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Authors: Saxena A, Jacovides C, Siad J, Ghanem E, Ketonis C, Azzam K, Kelly C, Parvizi J., Philadelphia, PA

Title: Cell Count and Differential of Aspirated Fluid in Acute Postoperative Period Total Knee Arthroplasty

Purpose: The purpose of this study is to establish values to help surgeons differentiate between an acute infection and normal postoperative levels.

Methods: We retrospectively analyzed the synovial fluid aspirated less than 6 weeks after total knee arthroplasty from 245 patients who underwent 257 procedures. We excluded patients whose aspirate results may have been inaccurate due to clotting. We also evaluated the blood test results of C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), white blood cell count (WBC), and neutrophil percentage in these patients. The mean joint fluid white cell count and neutrophil percentages were compared between patients who had an acute PJI and those who did not.

Results: So far, we have completed chart review on 70 synovial fluid tests from 61 patients, of whom 9.8% are infected (6 patients), 83.6% are uninfected (51 patients), and 6.6% need to be re-checked/indeterminate (4 patients). With our current data, we observed no significant differences between red blood cell count or neutrophil percentage in joint fluid aspirate. Also, infected patients had significantly lower WBC counts in their fluid aspirates than did infected patients (p = 0.0008). However, infected patients were also more likely to have positive fluid cultures than were uninfected patients (p < 0.0001). For serology tests separate from the joint aspirate tests, ESR, CRP, and WBC counts were not significantly different between the two groups; however, the neutrophil percentages for these tests was significantly higher in infected patients than in uninfected patients and, again, infected patients were more likely to have positive anaerobic cultures than were uninfecte! d patients (p = 0.0189 and p = 0.0223, respectively).

Discuassion and Conclusion: As serology is unreliable, the analysis of joint fluid, to determine cell count and percentage of neutrophils, is very useful in the evaluation of patients presenting with suspected periprosthetic infection in the acute postoperative period. Because of a significant difference in absolute numbers and the percentage of neutrophils between infected and uninfected cases, analysis of joint fluid can be relied upon for clinical decision-making.

Musculoskeletal Infecton Society
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