Synovial Fluid Alpha-Defensin Is an Adjunctive Tool in the Equivocal Diagnosis of Periprosthetic Joint Infection

Background: Synovial fluid alpha-defensin has shown to be a reliable diagnostic test for the diagnosis of periprosthetic joint infection (PJI), but its use in equivocal cases has yet to be established. The purpose of this study was to determine the reliability of alpha-defensin testing in patients, where the diagnosis of PJI was unclear. Methods: A consecutive series of 41 synovial aspirations by a single surgeon that were sent for alpha-defensin testing in equivocal cases of PJI were retrospectively reviewed. Indications for alpha-defensin testing included recent antibiotic use, borderline synovial fluid cell count, and differential, suspected culture-negative infection, and suspected false-positive culture. PJI was diagnosed using the Musculoskeletal Infection Society (MSIS) criteria. Results: Of the 39 aspirations in 32 patients included in the study, there were 33 (85%) knee and 6 (15%) hip samples. Eleven (28%) samples met MSIS criteria for PJI. Of the 23 samples that had recent antibiotic use (6 MSIS positive, 17 negative), alpha-defensin results confirmed the correct MSIS diagnosis in 19 (83%) samples. Of the 11 samples in which alpha-defensin testing was performed for a borderline cell count (3 MSIS positive, 8 negative), alpha-defensin confirmed the MSIS diagnosis in 10 of 11 (91%) samples. Finally, among the 5 samples with suspected false-positive or false-negative cultures (2 MSIS positive, 3 negative), alpha-defensin confirmed the correct diagnosis in 3 (60%) samples. The overall sensitivity, specificity, negative predictive value, and positive predictive value of synovial alpha-defensin were 82%, 82%, 92%, and 64%, respectively. Conclusion: In patients for whom the diagnosis of PJI is unclear because of recent antibiotic use, equivocal laboratory findings, or suspected false-negative or false-positive cultures, synovial fluid alpha-defensin can provide an additional data point to assist the clinician in determining whether PJI is present but is prone to false-positive results in this challenging population.