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Pain sensitivity is increased with higher disease activity in patients with rheumatoid arthritis (RA), a high proportion of whom also satisfy criteria for fibromyalgia, according to researchers from the U.K.

They suggest that the augmented pain processing in people with RA can confound assessment of disease activity as measured by the (DAS28). They note, however, that DAS-P, which reflects the contribution of patient-reported components to the DAS28, "has potential to be used as a marker for augmented pain processing in people with established RA during stable treatment with disease-modifying agents."

Their cross-sectional study, which appears in included 50 consecutive patients with stable RA for a duration >2 years and who reported current pain and had a DAS28 >3.1, consistent with active RA. The aim was to explore associations between pain sensitivity, fibromyalgia classification, and mental health, and their effects on disease activity assessment.

The clinical assessment included questionnaires for pain status and other patient-reported characteristics. Patients were assessed for (PPT) at the more painful knee and the anterior tibia distal to and sternum distant from the index knee. The authors note that PPT assessment has been standardized at these sites in people with osteoarthritis.

Some 69% of patients were using analgesics, and all were using at least one traditional or biologic disease-modifying anti-rheumatic drug and/or a glucocorticoid. Eighty percent reported current knee pain.

A low PPT, indicating more sensitivity to pain, at each site assessed was associated with higher reported pain. A lower PPT at each site was also associated with higher DAS28 and DAS28-P. A positive correlation between the DAS28-P and DAS28 had the potential to strongly confound analyses, according to the authors, so each component of the DAS28 score was examined separately for association with PPTs.

The associations between PPT and DAS28 were attributed to significant associations with the patient-reported components of DAS28 -- the general health assessment using a visual analog core (VAS-GH) and tender joint count. Significant associations were not observed with ESR or swollen joint counts on either univariate or multivariable analyses. Lower PPTs were associated with low mood or somatic symptoms.

Almost half (48%) of participants satisfied fibromyalgia classification criteria based on their responses to the questionnaires.

Study subjects who satisfied criteria for fibromyalgia had lower PPT at each site, associations that were confirmed in logistic regression analyses.

Patients who fulfilled fibromyalgia classification criteria also had higher DAS28 and DAS28-P, and these associations were attributed to significant univariate associations with VAS-GH and tender joint count, but not ESR or swollen joint counts. Logistic regression using all four DAS28 components confirmed that only VAS-GH was significantly associated with fibromyalgia classification.

The authors note that low PPTs have been associated with higher pain in patients with osteoarthritis and fibromyalgia, in addition to RA patients as discovered in this study, and also with neuropathic pain characteristics. "However, similar associations were found with other pain characteristics, suggesting that low PPTs may be a feature of chronic musculoskeletal pain in general, rather than indicating specifically neuropathology," they wrote.

Among the potential limitations are the use of 28 swollen joint counts and ESR to measure inflammation in RA, as "ESR might be within the normal range in people with active RA and DAS28 does not address feet, which might be a site of synovitis." Further, a significant number of participants used analgesic medications, which may have affected pain reporting and PPTs.