What is the Diagnostic Accuracy of Aspirations Performed on Hips With Antibiotic Cement Spacers

Periprosthetic joint infection is a serious complication after THA and commonly is treated with a two-stage revision. Antibiotic-eluting cement spacers are placed for local delivery of antibiotics. Aspirations may be performed before the second-stage reimplantation for identification of persistent infection. However, limited data exist regarding the diagnostic parameters of synovial fluid aspiration with or without saline lavage from a hip with an antibiotic-loaded cement spacer. We asked: (1) For hips with antibiotic cement spacers, does saline lavage influence the diagnostic validity of aspirations? (2) What is the diagnostic accuracy of preoperative aspirations performed on hips with antibiotic cement spacers using the Musculoskeletal Infection Society (MSIS) criteria, stratified by saline and nonlavage? (3) For hips with antibiotic spacers, what are the optimal thresholds for synovial fluid white blood cell (WBC) count and polymorphonuclear neutrophil (PMN) percentage for diagnosing infections? One hundred seventy-four hips (155 patients) with antibiotic-eluting cement spacers inserted between October 2012 and July 2015 were reviewed. Of these, 98 hips (80 patients) met the inclusion criteria and were included in the analysis (77 nonlavage, 21 saline lavage aspirations). Laboratory data from the aspiration and preoperative workup and intraoperative details were collected. Infection status of each hip procedure was determined based on a modified MSIS criteria using serologic, histologic, and intraoperative findings (sinus tract communicating with the joint at surgery or two positive intraoperative periprosthetic cultures with the same organism or two of the three following criteria: elevated erythrocyte sedimentation rate [ESR] [> 30 mm/hour] and C-reactive protein [CRP] [> 10 mg/L], a single positive intraoperative periprosthetic tissue culture, or a positive histologic analysis of periprosthetic tissue [> 5 neutrophils per high power field]). The diagnostic parameters were calculated for the MSIS criteria thresholds for synovial fluid (ie, WBC count > 3000 cells/µL and PMN percentage > 80%). Optimal thresholds were calculated for the corrected synovial WBC count and PMN percentage with a receiver operating characteristic curve. Separate analyses were performed for the hips with successful aspirations (nonlavage group) and those with saline lavage aspirations. The WBC count and PMN percentage were higher in hips with infection than in hips without infection when nonlavage aspirations were done (WBC count, 6680 cells/µL ± 6980 cells/µL vs 2001 ± 4825; mean difference, 4679; 95% CI, 923-8436; p = 0.015; PMN percentage, 83% ± 13% vs 44% ± 30%; mean difference, 39%; 95% CI, 39%–49%; p < 0.001) and the findings between infected and noninfected aspirations were not different when saline lavage aspirations were done (WBC count, 782 cells/µL ± 696 vs 307 cells/µL ± 343; mean difference, 475; 95% CI, −253 to 1203; p = 0.161; PMN percentage, 67% ± 15% vs 58% ± 28%; mean difference, 10%; 95% CI, −11% to 30%; p = 0.331). Aspirations performed without lavage yielded good diagnostic accuracy in all parameters (WBC count, 78% [95% CI, 70%–86%]; PMN percentage. 79% [95% CI, 70%–88%]; positive culture: 84% [95% CI, 81%–90%]; at least one of the above: 79% [95% CI, 70%–88%]); but in the saline lavage group, none had WBC counts above the threshold (diagnostic accuracies for WBC count, 0%; PMN percentage, 71% [95% CI, 62%–86%]; positive culture, 76% [95% CI, 76%–86%]; at least one: 71% [95% CI, 57%–91%]). Because saline lavage did not result in differences between aspirations from infected and noninfected hips, we calculated the optimal thresholds in the nonlavage group only; the optimal threshold for synovial WBC count was 1166 cells/µL and for synovial PMN the percentage was 68%, which corresponds to WBC count diagnostic accuracy of 78% (95% CI, 69%–87%) and PMN percentage accuracy of 78% (95% CI, 69%–87%). Because the MSIS criteria thresholds resulted in suboptimal sensitivities owing to a higher number of false negatives, we recommend considering lower WBC count and PMN percentage thresholds for hip-spacer aspirations. Furthermore, the WBC count and PMN percentage results from aspirations performed with saline lavage are not reliable for treatment decisions. Level III, diagnostic study.