Diagnosing acute monoarthritis in adults: a practical approach for the family physician.

Acute monoarthritis can be the initial manifestation of many joint disorders. The first step in diagnosis is to verify that the source of pain is the joint, not the surrounding soft tissues. The most common causes of monoarthritis are crystals (i.e., gout and pseudogout), trauma, and infection. A careful history and physical examination are important because diagnostic studies frequently are only supportive. Examination of joint fluid often is essential in making a definitive diagnosis. Leukocyte counts vary widely in septic and sterile synovial fluids and should be interpreted cautiously. If the history and diagnostic studies suggest an infection, aggressive treatment can prevent rapid joint destruction. When an infection is suspected, culture and Gram staining should be performed and antibiotics should be started. Light microscopy may be useful to identify gout crystals, but polarized microscopy is preferred. Blood tests alone never confirm a diagnosis, and radiographic studies are diagnostic only in selected conditions. Referral is indicated when patients have septic arthritis or when the initial evaluation does not determine the etiology.