The minimally clinically important difference in generic utility-based measures

Abstract

Purpose: To evaluate the use of utility-based generic quality of life measures for establishing the minimally clinically important difference (MCID).

Background: Utility-based quality of life measures place levels of wellness on a continuum anchored by death (0.00) and optimum function (1.00). Preference measurement studies are used to define the meaning of points along the continuum. Health states that differ by less than 0.03 units cannot be discriminated by panels of judges as different from one another. Thus, 0.03 is a reasonable MCID for these measures.

Method: Three published studies of patients with Chronic Obstructive Pulmonary Disease (COPD) reported data on the Quality of Well-being Scale (QWB) before and after pulmonary rehabilitation. One of the studies also randomly assigned patients to lung volume reduction surgery or to maximal medical therapy. These patients were followed for an average of 29 months.

Results: All three evaluations of pulmonary rehabilitation showed changes on the QWB in excess of the proposed 0.03 MCID. QWB changes for patients assigned to lung volume reduction surgery were close to the MCID threshold at one year but grew stronger in subsequent years. Using Norman's 0.50 standard deviation method, all three estimates of rehabilitation effectiveness and the outcomes one year following surgery fall below the MCID.

Conclusion: Different methods for estimating MCID lead to different conclusions about the meaning of quality of life changes following pulmonary rehabilitation and lung volume reduction surgery. The preference scaling system in generic utility-based quality of life measures provides a metric that is directly interpretable and avoids many of the criticisms of MCID measures. The method is sensitive enough to suggest clinically meaningful benefits of rehabilitation and surgery. Further, quality adjusted life years offer a valuable metric for policy analysis. Utility-based measures of health related quality of life should gain greater use in COPD outcomes research.