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Review Question - QID 210261

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QID 210261 (Type "210261" in App Search)
A 64-year-old male is evaluated for a painful, swollen right knee 3 weeks after undergoing a right primary total knee arthroplasty (TKA). As part of his workup, a right knee aspiration is performed and the synovial fluid white blood cell count returns at 10,500 cells/microliter. How should this value be interpreted in the context of evaluating for a prosthetic joint infection (PJI)?

It is above the cutoff of ~3,000 cells/microliter in the perioperative period; a PJI is likely present

24%

712/2943

It is below the cutoff of ~30,000 cells/microliter in the perioperative period; a PJI is unlikely

36%

1069/2943

It is above the cutoff of ~10,000 cells/microliter in the perioperative period; a PJI is likely present

26%

779/2943

It is below the cutoff of ~50,000 cells/microliter in the perioperative period; a PJI is unlikely

3%

91/2943

It is not possible to interpret the synovial WBC count within 6 weeks of a primary TKA

9%

263/2943

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A synovial fluid WBC count of 27,800 cells/microliter (approximately 30,000) within 6 weeks after primary TKA has been shown to predict prosthetic joint infection.

Prosthetic joint infection (PJI) is a relatively rare but devastating complication in total joint arthroplasty. Despite technological advances, diagnosis of PJI remains difficult. This is especially true in the acute perioperative period when post-surgical hemarthrosis and inflammation can confound the interpretation of the synovial fluid WBC count. Given these physiologic changes, a synovial fluid WBC of approximately 30,000 cells/microliter has been established as the cutoff for infection in the first 6 weeks following primary TKA.

Bedair et al. evaluated the diagnostic performance of the synovial fluid WBC count within 6 weeks of a primary TKA. They found that a cutoff value of 27,800 cells/microliter had a positive predictive value of 94% and a negative predictive value of 98% for the diagnosis of a PJI in the first 6 weeks following a primary TKA. The authors concluded that the standard cutoff value of 3,000 cells/microliter used at their institution would have led to unnecessary operations.

Mason et al. sought to determine the value of the synovial WBC count that would most accurately predict infection in a total knee arthroplasty. They found that aspirates with a synovial fluid WBC count higher than 2,500 cells/microliter and 60% PMNs were highly suggestive of infection, although the time from the original surgery to the aspiration was not mentioned. The authors concluded that the synovial fluid WBC count and differential analysis is a relevant indicator for the presence or absence of infection in prosthetic joints.

Incorrect Answers:

Answers 1 and 3: The cutoff for synovial fluid WBC count within 6 weeks of a primary TKA is approximately 30,000
Answer 4: While a PJI is unlikely, the cutoff for synovial fluid WBC count within 6 weeks of a primary TKA is approximately 30,000, not 50,000
Answer 5: The synovial fluid WBC count can be interpreted within 6 weeks of a primary TKA

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