Polysymptomatic distress (PSD) is the underlying metric of fibromyalgia (FM), and levels of PSD can identify criteria-positive FM with > 90% accuracy. We used levels of the PSD scale to test whether symptom levels in primary FM (PFM) and secondary FM (SFM) were the same and whether symptoms were equivalent in persons not meeting FM criteria.
We studied 1525 patients with a clinical diagnosis of FM and 12,037 patients with rheumatoid arthritis (RA). We used regression models to compare patients with potential and actual PFM to RA patients with potential and actual SFM for 17 key clinical variables.
When controlled for PSD values, the widespread pain index, symptom severity scale, and pain, global, quality of life, and physical and mental component scores were essentially the same or only slightly different in PFM and SFM. Health Assessment Questionnaire-Disability Index scores were slightly higher in SFM (0.21 units), as was the painful joint count (1.6 joints). Overall, higher PSD scores were associated with more severe symptoms or abnormal status. PSD scores in patients not satisfying FM criteria and in patients satisfying criteria operated similarly.
PFM and SFM are equivalent regarding symptom burden. PSD scores are more informative about severity and severity within diagnosis than dichotomization into FM/non-FM. Studies of FM versus "healthy individuals," or FM versus other diseases, are inherently defective, while studies of FM and PSD in RA offer the opportunity to have meaningful comparison groups, because there are no readily available unbiased appropriate controls for PFM.