Procalcitonin Does Not Differentiate Between Bacterial Infections Versus Disease Flare in Children with Systemic Juvenile Idiopathic Arthritis— A Diagnostic Accuracy Study Using Area-Under-Curve Analysis

Background: We compared the diagnostic accuracy of procalcitonin, C-reactive protein (CRP), and ferritin in differentiating an infectious aetiology versus disease flare in children with systemic juvenile idiopathic arthritis (sJIA). Methods: This prospective diagnostic accuracy study included 52 consecutive hospitalisations of 25 children diagnosed with sJIA (International League of Associations for Rheumatology 2001) presenting with a fever to the rheumatology department of a quaternary care centre. We categorised children post-discharge as infectious versus sJIA flare, depending on the clinical course. A subset of sJIA flare developed macrophage activation syndrome (MAS). We compared the area-under-curve (AUC) with its 95% confidence interval (95%CI) of the three biomarkers. Results: The median age of the children was 7 ± 7 years. We categorised 12 hospitalisations (23.1%) as infectious and 40 hospitalisations (76.9%) as sJIA flare. Thirteen hospitalisations (32.5%) with sJIA flare developed MAS. Neither procalcitonin (AUC = 0.584 ± 0.101, 95% CI: 0.387–0.782) nor CRP (AUC = 0.363 ± 0.096, 95% CI: 0.174–0.551) nor ferritin (AUC = 0.419 ± 0.093, 95% CI: 0.236–0.602) discriminated infectious aetiology from sJIA flare adequately. Both serum procalcitonin (AUC = 0.828, 95% CI: 0.694–0.961) and ferritin (AUC = 0.937, 95% CI: 0.862–1) demonstrated higher diagnostic accuracy in ascertaining MAS in sJIA flares, compared to CRP (AUC = 0.709, 95% CI: 0.543–0.876). Conclusion: Procalcitonin can be elevated in children in sJIA flare with MAS, even without an infectious aetiology. Clinicians should continue to rely on clinical history and examinations to discern infectious versus non-infectious fevers in children with sJIA.