SYNOLACTATE-PLUS cross-sectional study: prospective validation of RESAS and synovial biomarkers in septic arthritis diagnosis.

OBJECTIVE: Prospective validation of the RESAS score and its components (glucose, lactate, and lactate/glucose ratio) for the diagnosis of septic arthritis. A secondary objective was to assess the diagnostic performance of additional synovial fluid (SF) biomarkers, including calprotectin, synovial CRP and total proteins. METHODS: Synovial fluid samples from patients with acute arthritis (< 30 days) were prospectively collected in the monocentric, cross-sectional SYNOLACTATE-PLUS study. Septic arthritis of native joints (SANJ) was defined according to Newman's criteria. Each sample underwent Gram staining, white blood cell count, bacterial culture and crystal analysis. Biochemical measurements included L-lactate, glucose, CRP, calprotectin and total proteins. RESAS was calculated for each patient. Diagnostic performance was evaluated using ROC curves, area under the curve (AUC), sensitivity (Se), specificity (Sp), and likelihood ratios (LR ±). RESULTS: Among 208 patients, 31 had SANJ, 117 crystal-induced arthritis, and 60 other inflammatory forms. RESAS achieved an AUC of 0.905 (0.836-0.975), Se 51.6%, Sp 97.2%, LR + 18.3, and LR- 0.50 at a threshold ≥  + 4. Calprotectin showed the best diagnostic accuracy (AUC 0.944, Se 93.6%, Sp 79.4%, LR + 4.5, LR- 0.08, threshold ≥ 500 mg/L). Synovial CRP (AUC 0.753) and total proteins (AUC 0.634) were not clinically relevant. A modified RESAS combining RESAS, calprotectin, and neutrophil percentage yielded improved performance (AUC 0.961, Se 93.6%, Sp 88.0%, LR + 7.8, LR- 0.07). CONCLUSION: RESAS demonstrated strong prospective validity for predicting septic arthritis. Among protein biomarkers, synovial calprotectin showed the highest diagnostic accuracy.