Diagnosis of Acute Infectious Arthritis Arthrocentesis with synovial fluid examination and culture Blood culture Usually complete blood count and erythrocyte sedimentation rate (or C-reactive protein) Molecular testing (eg, polymerase chain reaction) Sometimes imaging studies Infectious arthritis is suspected in patients with acute monarticular or oligoarticular arthritis and in patients with other combinations of symptoms characteristic of particular infectious arthritis syndromes (eg, migratory polyarthritis, tenosynovitis, and skin lesions typical of disseminated gonococcal infection; erythema migrans or other symptoms and signs of Lyme disease).