Approach Considerations
Leukocytosis is common in patients with acute bacterial arthritis. Approximately 50% of persons with acute disease exhibit white blood cell (WBC) counts higher than 10,000/µL. Blood culture results are positive in approximately 33%-50% of patients with nongonococcal bacterial arthritis. [17]
If indicated, arthrocentesis for synovial fluid analysis is the single most important diagnostic procedure for evaluating infectious arthritis. It allows culture and appropriate microscopic examination of the synovial fluid and tissue.
Diagnostic imaging modalities may be helpful but are often nonspecific. [5]
Arthrocentesis and Synovial Fluid Analysis
The synovial fluid cell count is generally higher than 50,000/µL, with neutrophils predominating (>90%) in persons with acute bacterial arthritis.
Results of a Gram stain of synovial fluid are positive in approximately 75% of patients with staphylococcal infections but in only 50% of patients with gram-negative infections.
A microscopic examination of synovial fluid for monosodium urate crystals and calcium pyrophosphate crystals is performed to exclude crystal-induced arthritis (eg, gout or pseudogout). It is important, however, to be mindful of the possibility that infectious arthritis and crystal-induced arthritis may be coexisting in a single joint, though such coexistence is reportedly very uncommon.
Culture of synovial fluid should be performed for aerobic and anaerobic organisms. Inoculation of blood culture bottles is more sensitive than culture on solid media, especially in patients pretreated with antibiotics. [18]
In infants and young children (especially those 6 to 48 months of age), consideration should be given to infection with Kingella kingae, which is notoriously fastidious and often fails to be detected by traditional culture methods. Polymerase chain reaction (PCR) testing of synovial fluid for the16S rRNA gene can markedly improve identification of K kingae. [8, 9]
Biopsy of synovial tissue for culture and histologic examination is important if mycobacterial or fungal infections are suggested. Culture of synovial fluid is an insensitive diagnostic test in this setting.
Radiography, CT, MRI, and Radionuclide Imaging
Plain radiography is generally nonspecific and may reveal only a joint effusion in the early stages of infection. Cartilage destruction and joint space narrowing are late findings and may be difficult to interpret if there is a preexisting joint disease.
Computed tomography (CT) may help diagnose sternoclavicular or sacroiliac joint infections. Magnetic resonance imaging (MRI) is most useful in assessing the presence of periarticular osteomyelitis as a causative mechanism.
Radionuclide studies (eg, bone scans) yield positive results for any inflammatory arthritis and thus have poor specificity. They may be useful for diagnosing sternoclavicular or sacroiliac joint infection.
