Somatization differentiates fibromyalgia from low back pain: a comparative, cross-sectional cohort study

To examine whether somatization, depression, anxiety, fatigue, coping dimensions, pain, physical and social function, or sociodemographic characteristics can differentiate fibromyalgia from low back pain in a cross-sectional cohort setting of our Zurzach Interdisciplinary Pain Programme.

Fibromyalgia and low back pain (not fulfilling the diagnostic criteria for fibromyalgia) were compared using the Symptom Checklist-90R (SCL-90R) Somatization scale, the Quantification Inventory for Somatoform Syndromes (QUISS) Number of somatoform symptoms and other standardized instruments. Standardized mean differences (SMDs) quantified the score differences, and binomial logistic regression modelling with various covariates differentiated fibromyalgia from low back pain.

The largest differences indicating worse health in fibromyalgia (n = 131) were in somatization (SCL-90R: SMD = -0.971, QUISS: SMD = -0.960), followed by affective health, pain and coping (SMDs between -0.632 and -0.280). Physical and social functioning were comparable in the two conditions (n = 262 low back pain). The two somatization scales both with odds ratios (OR) = 0.966 (P ≤ 0.002) plus female sex (OR = 3.396, P < 0.001) predicted 74.3% of the cases correctly (accuracy) with a positive predictive value of 65.3% and a specificity of 87.0% for fibromyalgia. In the female subsample (n = 280), the model remained stable with an accuracy of 71.9%.

Somatization stood out from all other somatic, psychosocial and coping dimensions and sociodemographics as the one significant specific predictor distinguishing fibromyalgia from low back pain. The fibromyalgia phenotype is characterized by the generalization of painful loci but equally prominently by generalized somatoform symptoms. Assessment of somatization is recommended to ensure accurate identification and understanding of the multifaceted syndrome of fibromyalgia.