What diagnostic studies are useful in selected patients in the initial evaluation of monoarticular arthritis?
• Cultures of blood, urine, or other possible primary sites of infection: mandatory when a septic joint is being considered.
• Serum prothrombin and partial thromboplastin time: useful if the patient is receiving anticoagulation or if a coagulation disorder is suspected.
• Erythrocyte sedimentation rate or C-reactive protein: although results are often nonspecific, significant elevation may suggest an inflammatory process.
• Serum uric acid levels: notoriously unreliable in making or excluding the diagnosis of gout. These may be spuriously elevated in acute inflammatory conditions or acutely diminished in a true gout attack.
• Serologic tests for antinuclear antibodies (ANAs) and rheumatoid factor: these tests are rarely if ever indicated in the initial evaluation of acute monoarticular arthritis. However, the ANA may be positive in the oligoarticular form of juvenile idiopathic arthritis.
