Diagnostic Workup for Suspected Periprosthetic Joint Infection

Introduction to Diagnostic Approach

The American College of Radiology recommends proceeding immediately with joint aspiration for synovial fluid analysis to definitively rule out or confirm periprosthetic joint infection, given the elevated CRP at 2 years post-surgery 1, 2.

Peripheral WBC counts are not elevated in most patients with infected prostheses, making a normal WBC count meaningless for excluding infection, as noted by the American College of Radiology 1, 2.
CRP has 73-91% sensitivity and 81-86% specificity for prosthetic knee infection when using a cutoff of 13.5 mg/L, according to the American College of Radiology 1, 2.
Pain is the most common presenting symptom of infection, though it's nonspecific; chronic infections may manifest with pain alone without fever, erythema, or warmth, as reported by the American College of Radiology 1, 2.
The American Academy of Orthopaedic Surgeons strongly recommends using ESR and CRP together for diagnostic evaluation, with a diagnostic accuracy reaching 93% sensitivity, 100% specificity, and 97% accuracy when at least 2 of 3 tests are abnormal 1.
Interleukin-6 provides higher predictive value than most other serologic markers and shows excellent sensitivity when combined with CRP, as noted by the American College of Radiology 1, 3.

Obtain radiographs first as the initial imaging evaluation, looking for signs of loosening, osteolysis, or component migration that may accompany infection, as recommended by the American College of Radiology 1, 2.
Withhold antibiotics for at least 2 weeks prior to aspiration if clinically feasible to avoid false-negative cultures, according to Praxis Medical Insights 3.
Synovial fluid culture is critical for identifying causative organisms, most commonly Staphylococcus aureus and coagulase-negative Staphylococcus, as reported by the American College of Radiology 1, 2.

Night pain or pain at rest is characteristic of infection, whereas pain on weight-bearing suggests mechanical loosening, as noted by the American College of Radiology 1, 2.
A large multicenter study found CRP and joint aspiration to be the most useful tools for diagnosing infection, making a combination approach optimal, according to the American College of Radiology 1.
Do not rely on the absence of fever, erythema, or warmth to exclude infection—chronic infections frequently present with pain alone, as reported by the American College of Radiology 1, 2.
Aseptic loosening typically presents with pain on weight-bearing rather than at rest, as noted by the American College of Radiology 1, 2.