To compare outcome-predictor relationships in fibromyalgia (FM) and rheumatoid arthritis (RA), to provide information regarding the competing hypotheses that FM is a continuum or a discrete disorder.
We studied 3 outcome variables (work disability, opioid use, depression) and 12 clinical predictor variables in 2,046 patients with FM and 20,374 with RA. We determined whether outcome-predictor relationships were stronger in FM or RA by measuring the areas under the receiver-operating curves. We used fractional polynomial logistic regression to create graphic models for the outcome-predictor relationships.
All measures of status and outcome were more abnormal in FM than in RA. Depression was reported in 33.4% of patients with FM compared with 15.1% of those with RA. The predictor-outcome relationship was significantly stronger in RA in 28 of the 36 tests, and not different in the remainder. The relationship between outcome and predictor variables was generally similar in patients with FM and RA. However, unmodeled depression that was not explained by study variables was noted in FM.
Our data are consistent with the hypothesis that FM is the end of a severity continuum, but that additional psychological factors are an integral part of the syndrome.