Septic Arthritis Workup Laboratory Studies Complete blood count

Septic Arthritis: Workup

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)

Quality Reference Range Noninflammator Inflammatory y Septic Volume, m. L 3. 5> 3. 5< Viscosity High Low Variable Color Clear Straw-yellow Yellow Variable Clarity Transparent Translucent Opaque WBC, µL 200> 200 -2, 000 -75, 000 Often >100, 000 PMN, % 25%> 50%< 75%< Culture result Negative Often positive Mucin clot Firm Friable Glucose ~Blood Decreased Very decreased

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.

• Anteroposterior view of the knee demonstrates patchy demineralization of the tibia and femur and joint-space narrowing caused by tuberculoid infection of the joint

• Hyperintense joint 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.

• Anteroposterior view of the shoulder demonstrates subchondral erosions and sclerosis in the humeral head.

• Septic arthritis with associated soft tissue abscess. Coronal T 2 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