History

There are many risk factors for septic arthritis including age >80, Diabetes, Rheumatoid Arthritis, recent joint surgery, prosthesis, cellulitis. The absence of risk factors does not make septic arthritis less likely in an acute monoarticular arthritis

Physical

Monoarticular arthritis is often characterized as a warm, painful, swollen joint with limited range of motion. No studies to date have quantified specificity data on the physical exam. Therefore, clinicians must use their own clinical gestalt when interpreting physical exam findings.

Serum Tests

Useless – unless ESR and CRP are negative. In this case, low negative likelihood ratios may help to exclude septic arthritis

Synovial Fluid Analysis

sWBC > 50,000 - essentially rules IN septic arthritis

septic arthritis sWBC < 50,000 - unhelpful for clinical diagnosis

Gram Stain: useful when positive, unhelpful when negative

Culture: not available when we need it in the ED

Lactate >10 - essentially rules IN septic arthritis

In the end, clinicians must determine their own pre-test probabilities and interpret history, physical exam, and labs as a whole picture. When in doubt, it is safest to treat the patient with antibiotics and consult Orthopaedic Surgery for further management.

For more detailed information check out this article by Margaretten, et al (2007) - "Does This Adult Patient Have Septic Arthritis"