A patient presents with a painful swollen total knee replacement 18 months after index surgery. Aspiration of the knee yields a synovial fluid white cell count of 4,200 cells per microlitre with 85 percent polymorphonuclear leukocytes. Cultures are pending. Based on established diagnostic thresholds for chronic periprosthetic joint infection beyond 90 days, this result is:
- A Consistent with aseptic failure, as any count under 5,000 cells per microlitre excludes infection
- B Diagnostic only if the erythrocyte sedimentation rate is also above 100 mm per hour
- C Suggestive of chronic periprosthetic joint infection, which exceeds both the cell count and neutrophil percentage thresholds ✓
- D Interpretable only in a cemented prosthesis
Correct answer: C. Suggestive of chronic periprosthetic joint infection, which exceeds both the cell count and neutrophil percentage thresholds
Explanation
For a prosthesis in place more than 90 days, the accepted diagnostic thresholds are a synovial fluid white cell count greater than 3,000 cells per microlitre with a polymorphonuclear percentage greater than 80 percent, both of which this aspirate exceeds. In contrast, acute postoperative infection uses higher count cutoffs near several thousand within the first weeks. These fluid criteria apply regardless of cementation, and serology supports but does not gate the interpretation of the aspirate.
Reference: Journal of Bone and Joint Surgery consensus criteria as summarized in Miller's Review of Orthopaedics, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.