Background: One of the challenges of health-related quality of life research is to translate statistically significant health-related quality of life changes into interpretable clinical or medically important ones.
Objective: To calculate the minimal important difference of the King's Health Questionnaire, a condition-specific health-related quality of life questionnaire for the assessment of men and women with lower urinary tract dysfunction.
Methods: The King's Health Questionnaire was administered to patients suffering from overactive bladder enrolled in two multinational studies. Minimal important differences were calculated using an anchor-based approach with both a global rating of patient-perceived treatment benefit and one of perceived disease impact. A distribution-based method using effect size was calculated for comparison purposes.
Results: Minimal important difference values varied slightly with each method. Using the anchor-based approach, the King's Health Questionnaire minimal important difference ranged between 5-10 points when the calculation factored out patients who reported no change and 6-12 points for patients who experienced a small improvement. The effect size method indicated a minimal important difference of 5 to 6 points for a small effect and 10 to 15 points for a medium effect.
Conclusions: In the case of the King's Health Questionnaire, the anchor-based approaches and the distribution-based approach provide similar results. A change from baseline of at least 5 points on King's Health Questionnaire domains indicates a change that is meaningful to patients and is indicative of a clinically meaningful improvement in health-related quality of life after treatment. Convergence of the estimates using different approaches should give us confidence in the values derived for the quality of life domains measured by the King's Health Questionnaire.