Are any blood tests useful in septic arthritis?
Blood cultures are probably the most useful as approximately 50% (24%–76%) of patients with nongonococcal septic arthritis have positive cultures. Whenever blood cultures are positive for S. aureus , a transthoracic echo should be performed to assess for endocarditis. Also consider endocarditis in patients with enterococcal or streptococcal septic arthritis who do not have an obvious source for septic arthritis. Blood cultures may be positive while synovial fluid cultures are negative in up to 10% of cases. Leukocytosis, elevated erythrocyte sedimentation rate (ESR), and elevated C-reactive protein (CRP) are seen in most individuals (60%–90%) but are not discriminative due to overlap with other inflammatory arthritic diseases. Over 90% of patients with septic arthritis have an elevated CRP >2 mg/dL (20 mg/L), so values lower than this are helpful in excluding septic arthritis. An elevated serum procalcitonin level may be supportive of the diagnosis but nondiagnostic by itself. Serial measurements of ESR and CRP are helpful in monitoring response to therapy.