Clinical Relevance of Changes in Pain Intensity in Patients with Specific Back Pain

Background Pain intensity is frequently measured on the 11-point numerical pain rating scale (NRS-PI), ranging from 0 (no pain) to 10 points (worst imaginable pain). However, it is difficult to interpret the clinical importance of changes from baseline to endpoint on this instrument. Objectives To estimate the minimal detectable change (MDC) and the minimal clinically important difference (MCID) for average pain intensity in patients with specific back pain. Materials and Methods Data on 1232 subjects with specific back pain from a German hospital were included in this study. A score combining the patient's (PGIC) and the physician's global impression of change (CGIC) over the in-patient length of stay was used as an external criterion. A priori, we considered the score value "slightly improved" as the MCID. MDC was calculated using the standard error of measurement (SEM) and the standard deviation (SD) of the sample. MCID was estimated by the mean value of PI-NRS change in patients who self-assess as "slightly improved", and by sensitivity/specificity analyses, computed by the receiver operating characteristic method (ROC). Results MDC was 1.77. The MCS and ROC methods consistently showed an MCID of 2 for the total sample. Both methods showed the dependence of the MCID on the initial pain: 1 for mild to moderate pain at baseline (1-4 NRS points), 2 for moderate to severe pain (5-7) and 3-4 for very severe to extreme pain (8-10). For patients with lumbosacral intervertebral disc disorders and patients in the acute phase (duration of pain <6 weeks), the ROC method resulted in a higher limit than the MCS method. Conclusions In order to facilitate the interpretation of changes and to take into account the patient's perspective, the global assessment of the success of treatment should be used as an anchor criterion. In addition to dealing with pain measurement, function-related and psychosocial aspects of pain symptoms should be kept in mind.