Synovial absolute polymorphonuclear neutrophil cell count: A simple and inexpensive marker for diagnosing periprosthetic hip and knee joint infections.

PURPOSE

This study aimed to establish the optimal cutoff value for synovial absolute polymorphonuclear neutrophil (APMN) count in distinguishing between septic and aseptic hip and knee revision arthroplasties. We also investigated its effectiveness as an indicator in revision arthroplasties with challenging microbiological findings, including (i) aseptic cases with a single unexpected positive intraoperative culture (UPICs), (ii) septic cases with unexpected negative ICs (UNICs) and (iii) infections caused by high- and low-virulent pathogens.

METHODS

A total of 616 revision arthroplasties (177:hip, 439:knee) included. Using European Bone and Joint Infection Society (EBJIS) criteria, 325 (52.8%) were classified as infection confirmed, 271 (44%) infection unlikely and 20 (3.2%) as infection likely. International Consensus Meeting (ICM) 2018 criteria classified 308 (50%) as infected, 269 (43.7%) not infected, and 39 (6.3%) as inconclusive. Diagnostic accuracy was assessed through receiver operating characteristic curves and area under the curve (AUC).

RESULTS

Optimal APMN count thresholds using EBJIS criteria in hip and knee joints were 783.6 cells/µL (AUC: 0.92) and 549 cells/µL (AUC: 0.91), respectively. With the ICM criteria, its optimal cutoff values remained unchanged, except for the knee, which shifted to 594.2 cells/µL. Comparing UPICs to other aseptic cases showed no significant median APMN count differences when both criteria's applied, potentially ruling out infection suspicion. In septic cases, APMN counts differed between UNICs and culture positives but were statistically significant with EBJIS criteria (Hip:p = 0.01, Knee:p = 0.03) but not with ICM (p = 0.08). Median APMN counts were significantly elevated in high-virulent compared to low-virulent organisms, with similar trends in most of the other markers. Compared to alpha-defensin, APMN count exhibited better AUC, sensitivity and negative predictive value.

CONCLUSIONS

The APMN count represents a simple and inexpensive method that may serve as a complementary diagnostic marker in hip and knee revision arthroplasties with challenging microbiological findings.

LEVEL OF EVIDENCE

Level III, retrospective study.