Abstract
Understanding changes in patient-reported outcomes is indispensable for interpretation of results from clinical studies. As a consequence the term "minimal clinically important difference" (MCID) was coined in the late 1980s to ease classification of patients into improved, not changed or deteriorated. Several methodological categories have been developed determining the MCID, however, all are subject to weaknesses or biases reducing the validity of the reported MCID. The objective of this study was to determine the reproducibility and validity of a novel method for estimating low back pain (LBP) patients' view of an acceptable change (MCID(pre)) before treatment begins. One-hundred and forty-seven patients with chronic LBP were recruited from an out-patient hospital back pain unit and followed over an 8-week period. Original and modified versions of the Oswestry disability index (ODI), Bournemouth questionnaire (BQ) and numeric pain rating scale (NRS(pain)) were filled in at baseline. The modified questionnaires determined what the patient considered an acceptable post-treatment outcome which allowed us to calculate the MCID(pre). Concurrent comparisons between the MCID(pre), instrument measurement error and a retrospective approach of establishing the minimal clinically important difference (MCID(post)) were made. The results showed the prospective acceptable outcome method scores to have acceptable reproducibility outside measurement error. MCID(pre) was 4.5 larger for the ODI and 1.5 times larger for BQ and NRS(pain) compared to the MCID(post). Furthermore, MCID(pre) and patients post-treatment acceptable change was almost equal for the NRS(pain) but not for the ODI and BQ. In conclusion, chronic LBP patients have a reasonably realistic idea of an acceptable change in pain, but probably an overly optimistic view of changes in functional and psychological/affective domains before treatment begins.