Measurement properties and interpretation of three shortened versions of the asthma control questionnaire.

The Asthma Control Questionnaire (ACQ) measures the adequacy of asthma treatment as identified by international guidelines. It consists of seven items (5 x symptoms, rescue bronchodilator use and FEV1% of predicted normal). A validation study suggested that in clinical studies measurement of FEV1 and bronchodilator use may not be needed but this has never formally been tested in a clinical trial. The aims of this analysis were (1) to examine the measurement properties of three shortened versions of the ACQ (symptoms alone, symptoms plus FEV1 and symptoms plus short-acting beta2-agonist) and (2) to determine whether using the shortened versions would alter the results of a clinical trial. In the randomised trial, 552 adults completed the ACQ at baseline and after 13 and 26 weeks of treatment. The analysis showed that the measurement properties of all four versions of the ACQ are very similar. Agreement between the original ACQ and the reduced versions was high (intraclass correlation coefficients: 0.94-0.99). Mean differences between the ACQ and the shortened versions were less than 0.04 (on the 7-point scale). Clinical trial results using the four versions were almost identical with the mean treatment difference ranging from -0.09 (P=0.17), to -0.13 (P=0.07). For interpretability, the minimal important difference for all four versions was close to 0.5. In conclusion, these three shortened versions of the ACQ can be used in large clinical trials without loss of validity or change in interpretation.