How much change is enough? Evidence from a longitudinal study on depression in UK primary care.
The Patient Health Questionnaire (PHQ9), the Beck Depression Inventory (BDI-II), and the Generalised Anxiety Disorder Assessment (GAD-7) are widely used in the evaluation of interventions for depression and anxiety. The smallest reduction in depressive symptoms that matters to patients is known as the Minimum Clinically Important Difference (MCID). Little empirical study of the MCID for these scales exists.
A prospective cohort of 400 patients in UK primary care were interviewed on four occasions, two weeks apart. At each time point, participants completed all three questionnaires and a ‘global rating of change’ scale (GRS). MCID estimation relied on the reduction in scores among those reporting improvement on the GRS scale, stratified by three categories of baseline severity assessed on the Clinical Interview Schedule (CIS-R).
For moderate baseline severity, those who reported improvement on the GRS had a change of 21% (95% confidence interval (CI) -26.7, -14.9) on the PHQ9; 23% (95% CI -27.8, -18.0) on the BDI-II and 26.8% (95% CI -33.5, -20.1) on the GAD-7. The corresponding threshold scores below which participants were more likely to report improvement were -1.7, -3.5 and -1.5 points on the PHQ9, BDI-II and GAD-7, respectively. Patients with milder symptoms at baseline require a much larger drop in their scores as percentage of their baseline to perceive any benefit.
An MCID representing 20% reduction of their baseline scores is a useful rough guide for any of these scales. Treatment benefits smaller than this are unlikely to be noticed by patients at low baseline severity.