From the Guidelines
Septic arthritis is diagnosed based on a combination of clinical, laboratory, and imaging findings, with the Kocher criteria being a widely applied tool to distinguish septic arthritis from transient synovitis, including fever >101.3 F, erythrocyte sedimentation rate of at least 40 mm/hour, white blood cell (WBC) count of at least 12,000 cells/mm3, and an inability to bear weight on the affected side, as supported by 1. The key criteria for diagnosing septic arthritis include joint pain, swelling, warmth, and limited range of motion, typically affecting a single joint (monoarticular), though multiple joints can be involved. Fever is often present. Some key points to consider in the diagnosis of septic arthritis include:
- Laboratory tests should include synovial fluid analysis, which is the gold standard for diagnosis, with a white blood cell count greater than 50,000 cells/mm³ (with >75% neutrophils), low glucose, high protein, and positive Gram stain or culture strongly suggesting septic arthritis, as noted in 1.
- Blood cultures should be obtained before starting antibiotics, as they are positive in about 50-70% of cases.
- Elevated inflammatory markers such as C-reactive protein and erythrocyte sedimentation rate support the diagnosis but are not specific.
- Imaging studies like X-rays may show joint effusion but are often normal early in the disease; ultrasound, MRI, or CT can better detect effusions and early bone changes.
- The definitive diagnosis requires identification of bacteria from synovial fluid, though empiric treatment should not be delayed while awaiting culture results, as emphasized in 1. These criteria help distinguish septic arthritis from other inflammatory joint conditions, which is crucial as delayed treatment of septic arthritis can lead to rapid joint destruction and significant morbidity. In terms of management, typical management of septic arthritis consists of antibiotics, arthrotomy, irrigation, and debridement, as stated in 1. It's also important to note that the distribution of septic arthritis and osteomyelitis varies by age, with children <2 years of age being more likely to have septic arthritis than osteomyelitis, as reported in 1. Overall, prompt diagnosis and treatment of septic arthritis are critical to preventing long-term morbidity and mortality, and the Kocher criteria and synovial fluid analysis are essential tools in making an accurate diagnosis, as supported by 1 and 1.
From the Research
Diagnostic Criteria for Septic Arthritis
The diagnosis of septic arthritis is based on a combination of clinical features, laboratory tests, and imaging studies. The key diagnostic criteria include:
- Acute onset of joint pain, swelling, and fever 2
- Limited range of motion and tenderness of the joint 3
- New joint swelling and effusion 3
- Positive synovial fluid culture or direct demonstration of bacteria in synovial fluid 4
- Elevated serum markers, such as erythrocyte sedimentation rate and C-reactive protein 2
Risk Factors for Septic Arthritis
Certain patient populations are at higher risk for developing septic arthritis, including:
- Older adults (age > 80 years) 2
- Patients with diabetes mellitus 2, 5
- Patients with rheumatoid arthritis 2
- Patients with recent joint surgery or prosthetic joint infection 4, 2
- Patients with skin infections or immunosuppressive medication use 2
Laboratory Tests and Imaging Studies
Laboratory tests, such as synovial fluid analysis and blood cultures, are essential for diagnosing septic arthritis. Imaging studies, such as X-rays and MRI, may also be helpful in supporting the diagnosis 3. The criterion standard for diagnosis is diagnostic arthrocentesis, which involves the collection and analysis of synovial fluid from the affected joint 3.
Causative Pathogens
The most common causative pathogens in septic arthritis are: