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

• Published in: Journal of bone and joint surgery. American volume 2008-08, Vol.90 (8), p.1637-1643
• Main Authors: Ghanem, Elie, Parvizi, Javad, Burnett, R. Stephen J, Sharkey, Peter F, Keshavarzi, Nahid, Aggarwal, Ajay, Barrack, Robert L
• Format: Article
• Language: English
• Subjects: Adult; Aged; Aged, 80 and over; Arthroplasty, Replacement, Knee; Biological and medical sciences; Blood Sedimentation; C-Reactive Protein - analysis; Confidence Intervals; Diagnosis, Differential; Diseases of the osteoarticular system; Female; Humans; Leukocyte Count; Male; Medical sciences; Middle Aged; Neutrophils; Orthopedic surgery; Predictive Value of Tests; Prosthesis-Related Infections - blood; Prosthesis-Related Infections - diagnosis; Reoperation; ROC Curve; Sensitivity and Specificity; Surgery (general aspects). Transplantations, organ and tissue grafts. Graft diseases; Synovial Fluid - cytology
• ISSN: 0021-9355; 1535-1386
• DOI: 10.2106/JBJS.G.00470

BackgroundAlthough there is no absolute diagnostic test for periprosthetic infection, the synovial fluid leukocyte count and neutrophil percentage have been reported to have high sensitivity and specificity. However, the cutoff values for these tests are not agreed upon. We sought to identify definite cutoff values for both the fluid leukocyte count and the neutrophil percentage that may help to diagnose infection at the site of a prosthetic joint.MethodsWe 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.ResultsThe cutoff values for optimal accuracy in the diagnosis of infection were >1100 cells/10cm 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.ConclusionsThe 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/10cm) 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 EvidenceDiagnostic Level I