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Difficulty: MediumRheumatology and Immunology

A 61-year-old man comes to the emergency department due to severe right knee pain and swelling that began abruptly overnight. He reports that the pain is excruciating, and he is unable to bear weight on the right leg. His medical history includes type 2 diabetes mellitus, chronic kidney disease (stage 3a), and a recent hospitalization two weeks ago for a chronic obstructive pulmonary disease exacerbation. His temperature is 38.2°C (100.8°F), blood pressure is 142/88 mm Hg, and pulse is 98/min. Physical examination reveals a markedly swollen, erythematous, and warm right knee. Range of motion is severely limited by pain. There are no skin breaks or ulcerations around the joint. Which of the following is the most appropriate next step in management?

  1. Perform arthrocentesis of the right kneeAnswer
  2. B
    Initiate intravenous vancomycin and ceftriaxone
  3. C
    Measure serum uric acid concentration
  4. D
    Obtain magnetic resonance imaging of the right knee
  5. E
    Prescribe high-dose oral ibuprofen and advise joint rest

Answer

Perform arthrocentesis of the right knee
The correct answer is to perform arthrocentesis because distinguishing between a severe crystal-induced arthropathy and septic arthritis relies entirely on synovial fluid analysis. Given the risk of rapid, irreversible joint destruction with septic arthritis, obtaining fluid for a leukocyte count, Gram stain, culture, and crystal analysis is the most critical initial step.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient has an acute monoarticular arthritis with severe inflammatory signs (warmth, erythema, exquisite pain) and a systemic fever.
Identifying the acuity and inflammatory nature of the joint presentation dictates the urgency of the workup.
2
Determine the differential diagnosis.
The presentation is highly suspicious for septic arthritis. However, crystal arthropathies such as gout or pseudogout can also present with fever and identical severe joint findings.
Both infectious and microcrystalline etiologies are medical urgencies that require definitive differentiation.
3
Identify the mandatory next diagnostic step.
Synovial fluid analysis via arthrocentesis is required to evaluate leukocyte count, perform a Gram stain and culture, and assess for crystals.
It is the only method to definitively rule in or rule out a joint infection prior to initiating targeted therapies.

Key Concept

The initial evaluation of acute monoarticular arthritis, particularly when accompanied by fever, requires prompt arthrocentesis and synovial fluid analysis to definitively rule out septic arthritis before initiating condition-specific therapies.
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Topics

Rheumatology and Immunology

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