Criteria for the interpretation of synovial fluid are well established for native joint disorders but lacking for the evaluation of prosthetic joint failure. Our aim was to define cutoff values for synovial fluid leukocyte count and neutrophil percentage for differentiating aseptic failure and prosthetic joint infection.

We performed a prospective study of 133 patients in whom synovial fluid specimens were collected before total knee arthroplasty revision between January 1998 and December 2003. Patients with underlying inflammatory joint disease were excluded.

Aseptic failure was diagnosed in 99 patients and prosthetic joint infection was diagnosed in 34 patients. The synovial fluid leukocyte count was significantly higher in patients with prosthetic joint infection (median, 18.9 × 10³/μL; range, 0.3 to 178 × 10³/μL) than in those with aseptic failure (median, 0.3 × 10³/μL; range, 0.1 to 16 × 10³/μL; P <0.0001); the neutrophil percentage was also significantly higher in patients with prosthetic joint infection (median [range], 92% [55% to 100%] vs. 7% [0% to 79%], P <0.0001). A leukocyte count of >1.7 × 10³/μL had a sensitivity of 94% and a specificity of 88% for diagnosing prosthetic joint infection; a differential of >65% neutrophils had a sensitivity of 97% and a specificity of 98%. Staphylococcus aureus was the only pathogen associated with leukocyte counts >100 × 10³/μL.

A synovial fluid leukocyte differential of >65% neutrophils (or a leukocyte count of >1.7 × 10³/μL) is a sensitive and specific test for the diagnosis of prosthetic knee infection in patients without underlying inflammatory joint disease.