The purposes of this study were as follows: (1) to evaluate the diagnostic testing performance of synovial WBC count, PMN%, synovial glucose, synovial protein, synovial LDH and synovial CRP levels, as diagnostic markers for diagnosis of septic arthritis after ACL reconstruction; (2) to define the ideal thresholds of the aforementioned tests, leading to the optimal sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy; (3) to define sensitivity of synovial fluid culture and synovial tissue sample culture, and if previous antibiotic treatment may affect the accuracy of these tests.
A retrospective analysis of all patients readmitted from January 2009 to September 2019 with suspicious signs of septic arthritis and undergoing a knee aspiration for synovial fluid analysis and culture was performed. ,. The receiver operating characteristic (ROC) curves and the associated area under the curve (AUC) were constructed for the aforementioned synovial markers. Sensitivity, specificity, PPV, (NPV) and accuracy were calculated for the obtained optimal values. Sensitivity was also calculated for synovial fluid culture and synovial tissue samples culture, and the influence of previous antibiotic treatments on culture sensitivity was evaluated.
Out of 3,408 cases of ACL reconstruction, after excluding 13 patients not meeting the inclusion criteria, 24 infected and 14 non-infected patients were reviewed and included in the analysis. Diagnosis was confirmed by the presence of two positive cultures with the same isolated microorganism, or by at least 3 out of 4 of the following criteria: elevated serum CRP and ESR, positive histological analysis of synovial tissue, macroscopic evidence of purulence, one positive culture. The ROC curve analysis demonstrated that the most reliable marker for the diagnosis of septic arthritis after ACL reconstruction was the synovial WBC count (AUC = 0.89). A cutoff value of 28,100 cells/ml presented the highest accuracy (0.85), the highest PPV (0.94) and the highest NPV (0.76); moreover, with the threshold set at 40,000 cells/mL, postoperative infection could be diagnosed with 100% specificity. The sensitivity of synovial fluid culture was significantly lower than the sensitivity of synovial tissue samples culture (0.63 vs 0.96, p =0.0045); moreover, the sensitivity further reduced if patient took antibiotics before aspiration (0.44 vs 0.73), although this decrease was not statistically significant .
Synovial WBC count is the most reliable test for diagnosis of septic arthritis after ACL reconstruction. While the sensitivity of synovial fluid culture is affected by previous antibiotic treatment, synovial WBC count is not influenced, and proved to be useful in the diagnosis of this uncommon complication.

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