using COPD assessment test (CAT) or modified Medical Research Council (mMRC) dyspnea scores: a cross-sectional analyses
© Kim et al.; licensee BioMed Central Ltd. 2013
Received: 23 January 2013
Accepted: 29 May 2013
Published: 3 June 2013
The GOLD 2011 document proposed a new classification system for COPD combining symptom assessment by COPD assessment test (CAT) or modified Medical Research Council (mMRC) dyspnea scores, and exacerbation risk. We postulated that classification of COPD would be different by the symptom scale; CAT vs mMRC.
Outpatients with COPD were enrolled from January to June in 2012. The patients were categorized into A, B, C, and D according to the GOLD 2011; patients were categorized twice with mMRC and CAT score for symptom assessment, respectively. Additionally, correlations between mMRC scores and each item of CAT scores were analyzed.
Classification of 257 patients using the CAT score vs mMRC scale was as follows. By using CAT score, 60 (23.3%) patients were assigned to group A, 55 (21.4%) to group B, 21 (8.2%) to group C, and 121 (47.1%) to group D. On the basis of the mMRC scale, 97 (37.7%) patients were assigned to group A, 18 (7.0%) to group B, 62 (24.1%) to group C, and 80 (31.1%) to group D. The kappa of agreement for the GOLD groups classified by CAT and mMRC was 0.510. The mMRC score displayed a wide range of correlation with each CAT item (r = 0.290 for sputum item to r = 0.731 for dyspnea item, p < 0.001).
The classification of COPD produced by the mMRC or CAT score was not identical. Care should be taken when stratifying COPD patients with one symptom scale versus another according to the GOLD 2011 document.
KeywordsCOPD CAT mMRC scores
Chronic obstructive pulmonary disease (COPD) is characterized by persistent airflow limitation . The degree of airflow limitation is associated with many disease outcomes but was poorly predictive of dyspnea and quality of life [2–4]. Lung function alone does not explain the heterogeneous features of COPD [5, 6]. Therefore, the Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2011 document proposed a new classification system for COPD, combining symptom assessment and exacerbation risk including spirometry to identify disease severity .
For assessing symptoms, GOLD 2011 primarily recommends the use of the Modified.
British Medical Research Council (mMRC) questionnaire or the COPD Assessment Test (CAT). The mMRC scale is a 5-point (0–4) scale based on the severity of dyspnea . The CAT comprises eight items relating to the severity of cough, sputum, dyspnea, chest tightness, capacity for exercise and activities, confidence, sleep quality and energy levels  while mMRC scale is a quantitative assessment tool only for breathlessness. These questionnaires are used to distinguish patients with less severe symptoms from patients with more severe symptoms; low vs high symptoms in the new GOLD 2011. However, a recent report indicated that group assignment of COPD patients could be different by the symptom scale that is used ; Han ML et al. used mMRC and SGRQ scores (as a surrogate of CAT scores) for assessing symptoms of COPD. The group assignment of COPD patients by mMRC vs SGRQ was not identical .
Hence, this study was performed to see whether the classification of COPD group according to GOLD 2011 would be identical irrespective of symptom scales (mMRC vs CAT score), and to evaluate the associations between two symptom scales.
Symptoms were quantified with both the mMRC  and the CAT  (http://www.catestonline.org). The mMRC scale 0–4 was developed by the American Thoracic Society as a modification of the originally proposed British Medical Research Council dyspnea index (scale 1–5) . The CAT consists of 8 items. Each item is scored from 0 to 5. An overall score is calculated by adding the score from each item; a total score ranging from 0 to 40. We determined the distribution of COPD patients with the mMRC and with the CAT. According to the GOLD 2011 classification , patients were stratified with either mMRC score (0–1 vs ≥2) or CAT score (<10 or ≥10) resulting in two low-symptom categories (A and C) and two high-symptom categories (B and D). Exacerbation risk was assessed with either FEV1% predicted (<50% or ≥50%), or COPD exacerbation history (0–1 vs ≥2) in the previous year to stratify patients into low-risk groups (A and B) versus high-risk groups(C and D). An exacerbation was defined as an acute event characterized by a worsening of the patient’s respiratory symptoms that is beyond normal day to day variations and leads to a change in medication [1, 11]. The number of exacerbations in the previous year was taken from patients’ history at the same time when mMRC and CAT score were measured. Patients underwent standardized spirometry before and after the inhalation of 10 mg of salbutamol by nebulizer . Finally, the patients with COPD were categorized into A, B, C, and D combining symptom assessment by COPD assessment test (CAT) or modified Medical Research Council (mMRC) dyspnea scores, and exacerbation risk according to the 2011 GOLD report.
