From the Guidelines
Synovial fluid analysis is crucial in diagnosing septic arthritis, and it should include a total cell count and differential leukocyte count, as well as culture for aerobic and anaerobic organisms. When evaluating a potentially infected joint, the synovial fluid should be sent for cell count with differential, Gram stain, culture, crystal analysis, and glucose level 1. Normal synovial fluid is clear, straw-colored with <2,000 white blood cells (WBCs)/mm³. In septic arthritis, the fluid typically appears cloudy or purulent with WBC counts exceeding 50,000/mm³, though counts >20,000/mm³ with >90% neutrophils are highly suspicious for infection.
Key Components of Synovial Fluid Analysis
- Cell count with differential
- Gram stain
- Culture for aerobic and anaerobic organisms
- Crystal analysis if clinically indicated
- Glucose level Joint aspiration should always be performed before starting antibiotics when septic arthritis is suspected, as empiric therapy should target Staphylococcus aureus, streptococci, and gram-negative organisms based on patient risk factors 1. The infected joint requires both antimicrobial therapy and drainage, either through serial aspirations, arthroscopic lavage, or open surgical drainage depending on the joint involved and severity.
Importance of Prompt Intervention
- Bacterial arthritis can destroy cartilage within days, leading to permanent joint damage if not promptly addressed
- Urgent intervention is necessary to prevent long-term morbidity and mortality
- A diagnostic arthrocentesis should be performed in all patients with suspected acute septic arthritis unless the diagnosis is evident clinically and surgery is planned and antimicrobials can be safely withheld prior to surgery 1.
From the Research
Synovial Fluid Analysis in Septic Joint
- Synovial fluid analysis is a crucial procedure for the diagnosis and treatment of joint disease, including septic arthritis 2, 3.
- The gross appearance of synovial fluid can provide useful diagnostic information, such as the degree of joint inflammation and presence of haemarthrosis 2, 3.
- Microbiological studies of synovial fluid are essential for confirming infectious conditions, including septic arthritis 2, 3.
Diagnostic Criteria for Septic Arthritis
- A synovial cell count greater than 50,000/mm3 is commonly used as a threshold to diagnose septic arthritis, but this may be nonspecific in the presence of crystalline arthropathy 4.
- Increasing the white blood cell (WBC) count cutoff to 85,000/mm3 may be more appropriate to diagnose concomitant septic arthritis and crystalline arthropathy 4.
- The optimal synovial leukocyte count cutoff for diagnosing septic arthritis may vary depending on whether the patient has received antibiotics, with a cutoff of >16,000 cells suggested for patients who have received antibiotics and >33,000 cells for those who have not 5.
Risk Factors and Treatment of Septic Arthritis
- Risk factors for septic arthritis include age older than 80 years, diabetes mellitus, rheumatoid arthritis, recent joint surgery, hip or knee prosthesis, skin infection, and immunosuppressive medication use 6.
- Prompt diagnosis and treatment of septic arthritis are critical to prevent permanent morbidity and mortality 6.
- Empiric antibiotic therapy should be initiated if there is clinical concern for septic arthritis, with oral antibiotics being suitable for most cases 6.