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What is the diagnosis?

The differential includes septic arthritis, gout, pseudogout, cellulitis, abscess, reactive arthritis, rheumatoid arthritis, and Lyme disease. Septic arthritis, which this patient has, is the "can't miss" diagnosis.

• Pearl: Septic arthritis typically affects one joint but is seen in multiple joints in 20% of cases. The most commonly affected joint is the knee [1]. 

What is your differential? What is the “can’t miss” diagnosis?

What evaluation should be performed?

Arthrocentesis, and evaluation of the synovial fluid (culture, cell analysis, and Gram stain) is required to make the diagnosis [1,2].

Blood tests such as CBC, ESR, CRP can be obtained for evaluation, however, their results are inadequate to rule out septic joint and should not influence the decision to obtain synovial fluid [1].

• Pearl: Blood cultures are positive in over one-third of patients with septic joints, and about one-sixth of patients with negative synovial fluid cultures have positive blood cultures [1].
• Pearl: Synovial fluid analysis demonstrating WBC >50,000 cells/mm3 is concerning for septic joint. WBC <50,000 cells/mm3 makes the diagnosis less likely but does not completely exclude it [2]. 

What is your management in the Emergency Department? Do you need to consult Orthopedics?

An arthrocentesis should be performed and the synovial fluid sent for testing. If concern for septic arthritis exists, consult orthopedic surgery early as patients may require repeat arthrocentesis, arthroscopy, or arthrotomy [1]. Definitive treatment for septic arthritis is OR washout. Discuss antibiotic coverage with your Orthopedic team, often they will want to wait until OR cultures can be obtained prior to starting antibiotics if the patient is not septic.

• Pearl: Administration of antibiotics prior to arthrocentesis results in decreased sensitivity of microscopy and culture; thus, if the patient is stable, antibiotics should not be administered prior to fluid aspiration [3].
• Pearl: Traditional teaching is to avoid performing an arthrocentesis through overlying cellulitis. A recent review suggests there may not be harm but that one should still avoid aspirating through an abscess [1]. If concern exists, discuss with your orthopedic surgeon.