['Previous studies have demonstrated that the administration of antibiotics to patients before performing diagnostic testing for periprosthetic joint infection (PJI) can interfere with the accuracy of test results. Although a single-institution study has suggested that alpha-defensin maintains its concentration and sensitivity even after antibiotic treatment, this has not yet been demonstrated in a larger multiinstitutional study.', '(1) For the evaluation of PJI, is prior antibiotic administration associated with decreased alpha-defensin levels? (2) When prior antibiotics are given, is alpha-defensin a better screening test for PJI than the traditional tests (erythrocyte sedimentation rate [ESR], C-reactive protein [CRP], fluid white blood cells, fluid polymorphonuclear cells [PMNs], and fluid culture)?', "This retrospective study included data from 106 hip and knee arthroplasties with Musculoskeletal Infection Society-defined PJI from four centers. Of the 106 patients in this study, 30\xa0(28%) were treated with antibiotics for PJI before diagnostic workup (ABX group), and 76\xa0(72%) were not treated before the diagnostic workup (NO-ABX group). There were no differences in age, sex, joint, culture-negative rate, or bacteriology between groups. The patients in the ABX group had antibiotics initiated by physicians who commenced care before assessment for PJI by the treating surgeon's service. We compared the alpha-defensin levels and sensitivity between the ABX and NO-ABX groups. Additionally, the sensitivity of the alpha-defensin test was compared to that of traditional tests for PJI among patients on antibiotics.", 'The administration of antibiotics before performing the alpha-defensin test for PJI was not associated with a decreased median alpha-defensin level (ABX group, median 4.2 [range, 1.79-12.8 S/CO] versus NO-ABX, median 4.9 [range, 0.5-16.8 S/CO], difference of medians: 0.68 S/CO [95% confidence interval {CI}, -0.98 to 1.26], p = 0.451). Furthermore, the alpha-defensin test had a higher sensitivity (100%; 95% CI, 88.4%-100.0%) in diagnosing PJI among patients on antibiotics when compared with the ESR (69.0% [95% CI, 49.17%-84.72%], p = 0.001), the CRP (79.3% [95% CI, 60.3%-92.0%], p = 0.009), the fluid PMN% (79.3% [95% CI, 60.3%-92.0%), p = 0.009), and fluid culture (70.0% [95% CI, 50.6%-85.3%], p = 0.001).'] Is the Alpha-defensin Test for Periprosthetic Joint Infections Affected by Prior Antibiotic Administration?