Interlaboratory Reliability of Synovial Fluid Analysis in Prosthetic Knee Aspiration

Synovial fluid analysis is a critical step in the diagnosis of periprosthetic knee joint infection. We hypothesized that clinically meaningful differences between white blood cell (WBC) count, polymorphonuclear leukocyte (PMN) cell percentage, and culture results would exist between 2 independent laboratories when the same synovial fluid aspirate is analyzed.

A retrospective chart review was completed on total knee arthroplasty patients who underwent knee arthrocentesis between 2017 and 2025. The synovial fluid aspirate from a single knee arthrocentesis was divided into identical samples and analyzed by 2 independent laboratories. WBC count, PMN percentage, and culture results were recorded.

In total, 202 consecutive prosthetic knee aspirates were analyzed. Agreement was found in 14 of 20 samples (70%) that tested ≥3000 WBC/μl (k = 0.81; 95% confidence inverval [CI], 0.66-0.96), 9 of 13 samples (69%) that tested ≥80% PMNs (k = 0.81; 95% CI, 0.62-0.99), 7 of 12 cases (58%) reported both ≥80% PMNs and ≥3000 WBC/μl, 8 of 9 cases (89%) that tested ≥90% PMNs (k = 0.94; 95% CI, 0.82-1.00), 3 of 23 cases (13%) with positive culture results (k = 0.18; 95% CI, -0.05-0.40).

Two independent laboratories displayed clinically meaningful variability in WBC count, PMN percentage, and culture results. The difficulty in diagnosing periprosthetic joint infection may at least in part be due to poor interlaboratory reliability concerning synovial fluid analysis. Refinement of interlaboratory agreement, further research into advanced diagnostic methods, and measures to improve the accuracy of synovial fluid analysis are all critical steps toward refining the diagnosis of periprosthetic joint infection.

Retrospective Cohort study, Diagnostic.