From the Guidelines
Synovial fluid infected with bacteria typically shows characteristic changes in laboratory values, with WBC counts usually exceeding 50,000/mm³, glucose levels decreased, protein levels elevated, and lactate levels increased, as noted in the most recent study by 1.
Key Laboratory Findings
- Normal synovial fluid is clear, straw-colored with less than 200 white blood cells (WBCs) per cubic millimeter.
- In septic arthritis, the fluid appears cloudy or purulent with WBC counts usually exceeding 50,000/mm³, often reaching 100,000/mm³ or higher, with more than 75% neutrophils.
- Glucose levels in infected synovial fluid are decreased compared to serum glucose (typically less than 40 mg/dL or a synovial-to-serum glucose ratio less than 0.5).
- Protein levels are elevated above 3 g/dL.
- Lactate levels are typically increased above 5.0 mmol/L.
- The fluid may show positive Gram stain in 50-75% of cases and positive cultures in 60-80% of bacterial infections.
Diagnostic Approach
- Synovial fluid analysis should include cell count with differential, Gram stain, culture, crystal examination, protein, and glucose measurements, as recommended by 1 and 1.
- The use of alpha-defensin laboratory test has been described for the diagnosis of periprosthetic joint infection, with high sensitivity and specificity, as noted in 1 and 1.
- The American Academy of Orthopaedic Surgeons (AAOS) guidelines strongly recommend the use of ESR, CRP, and serum interleukin-6 testing for patients being assessed for peri-prosthetic joint infection, as mentioned in 1.
Clinical Implications
- These findings help distinguish infectious arthritis from other inflammatory conditions like gout or rheumatoid arthritis.
- Septic arthritis requires urgent antibiotic therapy and often surgical drainage to prevent joint destruction, highlighting the importance of prompt and accurate diagnosis, as emphasized in 1 and 1.
From the Research
Synovial Fluid Analysis
- Synovial fluid analysis is a vital tool in the diagnosis and prognosis of joint diseases, including infections 2.
- The analysis of synovial fluid can help distinguish between different types of joint diseases, such as degenerative, inflammatory, autoimmune, traumatic, congenital, and neoplastic joint diseases 2.
Infected Synovial Fluid Values
- A synovial white blood cell (WBC) count of ≥50,000/mm³ is often used as a threshold to diagnose joint infections, with a sensitivity of 0.727 and specificity of 0.923 3.
- However, the percentage of polymorphonuclear cells (%PMNs) in the synovial fluid may be a more sensitive predictor of joint infection, with a sensitivity of 0.932 when %PMNs is at least 80% 3.
- Synovial fluid viscosity has also been proposed as a potential marker for the diagnosis of periprosthetic joint infection, with a sensitivity of 93.33% and specificity of 66.67% 4.
Comparison of Infected and Non-Infected Synovial Fluid
- Infected synovial fluid tends to have a higher WBC count and %PMNs compared to non-infected fluid 3, 5.
- Contaminated synovial fluid cultures can be distinguished from true infections based on clinical characteristics, such as age, peripheral polymorphonuclear lymphocytes, and length of stay 5.
- Patients with contaminated synovial fluid cultures tend to have less severe disease and better outcomes compared to those with true infections 5.
Diagnostic Thresholds
- A threshold of 11.80 mPa·s for synovial fluid viscosity has been proposed for the diagnosis of periprosthetic joint infection 4.
- A %PMNs threshold of at least 80% has been suggested as a sensitive predictor of joint infection 3.
- A synovial WBC count of ≥50,000/mm³ is often used as a diagnostic threshold for joint infections, but may not be as sensitive as %PMNs 3.