Minimal clinically important difference for pain on the VAS scale and the relation to quality of life in women with endometriosis.

Objectives: The minimal important difference can be helpful in interpreting data from clinical trials. The objective of the study was to calculate the minimal important difference for improvement on the VAS scale for women with endometriosis.

Study Design: A prospective study was conducted to evaluate the effect of pertubation with lignocaine on dysmenorrhea and quality of life in women with endometriosis. Data collected in the trial were used for additional analyses in the present descriptive study. Eligible women (n=37) had endometriosis with pain>VAS 50mm (visual analogue scale).

Main Outcome Measures: In a questionnaire, women evaluated their maximum pain on the VAS- scale during every menstrual period before and after treatment. They also estimated the changes in overall pain level by answering the response categories "much better", "somewhat better", "about the same", "somewhat worse" or "much worse". The women were grouped according to their own estimation of change in pain intensity after four months. The minimal important differences for change on the VAS scale correlate to the mean change for women who felt "somewhat better" (n=18) excluding those who were pain free (n=2).

Results: The minimal important difference for improvement on the VAS scale was found to be -39mm and/or -49%.

Conclusion: If the patients have a pain level of at least 50mm on VAS scale at inclusion, the cut off for success in clinical trials is suggested to be defined as an either >40mm or a >50% decrease on VAS scale. Trial registry ClinicalTrials.gov Identifier: NCT01329796.