Diagnostic Approach to Septic Arthritis
Joint aspiration is the definitive diagnostic procedure for septic arthritis, which should be performed immediately prior to antibiotic administration to maximize the likelihood of identifying the causative organism. 1, 2
Clinical Presentation and Initial Evaluation
- Septic arthritis is an orthopedic emergency as bacterial proliferation can rapidly cause cartilage damage and permanent joint dysfunction 2, 1
- The acute onset of monoarticular joint pain, erythema, heat, and immobility should raise suspicion of sepsis 3
- Constitutional symptoms such as fever, chills, and rigors are poorly sensitive for septic arthritis 3
- Initial evaluation should determine if the case is a surgical emergency based on clinical presentation, such as presence of sepsis, immunocompromised status, or evidence of extensive joint destruction 1, 4
Diagnostic Algorithm
Step 1: Imaging
- Plain radiographs should be obtained initially but may only show soft tissue swelling or joint effusion in early infection 1
- Ultrasound is recommended as the initial imaging modality, especially for detecting effusions and guiding aspiration 1, 5
- MRI has high sensitivity for detecting joint effusions, soft tissue involvement, and concomitant osteomyelitis when clinical suspicion remains high despite negative joint aspiration 1, 5
Step 2: Joint Aspiration
- Image-guided aspiration (ultrasound or fluoroscopy) is preferred to ensure proper needle placement and reduce complications 1, 5
- Synovial fluid analysis should include 2, 1:
- White blood cell count and differential
- Gram stain
- Culture
- Crystal analysis to rule out crystalline arthropathy
Step 3: Laboratory Interpretation
- Synovial fluid white blood cell count ≥50,000 cells/mm³ is highly suggestive of septic arthritis in patients who have not received antibiotics 2, 6
- For patients who received antibiotics before aspiration, a lower threshold of >16,000 cells/mm³ should be used 6
- Neutrophil percentage >90% supports the diagnosis of septic arthritis 6, 7
- Synovial fluid culture is positive in approximately 80% of non-gonococcal septic arthritis cases 2
- Elevated serum CRP (>10 mg/dL) combined with synovial fluid findings significantly increases the likelihood of septic arthritis 7
Special Diagnostic Considerations
- Crystalline arthropathy (gout or pseudogout) can coexist with septic arthritis - the presence of crystals does not rule out infection 7
- In patients with prosthetic joints, the intra-articular cutoff values for infection may be as low as 1,100 white blood cells per mm³ with a neutrophil differential of greater than 64% 3
- Negative joint aspirate culture does not rule out infection, especially if antibiotics were administered prior to aspiration 2, 4
- Consider specific pathogens based on patient demographics 1, 4:
- Kingella kingae in children <4 years
- Salmonella species in patients with sickle cell disease
- Gonococcal arthritis in sexually active young adults
Pitfalls and Caveats
- Delayed diagnosis was associated with longer hospitalizations 5
- If septic arthritis is in the differential diagnosis, aspiration for Gram stain and culture remains an essential part of the evaluation, even when using clinical algorithms 5
- Concomitant osteomyelitis occurs in up to 30% of children with septic arthritis and requires longer treatment 2, 4
- In the setting of crystalline arthropathy, a synovial TNC >50,000 cells, PMN >90%, and serum CRP >10mg/dL indicates high risk for concurrent septic arthritis 7
- MRI findings of synovial enhancement, joint effusion, and T1 hypointense bone marrow signal changes have a high correlation with infection 5
By following this diagnostic approach, clinicians can rapidly identify septic arthritis and initiate appropriate treatment to prevent joint destruction and other complications.