Correlations between pain and function in a longitudinal low back pain cohort.

PURPOSE Most studies of low back pain (LBP) and functional limitation have been cross-sectional, and show only modest correlations between pain and function. Though functional limitation may be superior for predicting disability outcomes, there is a need to understand better the gap between pain and function. This study analysed changes in intra-individual correlations between pain and function over time. METHOD Seventeen men and 16 women currently experiencing LBP provided self-reports of LBP (0-to-10 scale) and functional status (Back Pain Functional Scale) for a maximum of 8 weeks. Spearman correlation coefficients between pain and function scores were calculated for each individual. The effects of pain history, pain intensity, variability and trends over time on pain-function correlations were assessed. RESULTS There were no significant differences in correlation due to gender, age or pain intensity (low versus high). Participants with steeper slopes in change in pain score over the study period had significantly stronger correlations to function than those with weaker trends, r = -0.91 and r = -0.45, respectively. Participants with at least one pain-free score during the reporting period had significantly stronger correlations than those with no pain-free reports, r = -0.80 and r = -0.51, respectively. Participants having the first episode LBP had stronger correlations (r = -0.85) than those with persistent symptoms of LBP (r = -0.62). CONCLUSIONS The results suggest that over the course of LBP, within-person pain-function correlations are stronger than those reported in cross-sectional, population-based studies. Changes in pain ratings over time may have more clinical relevance than differences in pain levels between individuals. Among those with more long-standing pain, factors other than pain intensity, such as pain catastrophising or fear avoidant beliefs, may have a greater effect on day-to-day perceptions of functional limitation.