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Difficulty: EasyRheumatology and Immunology

A clinic evaluates a 55-year-old male who developed sudden, severe right knee swelling and pain 48 hours ago. He denies any recent trauma, falls, or prior joint issues. His current temperature is 38.2°C (100.8°F). Examination reveals a visibly enlarged, erythematous right knee that is exquisitely tender to palpation. Range of motion is severely limited by pain. Which of the following is the most appropriate initial step in the evaluation of this patient?

  1. A
    Measurement of serum uric acid concentration
  2. B
    Initiation of empiric intravenous vancomycin and ceftriaxone
  3. Arthrocentesis and synovial fluid analysisAnswer
  4. D
    Magnetic resonance imaging of the right knee
  5. E
    Intra-articular injection of a long-acting glucocorticoid

Answer

Arthrocentesis and synovial fluid analysis
The patient presents with an acute, atraumatic, inflammatory monoarthritis accompanied by systemic signs (fever). This clinical picture is highly suspicious for septic arthritis, which is a joint-threatening medical emergency, as well as acute crystal arthropathy (such as gout or pseudogout). The most critical and urgent initial step is to perform arthrocentesis and synovial fluid analysis (including cell count, Gram stain, and crystal analysis). This procedure definitively establishes the diagnosis and rules out a bacterial infection, which dictates the subsequent course of management.

Step-by-Step Solution

1
Identify the primary clinical syndrome based on the patient's presentation.
The patient has an acute, atraumatic monoarticular arthritis accompanied by fever.
Recognizing the pattern of acute monoarthritis is the first step in formulating a differential diagnosis, which primarily includes septic arthritis, gout, and pseudogout.
2
Determine the most critical, time-sensitive condition to rule out.
Septic arthritis is a medical emergency that must be rapidly identified or excluded.
Failure to diagnose and treat a septic joint can lead to irreversible joint destruction within days and carries significant morbidity.
3
Select the appropriate diagnostic modality to differentiate the possible etiologies.
Arthrocentesis with synovial fluid analysis is the definitive test.
Direct examination of the synovial fluid allows for the immediate identification of crystals (diagnosing gout/pseudogout) or microorganisms and elevated leukocytes (diagnosing infection).

Key Concept

The initial evaluation of any acute, inflammatory monoarthritis must always include arthrocentesis to definitively rule out septic arthritis before initiating specific treatments.
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Topics

Rheumatology and Immunology

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