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)

Septic Inflammatory Noninflammatory

Reference Range Quality

>3.5 >3.5 >3.5 <3.5 Volume, mL

Variable Low High High Viscosity

Variable Yellow Straw-yellow Clear Color

Opaque Translucent Transparent Transparent Clarity

Often >100,000 2,000-75,000 200-2,000 <200 WBC, µL

>75% >50% <25% <25% PMN% ,Often positive Negative Negative Negative Culture result

Friable Friable Firm Firm Mucin clot

Very decreased Decreased ~Blood ~Blood Glucose

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 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