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Cell Count and Differential of Aspirated Fluid in the Diagnosis of Infection at the Site of Total Knee Arthroplasty

¹ Rothman Institute of Orthopedics at Jefferson, 925 Chestnut Street, Philadelphia, PA 19107. E-mail address for J. Parvizi: parvj{at}aol.com
² The Barnes-Jewish Hospital, One Barnes-Jewish Hospital Plaza, St. Louis, MO, 63110

Investigation performed at the Rothman Institute of Orthopedics, Thomas Jefferson University, Philadelphia, Pennsylvania

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Stryker Orthopedics. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Smith and Nephew). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

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Methods: We analyzed synovial fluid that had been aspirated preoperatively from 429 knees that had undergone revision arthroplasty at three different academic institutions; 161 knees were found to be infected, and 268 knees were not. Using receiver operating characteristic curves, we determined cutoff values for the fluid leukocyte count and neutrophil differential with an optimal balance of sensitivity and specificity for the diagnosis of periprosthetic infection. The sensitivity, specificity, and predictive values were calculated for those cutoff values. The erythrocyte sedimentation rate and C-reactive protein level cutoff values of 30 mm/hr and 10 mg/L, respectively, were combined with the cutoff values for the fluid leukocyte count and neutrophil percentage.

Results: The cutoff values for optimal accuracy in the diagnosis of infection were >1100 cells/10^–3cm³ for the fluid leukocyte count and >64% for the neutrophil differential. When both tests yielded results below their cutoff values, the negative predictive value of the combination increased to 98.2% (95% confidence interval, 95.5% to 99.5%), whereas when both tests yielded results greater than their cutoff values, infection was confirmed in 98.6% (95% confidence interval, 94.9% to 99.8%) of the cases in our cohort. Similarly, when both the neutrophil percentage and the C-reactive protein level were less than the cutoff values of 64% and 10 mg/L, respectively, the presence of periprosthetic infection was very unlikely.

Conclusions: The synovial fluid leukocyte count and differential are useful adjuncts to the erythrocyte sedimentation rate and the C-reactive protein level in the preoperative workup of infection at the site of a total knee arthroplasty. The present study identified cutoff values for the leukocyte count (>1100 cells/10^–3cm³) and neutrophil percentage (>64%) that can be used to diagnose infection. Combining the peripheral blood tests with the synovial fluid cell count and differential can improve their diagnostic value.

Level of Evidence: Diagnostic Level III. See Instructions to Authors for a complete description of levels of evidence.

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