The COPD Assessment Test (CAT) is a short, simple questionnaire developed to assess the impact of chronic obstructive pulmonary disease (COPD) on patients' health status. It was developed through qualitative research with COPD patients and validated using data from three international studies involving 1,503 participants. Psychometric and Rasch analyses identified eight items that fit a unidimensional model, forming the CAT. The questionnaire has excellent internal consistency (Cronbach's α = 0.88) and good test-retest reliability (intra-class correlation coefficient 0.8). It showed strong correlation with the COPD-specific version of the St George's Respiratory Questionnaire (r = 0.80). The CAT demonstrated significant differences between stable and exacerbation patients (5 units on a 40-point scale, 12% of the scale). The CAT is a reliable and valid measure of COPD health status, with good measurement properties and sensitivity to changes in health status. It is designed to be used in routine clinical practice for assessing and monitoring COPD. The CAT covers a wide range of COPD-related symptoms and health impacts, including cough, phlegm, chest tightness, breathlessness, activity limitation, confidence in leaving home, sleep, and energy. It is a standardized tool that can be used internationally and is not a diagnostic tool but rather a supplementary measure to assess COPD impact. The CAT has good repeatability and is sensitive to treatment effects at a group level. It is expected to improve communication between clinicians and patients, enabling a better understanding of the severity and impact of COPD. The CAT is supported by GlaxoSmithKline and has been validated in multiple studies. It is a valuable tool for assessing COPD health status and monitoring treatment outcomes.The COPD Assessment Test (CAT) is a short, simple questionnaire developed to assess the impact of chronic obstructive pulmonary disease (COPD) on patients' health status. It was developed through qualitative research with COPD patients and validated using data from three international studies involving 1,503 participants. Psychometric and Rasch analyses identified eight items that fit a unidimensional model, forming the CAT. The questionnaire has excellent internal consistency (Cronbach's α = 0.88) and good test-retest reliability (intra-class correlation coefficient 0.8). It showed strong correlation with the COPD-specific version of the St George's Respiratory Questionnaire (r = 0.80). The CAT demonstrated significant differences between stable and exacerbation patients (5 units on a 40-point scale, 12% of the scale). The CAT is a reliable and valid measure of COPD health status, with good measurement properties and sensitivity to changes in health status. It is designed to be used in routine clinical practice for assessing and monitoring COPD. The CAT covers a wide range of COPD-related symptoms and health impacts, including cough, phlegm, chest tightness, breathlessness, activity limitation, confidence in leaving home, sleep, and energy. It is a standardized tool that can be used internationally and is not a diagnostic tool but rather a supplementary measure to assess COPD impact. The CAT has good repeatability and is sensitive to treatment effects at a group level. It is expected to improve communication between clinicians and patients, enabling a better understanding of the severity and impact of COPD. The CAT is supported by GlaxoSmithKline and has been validated in multiple studies. It is a valuable tool for assessing COPD health status and monitoring treatment outcomes.