Identifying septic arthritis superimposed on crystalline arthropathy: Is synovial cell count the answer?

Septic arthritis (SA) is an orthopedic urgency with significant morbidity and potential mortality. Differentiating SA and crystalline arthritis (CA) is difficult as concurrent disease may exist. This study sought to identify the prevalence of and define diagnostic laboratory markers for SA in the setting of CA.

A retrospective review was conducted of adult patients presenting to a single medical center between January 2012 and March 2023 with monosodium urate (MU) or calcium pyrophosphate (CPP) in synovial knee joint fluid of a native knee. Categorical variables were assessed using chi-square and Fisher exact tests. Receiver operating characteristic curves were computed, and diagnostic accuracy was determined from associated area under the ROC curve (AUC). The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated and optimal diagnostic thresholds determined by Youden index.

225 knee aspirations were analyzed, 56.4 % CPP and 43.6 % MSU. There were 12 cases of superimposed SA (5.3 %), 11 culture confirmed and 1 suspected. Immunosuppression was associated with a higher incidence of SA (41.7 % versus 14.6 %, p = 0.0271). SA had a significantly higher mean synovial WBC (135,796 vs 22,510; p = 0.0007). There was no difference in percentage of PMNs between the septic and aseptic groups (90.6 % vs 69.7 %; p = 3.327). Of the 12 cases of concomitant SA, 10 (83.3 %) had a synovial WBC ≥50,000/mm3, compared to 21 (9.9 %) of the aseptic aspirations (p < 0.0001). Nine (75 %) aspirations in the septic group had ≥90 % PMNs, compared to 78 (36.6 %) in the aseptic group (p = 0.0123). The optimal diagnostic threshold for synovial WBC was 50,000/mm3 (91.7 % sensitivity, 90.1 % specificity, AUC = 0.960). The optimal synovial percentage of PMNs was 64 % (100 % sensitivity, 6.7 % specificity, AUC = 0.731).

In the setting of CA, synovial WBC may provide a useful diagnostic marker for SA with an optimal threshold of 50,000 cells/mm3.