From the Guidelines
Synovial fluid analysis is crucial for diagnosing joint infections, with infected joints typically having significantly elevated WBC counts >50,000/mm³, often exceeding 100,000/mm³, with >75% neutrophils, as supported by the most recent study 1. The normal synovial fluid is clear, straw-colored with <200 white blood cells (WBCs)/mm³. In contrast, infected joints usually appear cloudy or purulent.
- Key parameters for synovial fluid analysis include:
- Total cell count and differential leukocyte count
- Culture for aerobic and anaerobic organisms
- Glucose levels, which are decreased (<40 mg/dL or less than half of serum glucose) in infected joints
- Protein levels, which are elevated (>3 g/dL) in infected joints
- Lactate level, typically elevated (>10 mmol/L) in septic arthritis
- Crystal analysis to rule out crystalline arthropathies
- pH, usually decreased (<7.2) in infected fluid These parameters help differentiate septic arthritis from other inflammatory conditions like gout or rheumatoid arthritis, which may have elevated WBCs but typically not to the same degree, as noted in 1 and 1. Prompt analysis is essential as untreated joint infections can lead to rapid joint destruction and systemic complications, emphasizing the importance of timely diagnosis and treatment, as highlighted in 1 and 1. In cases where the diagnosis is not clinically evident, a test for sedimentation rate or C-reactive protein (CRP) should be performed, and a diagnostic arthrocentesis is advised, as recommended in 1 and supported by the findings in 1.
From the Research
Synovial Infected Fluid Values
- The analysis of synovial fluid is a crucial diagnostic procedure for detecting the presence of pathogenic agents, such as microorganisms, and evaluating the degree of inflammation in joint diseases 2.
- The gross appearance of synovial fluid can provide useful diagnostic information, including the degree of joint inflammation and the presence of haemarthrosis 3, 4.
- Microbiological studies of synovial fluid are essential for confirming infectious conditions, and the detection of monosodium urate and calcium pyrophosphate dihydrate crystals can allow for a precise diagnosis of gout and calcium pyrophosphate crystal-related arthritis 3, 4.
- The white blood cell count (WBC) in synovial fluid can be used to diagnose joint infections, with a cut-off value of 2600 leucocytes/mm3 showing a sensitivity of 85.7% and a specificity of 100.0% 5.
- Joint aspiration and synovial fluid analysis can be used to identify culture-positive infected hip arthroplasty, with a sensitivity of 59% and a specificity of 100% when using synovial fluid aspiration and/or synovial biopsy 6.
Diagnostic Values
- The sensitivity and specificity of preoperative joint aspiration were 58.3% and 88.9%, respectively, while the sensitivity and specificity of interstage aspiration were 0.0% and 88.5%, respectively 5.
- The use of synovial biopsy alone resulted in a sensitivity of 41%, a specificity of 100%, and an accuracy of 91% in detecting culture-positive periprosthetic infection 6.
- The combination of synovial fluid aspiration and synovial biopsy showed a sensitivity of 59%, a specificity of 100%, and an accuracy of 94% in detecting culture-positive periprosthetic infection 6.
Clinical Applications
- Joint aspiration and synovial fluid analysis are invaluable procedures for the diagnosis and treatment of joint diseases, including septic arthritis and crystal arthritis 2, 3, 4.
- The use of ultrasonography can facilitate arthrocentesis in difficult cases and provide useful diagnostic information about the presence of synovial fluid and its characteristics 2.
- The analysis of synovial fluid can help guide targeted antibiotic therapy and empiric antibiotic therapy should cover common pathogens, even if aspiration shows negative microbiological cultures 5.