Acute monoarthritis in adults (septic arthritis to rule out, gout, chondrocalcinosis)

• Refer the same day (emergency department or rheumatology): joint aspiration and blood cultures BEFORE any antibiotic.
• No blood tests in the community that delay referral.

Joint fluid (aspiration in the community if the practitioner is competent, otherwise at the rheumatologist)

• Joint fluid examination: cytology (leucocytes), bacteriology (direct examination and culture), crystals
• Transport the fluid to the laboratory without delay (cytology and crystals the same day); if it has to wait, keep it in the refrigerator (+4 °C).

Laboratory tests

• FBC
• CRP
• Serum urate
• INR (if on a vitamin K antagonist, before aspiration)if neededSupratherapeutic anticoagulant treatment is a contraindication to joint aspiration, especially of a deep joint ^[2].
• Chlamydia trachomatis and gonococcus NAAT (PCR) on first-void urine (if young patient, urethritis, at-risk intercourse, pustular rash)if neededA febrile oligoarthritis in a young person with sexual risk behaviour points to gonococcal infection; urethritis or infectious diarrhoea to reactive arthritis ^[2]. The NABM covers PCR at all possibly infected sites and for the diagnosis of reactive arthritis ^[7].
• Lyme serology (if tick bite in an endemic area or history of erythema migrans)if needednon-French guidelineAn American Family Physician review (2025) lists Lyme arthritis among the diagnoses to consider in acute monoarthritis ^[6]; the American College of Rheumatology (Choosing Wisely campaign) says to look for Lyme disease in joint symptoms only in case of exposure and suggestive signs ^[8]. In France, the COFER cites a history of erythema migrans among the guiding signs and Lyme disease among the causes of chronic monoarthritis ^[2]. See the page “Lyme borreliosis: tick bite, erythema migrans, disseminated forms”.