Defining optimal synovial aspirate thresholds and utility of frozen sections in predicting bacterial presence at the time of revision shoulder arthroplasty

Background: The discriminatory utility of synovial aspirates and frozen sections in the evaluation of a failed shoulder arthroplasty is uncertain. We performed a multi-institutional study of consecutive revision shoulder arthroplasties to define the utility and optimal cutoffs of synovial aspirates and frozen sections to predict positive intraoperative cultures. Methods: Multicenter data were collected on 425 revision shoulder arthroplasties in which a synovial aspirate, frozen section, and a minimum of 3 deep intraoperative cultures were reported. Analysis was stratified based on International Consensus Meeting definite and nondefinite periprosthetic joint infection (PJI) and 2 different thresholds of culture positivity (≥2 or ≥3). Receiver operating characteristic curves were constructed, and area under the curve (AUC), optimal thresholds, and diagnostic utility for each test were calculated. Results: Fifty-five patients (13%) had definite PJI per International Consensus Meeting criteria while 370 (87%) had nondefinite PJI. Cutibacterium was the most common bacteria recovered in both definite PJI and nondefinite PJI. Synovial neutrophil percentage had the highest AUC at 0.707 and 0.714 for definite PJI using a threshold of ≥2 and ≥ 3 positive cultures, respectively, and optimal cutoff values were 71.9% and 76.7%, respectively. For Nondefinite PJI, the AUC was 0.625 and 0.595 using a threshold of ≥2 and ≥ 3 positive cultures, respectively, with optimal cutoff values of 74.5% and 77.7%, respectively. Synovial cell count had poor discriminatory utility for Definite PJI (AUC 0.448-0.584) but slightly higher for nondefinite PJI (AUC 0.600-0.664) with optimal cutoff of 3,800-4,100 cells. Frozen section had moderate discriminatory utility for definite PJI (sensitivity 0.667-0.708, specificity 0.500-0.778) but poor sensitivity for nondefinite PJI (sensitivity 0.197-0.282, specificity 0.943-0.948). Conclusion: This is the first large-scale, multicenter study of consecutive revision shoulder arthroplasties analyzing the utility of synovial aspirate analysis and frozen sections. Synovial neutrophil percentage provided the greatest discriminatory utility with optimal cutoff values ranging from 71.9% to 77.7%. Frozen section also provided utility in predicting positive cultures in patients with definite PJI.