Diagnosis of native joint septic arthritis

In patients with suspected native joint septic arthritis, take diagnostic samples before starting antibiotic therapy. This allows alternative or coexisting diagnoses (such as an acute crystal arthropathy) to be excluded and antibiotic therapy to be directed. Appropriate diagnostic samples include blood for culture, and a joint aspirate for Gram stain, culture and microscopy.

Prolonged incubation of cultures and/or nucleic acid amplification tests (NAATs) may be appropriate in certain circumstances (eg culture-negative septic arthritis) after discussion with a clinical microbiologist. False-negative synovial fluid culture results may be seen in patients who have been previously treated with antibiotic therapy or have an infection with a fastidious organism (eg Kingella kingae, Cutibacterium acnes).

Joint aspiration should be performed by clinicians experienced in the procedure. When performing diagnostic aspiration, aspirating the bulk of the effusion can offer therapeutic benefits (eg analgesia). Aspiration should be as complete as possible without compromising patient comfort or causing damage to the joint. Presence of crystals in synovial fluid does not exclude septic arthritis. Specific considerations for joint aspiration are outlined in Utility of joint aspiration and synovial fluid analysis for rheumatological diseases.

Infective endocarditis should be excluded if haematogenous seeding is likely, particularly if any of the following pathogens are identified: Staphylococcus aureus, nongroupable oral streptococci (eg viridans group streptococci), Streptococcus gallolyticus subspecies Gallolyticus (formerly known as Streptococcus bovis) or Enterococcus faecalisCouderc, 2020.

Acute rheumatic fever may present as an acute monoarthritis and should be considered as a differential diagnosis, particularly in Aboriginal and Torres Strait Islander people living in rural or remote settings, and other populations who may be at high risk (eg Aboriginal and Torres Strait Islander people living in urban settings, Māori and Pacific Islander people).