Diagnosing joint infections: synovial fluid differential is more sensitive than white blood cell count

In order to identify the predictive value of synovial fluid white blood cell (WBC) count and differential white blood cell count in identifying nonprosthetic joint infection in immunocompetent and immunosuppressed populations, we retrospectively reviewed 96 adult patients who underwent hip or knee aspiration because of symptoms suggesting a possible nonprosthetic joint infection. Medical history, including immunosuppressive disease or drugs, was recorded, and synovial fluid cell count, differential, and culture results were compared. There were 44 patients with positive synovial cultures. Of 36 patients who had a synovial WBC ≥50,000/mm³, 89% had positive cultures. The sensitivity to synovial WBC ≥50,000/mm³ was 0.727 (95% CI 0.570-0.845), and specificity was 0.923 (95% CI 0.806-0.975). There were 12 patients with a synovial WBC <50,000/mm³ that had positive cultures. The sensitivity of percentage polymorphonuclear cells (%PMNs) to predict positive cultures when the %PMNs were at least 80, 85, and 90% was 0.932, 0.886, and 0.818, respectively. The specificity when the %PMNs was at least 80, 85, and 90% was 0.598, 0.577, and 0.673, respectively. Among the 29% of immunocompromised patients, the sensitivity to synovial WBC ≥50,000/mm³ was 0.714 (95% CI 0.420-0.904), and specificity was 1.000 (95% CI 0.732-1.000). Twenty-nine percent of patients with a synovial WBC <50,000/mm³ had positive cultures. The sensitivity of %PMNs to predict positive cultures when the %PMNs was at least 80, 85, and 90% was 1.000, 0.929, and 0.786, respectively. The specificity when the %PMNs were at least 80, 85, and 90% was 0.500, 0.643, and 0.714, respectively. We found that the synovial WBC differential (percentage synovial fluid PMNs) is a more sensitive predictor for nonprosthetic adult joint infection than the synovial absolute WBC count. This was true in both the general population and the immunosuppressed population.