Predicting Septic Arthritis in the Setting of Crystalline Arthropathy in the Native Joint Using Laboratory Data.

Background: Septic arthritis is an orthopedic emergency. Diagnosis is difficult in patients with concomitant crystalline arthropathy (gout or pseudogout). The symptomatology of crystal arthritis mimics septic arthritis, clouding clinical diagnosis. Arthrocentesis and synovial fluid analysis are the standard diagnostic tests for both pathologies. Crystals on microscopy are diagnostic of crystal arthritis, however their presence does not rule out septic arthritis. Septic arthritis is diagnosed by positive microbiology culture. Though septic arthritis is associated with elevated synovial total nucleated count (TNC), TNC elevations can also occur with gout. The literature suggests that a TNC count of > 50,000 cells in a crystal-positive joint should raise suspicion for concurrent septic arthritis, however data is limited. Further diagnostic indicators are needed to help clinicians promptly identify crystal positive septic arthritis as the treatments and prognoses are different.

Methods: Patients were retrospectively identified who had arthrocentesis of a native joint positive for monosodium urate (MSU) and/or (CPPD) crystals. Laboratory data was collected including synovial fluid cultures, total nucleated cell count (TNC), percent polymorphic neutrophils (%PMN), and crystal analysis; and serum CRP, ESR, and white blood cell count (WBC). Statistical analysis performed using Spearman correlation, Univariate-Fischer's exact and Wilcoxon tests, and multivariate analysis.

Results: 442 joints identified with positive CPPD and/or MSU crystals, 31% female, 69% male. Of 442 aspirates, 58 had positive cultures. Patients were more likely to have positive cultures if synovial TNC > 50,000 (odds ratio 7.7), CRP > 10 mg/dL (OR 3.2), PMN > 90% (OR 2.17), and if the patient was female (OR 1.9), all were statistically significant with p < 0.05. There were 55 patients who underwent irrigation and debridement based on clinical suspicion or a positive gram stain, 37 of these ultimately had a positive culture (67%), the remaining 18 had negative cultures.

Conclusion: Results are consistent with the literature, a TNC > 50,000 warrants a high suspicion for concurrent septic arthritis and should prompt providers to critically evaluate other patient laboratory data. Results further suggests that a patient with positive crystals, synovial TNC > 50,000 cells, PMN > 90%, and serum CRP > 10mg/dL is at high risk for having a concurrent septic arthritis and may warrant urgent irrigation and debridement and antibiotic therapy. This data serves as a supporting to develop an infection risk calculator for crystal positive septic arthritis. Level of Evidence: III.