The diagnostic thresholds for synovial fluid analysis in late periprosthetic infection of the hip depend on the duration of symptoms.
OBJECTIVE
We hypothesised that the synovial white blood cell (WBC) count in patients with a late periprosthetic joint infection (PJI) of the hip would depend on the duration of a patient's symptoms, and that the optimal diagnostic threshold would also depend on this period of time.


METHODS
The synovial WBC count and percentage of polymorphonuclear cells (%PMN), and the serum CRP and ESR levels obtained > six weeks after primary THA were compared between 50 infected and 88 non-infected THAs, and in patients with symptoms for more than or less than two weeks. Diagnostic thresholds for the synovial WBC count were calculated using area under the curve calculation.


RESULTS
The synovial WBC count was significantly higher in patients with symptoms for ≤ two weeks compared with those with symptoms for > two weeks (p = 0.03). The optimal threshold for diagnosing PJI for the synovial WBC count was 5750 cells/µL (sensitivity 94; specificity 100; PPV 100%; NPV 89%; AUC 99%) and 1556 cells/µL (sensitivity 91; specificity 94; PPV 87% and NPV 97%; AUC 95%), respectively. The thresholds for the cut-offs based on duration of symptoms improved the diagnostic performance of this test.


CONCLUSIONS
This study shows that the diagnostic thresholds for synovial fluid analysis in late periprosthetic infection following THA may depend on duration of symptoms. Cite this article: Bone Joint J 2016;98-B:1355-9.
Massachusetts General Hospital, Boston, Massachusetts, USA.
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HR
Choi
Massachusetts General Hospital, Boston, Massachusetts, USA.
K
K
Agrawal
Massachusetts General Hospital, Boston, Massachusetts, USA.
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H
Bedair
D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects*; D001799:Blood Sedimentation; D002097:C-Reactive Protein / Q000032:analysis*; D005260:Female; D006622:Hip Prosthesis; D006801:Humans; D007958:Leukocyte Count; D008297:Male; D008875:Middle Aged; D009504:Neutrophils / Q000473:pathology; D016459:Prosthesis-Related Infections / Q000175:diagnosis* / Q000378:metabolism; D012372:ROC Curve; D012189:Retrospective Studies; D013582:Synovial Fluid / Q000166:cytology* / Q000378:metabolism
D002097:C-Reactive Protein
Periprosthetic joint infection; Symptom duration; Synovial WBC count; Total hip arthroplasty