Laboratory Studies • Complete blood count with differential - Often reveals leukocytosis with a left shift • Erythrocyte sedimentation rate and C-reactive protein - Helpful in monitoring treatment course • Blood cultures • May be positive in up to 50% of S aureus infections • Very poor in detecting N gonorrhoeae (Approximately 10% of cases prove positive.) • Urethral, cervical, pharyngeal, and rectal cultures - Much higher yield for N gonorrhoeae than in blood cultures • Synovial fluid analysis – Gram stain, culture, cell counts, and crystal analysis Synovial Fluid Classification (Modified from Schumacher HR. Pathologic Findings in Rheumatoid Arthritis)
Imaging Studies • Plain radiography - Anteroposterior and lateral views • Findings are often normal. • Radiography may be helpful when considering hip involvement in young children. • Look for soft-tissue swelling around the joint, widening of the joint space, and displacement of tissue planes. • In later stages of progression, look for bony erosions and joint space narrowing.
Ultrasonography • This study is very sensitive in detecting joint effusions generated by septic arthritis. • Ultrasound can be used to define the extent of septic arthritis and help guide treatment. • Ultrasound helps to differentiate septic arthritis from other conditions (eg, soft-tissue abscesses, tenosynovitis) in which treatment may differ.
Nuclear scanning: • This study may be helpful to differentiate transient synovitis from septic arthritis.
Anteroposteriorview of the knee demonstrates patchy demineralization of the tibia and femur and joint-space narrowing caused by tuberculoid infection of the joint
Hyperintensejoint effusion and increased signal intensity in the bone marrow of the pubic rami shown in septic arthritis with associated osteomyelitis and inflammatory changes in the soft tissues.
Anteroposteriorview of the shoulder demonstrates subchondral erosions and sclerosis in the humeral head.
Septic arthritis with associated soft tissue abscess. Coronal T2-weighted fat-saturated MRI of the shoulder demonstrates a joint effusion, bone marrow edema, and marked adjacent soft tissue inflammation with a fluid collection in the infraspinatus muscle.
Diagnostic Procedures • Needle aspiration • May be the initial best diagnostic and therapeutic procedure in the vast majority of cases • May allow thorough decompression of joint • Can be repeated serially to achieve relief of symptoms, decrease joint effusion, and clear bacteria and synovial WBCs. • Poor choice in joints with loculations