From the Research
In septic arthritis, the typical cell count on gram stain of synovial fluid is greater than 50,000 white blood cells (WBCs) per cubic millimeter, often reaching 100,000 WBCs/mm³ or higher, as supported by the most recent and highest quality study 1.
Key Findings
- A synovial fluid WBC count above 50,000/mm³ is highly suggestive of bacterial infection, though counts between 20,000-50,000/mm³ can also be seen in septic arthritis.
- The percentage of polymorphonuclear neutrophils (PMNs) is typically greater than 75%, reflecting the acute inflammatory response to bacterial invasion.
- While gram stain positivity varies (only 50-75% of cases show visible organisms), the high cell count remains a critical diagnostic feature.
- Lower cell counts may be seen in patients who are immunocompromised, have received prior antibiotics, or have infections with less virulent organisms.
Diagnostic Considerations
- When evaluating synovial fluid, other parameters should also be assessed, including:
- Turbidity (typically purulent in septic arthritis)
- Protein levels (elevated)
- Glucose (decreased compared to serum)
- Culture results, which remain the gold standard for definitive diagnosis despite taking longer to obtain.
- The sensitivity and specificity of gram stain results should be considered, with a study finding a sensitivity of 22% and specificity of 99.6% 2.
Clinical Implications
- Prompt diagnosis and treatment of septic arthritis are crucial to prevent significant morbidity and mortality, with the best results obtained when treatment is instituted within 1 week of the onset of symptoms 3.
- A low threshold for overlooking a negative gram stain result and placing greater emphasis on clinical findings in conjunction with biochemical markers is recommended 2.