From the Guidelines
A synovial fluid white blood cell count of 50,000 cells/mm³ or higher is suggestive of septic arthritis. According to the most recent study 1, a synovial fluid white blood cell count of 3 (50 × 10^9/L) or higher is indicative of septic arthritis. However, it's essential to consider that this number is not conclusive and can be influenced by different factors.
Key Considerations
- The diagnostic range for septic arthritis can start as low as 20,000 cells/mm³, with counts above 100,000 cells/mm³ being highly suggestive of infection 1.
- The percentage of polymorphonuclear cells (PMNs) is also important, with greater than 75% PMNs supporting an infectious etiology.
- Patients with inflammatory arthritis (like rheumatoid arthritis) or crystal arthropathies may have elevated cell counts in the 10,000-50,000 cells/mm³ range, creating some diagnostic overlap.
- Immunocompromised patients may present with lower cell counts despite having septic arthritis.
Diagnostic Approach
When evaluating joint fluid, other parameters should also be considered alongside cell count, including:
- Culture results
- Glucose levels (typically decreased in infection)
- Protein levels
- The presence of crystals Prompt joint aspiration and analysis are essential when septic arthritis is suspected, as delayed treatment can lead to significant joint destruction and systemic complications 1.
From the Research
Cell Count in Septic Arthritis
- A synovial fluid cell count of 50,000 cells/mm3 has been used as a diagnostic cutoff for acute septic arthritis, although data supporting this is lacking 2.
- Studies have shown that a cell count of > 50,000 cells/mm3 may be associated with septic arthritis, especially in the setting of crystalline arthropathy 3, 4.
- However, some studies have found that this cutoff may not be reliable in all cases, such as in immunosuppressed patients 5.
- A higher cell count cutoff of 85,000/mm3 may be more specific for diagnosing septic arthritis in the presence of crystalline arthropathy 6.
- The optimal diagnostic threshold for synovial WBC in the setting of crystalline arthropathy may be 50,000/mm3, with a sensitivity of 91.7% and specificity of 90.1% 4.
Key Findings
- A cell count of > 50,000 cells/mm3 is associated with an increased risk of septic arthritis in patients with crystalline arthropathy 3, 4.
- The presence of crystals does not rule out septic arthritis, and a high cell count should prompt further evaluation 3, 6.
- Immunosuppressed patients may have a lower cell count in cases of septic arthritis, making diagnosis more challenging 5.
- The percentage of polymorphonuclear cells (PMNs) may also be useful in diagnosing septic arthritis, with a threshold of 64% showing high sensitivity but low specificity 4.