Fibromyalgia (FM) is a rheumatic disease characterised by musculoskeletal pain, chronic diffuse tension and/or stiffness in joints and muscles, fatigue, sleep and emotional disturbances and pressure pain sensitivity in at least 11 of 18 tender points. There are currently no instrumental tests or specific diagnostic markers, and the characteristic symptoms of the disease overlap those of many other conditions classified in a different manner. FM is often associated with other diseases that act as confounding and aggravating factors, including primary Sjögren's syndrome (pSS), systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). It has been reported to coexist in 25% of patients with RA, 30% of patients with SLE and 50% of patients with pSS. Its clinical diagnosis is not easy because FM-like symptoms are frequent, and its differential diagnosis with other causes of chronic diffuse pain is difficult. This is even more true in the case of patients who are positive for antinuclear antibodies (ANAs) because, although sensitive, ANA positivity is not specific for SLE or connective tissue diseases, and can also be found in 10-15% of FM patients. Furthermore, composite indices such as the disease activity score (DAS)-28, which are widely used in everyday clinical practice and clinical trials, may be insufficient to evaluate real inflammatory activity in patients with RA associated with chronic pain syndromes such as FM, and can lead to an overestimate of disease activity in RA. The presence of diffuse pain in autoimmune rheumatic diseases compromises the quality of life of the patients, although overall mortality is not increased. A misdiagnosis harms the patients and the community. Rheumatologists should be able to recognise and distinguish primary and secondary FM, and need new guidelines and instruments to avoid making mistakes.
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