All statistical analyses were done with IBM SPSS Windows version 20.0. All data are presented as mean (SD) where appropriate. Kappa coefficient is used to interpret the extent of agreement between two symptom scales (mMRC vs CAT). The discussion of agreement in this study is based on kappa value as described previously in the literature in which k < 0.00 is “poor,” 0 < k < 0.02 is “slight,” 0.21 < k < 0.40 is “fair,” 0.41 < k < 0.60 is “moderate,” 0.61 < k < 0.80 is “substantial,” 0.81 < k < 1.00 is “almost perfect”, and k = 1 is “perfect” agreement . The correlation between mMRC dyspnea score and CAT score was examined using the Spearman rank correlation coefficient (rho), because the MRC dyspnea score is an ordinal categorical variable. Differences were considered statistically significant when p < 0.05.
Clinical characteristics and demographics of patients (n = 257)
Mean or number of patients
SD or %
Sex (M:F), n
ever smoker, n
never smoker, n
FEV1, % predicted
FEV1 ≥ 50%, n
FEV1 < 50%. n
Frequent exacerbator, n
Classifications of COPD groups using mMRC scores (n = 257)
mMRC ≥ 2
Classifications of COPD groups using CAT scores (n = 257)
CAT < 10
CAT ≥ 10
Distributions of CAT total scores at each mMRC level (n = 257)
CAT total scores
% of Total
% of Total
% of Total
% of Total
Correlations between mMRC and each item of CAT scores
Each item of CAT
A main finding of this study was that the group classification of COPD produced by each symptom scale was not identical because the distributions of CAT scores were heterogeneous at every mMRC level. The group classification is important because treatment is recommended according to the groups of COPD in the 2011 GOLD.
The mMRC scale is widely used to measure breathlessness because of brevity and simplicity . However, it is a unidimensional measurement to quantify only dyspnea. The CAT score is a multidimensional method, which assess 8 items; not only dyspnea but also other symptoms and health status [8, 14]. It has proposed as a new tool for assessment of COPD health status in daily practice since it has the advantage of being easy to perform and it can be associated with clinically important variables (FEV1%, exacerbation as well as MRC dyspnea scale) [15, 16]. And, the CAT has good repeatability  and is responsive to exacerbation onset and recovery .
GOLD 2011 recommends the CAT or mMRC scores to distinguish the symptom groups (high vs low symptoms). And, GOLD mentioned that it is unnecessary to use more than one symptom scale. However, a wide range of CAT scores was seen at each mMRC score in our study. The group assignment of COPD patients by each symptom scale was different. As the symptomatic cutpoint to differentiate the symptom groups, GOLD proposed either CAT score of 10 or mMRC score of 2. But, this study showed that an mMRC of 2 did not correspond with a CAT of 10. An mMRC of 1 was comparable to a CAT of 10. These results support a recent report  Han et al. used St George’s Respiratory Questionnaire (SGRQ) score as a surrogate for the CAT score to assess symptoms. The choice of symptom measure influenced category assignment of COPD. And, the mMRC score of 1 corresponded with an SGRQ of 25 that is similar with a CAT score of 10 .
Classifications of COPD groups using mMRC scores (cutpoint of mMRC = 1)* (n = 257)
mMRC < 1
mMRC ≥ 1
Secondary finding of this study is that all of each CAT item was found to have significant correlations with mMRC scores. However, it had a wide range of correlation coefficients. The mMRC score is the most strongly correlated with dyspnea item of CAT while the correlations of mMRC with other items of CAT were less strong. This is not a surprising result since mMRC is an indicator of breathlessness. Also, this could suggest that mMRC scale only for breathlessness cannot be identical with CAT score and it cannot be used as a surrogate of CAT score.
We acknowledge a major limitation of this study. The subjects were the patients who referred from primary clinics and were recruited from a single center. Our results, however, are comparable with a recent, multicenter study. An advantage of this study is that the mMRC and CAT were compared directly regarding to the recent GOLD report. The other weakness of this study is that the prospective analysis of exacerbation was not included. A prospective study will be needed to explore the clinical impacts of stratification of COPD on future exacerbations and treatments in patients with COPD.
In conclusion, the GOLD 2011 recommends the use of mMRC or CAT scores for assessing symptoms. However, we showed that choice of symptom scale can alter group assignment of COPD because mMRC and CAT do not behave identically in distinguishing symptom groups. A refinement of the GOLD classification should be considered in the future.
Chronic obstructive pulmonary disease
COPD assessment test
- mMRC scores:
modified Medical Research Council (mMRC) dyspnea scores.
This study was supported by a grant (CRI11008-1) of the Chonnam National University Hospital Research Institute of Clinical Medicine.
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