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How helpful are the synovial fluid analysis and culture in nongonococcal septic arthritis?
Arthrocentesis with demonstration of the bacteria on Gram stain or culture establishes the diagnosis of septic arthritis. Of the tests that can be done on the synovial fluid, culture, Gram stain, and leukocyte (white blood cell [WBC]) counts are the most helpful. Universal polymerase chain reaction (PCR) can be sent to reference labs and when available, can be helpful in identifying organisms in the setting of antibiotic use or when organisms are unable to be grown in culture.
Laboratory Tests of Synovial Fluid in Nongonococcal Septic Arthritis:
| Procedure | Technical Aspects | Diagnostic Yield |
|---|---|---|
| Culture | Plate or inoculate culture bottles immediately | 70%–90% positive in nongonococcal arthritis |
| Gram stain | May increase yield by centrifuging synovial fluid | Sensitivity 30%–50% |
| WBC count | Usually >50,000 cells/mm 3 with neutrophilic predominance | Counts often overlap other inflammatory disease (gout, RA, ReA) |
| Glucose | <50% of serum glucose | Helpful if present |
| Miscellaneous tests | Alpha defensin very sensitive for prosthetic joint infections | NAAT if concerned for Mycoplasma , Ureaplasma , Brucella , Yersinia , or Chlamydia |
NAAT, nucleic acid amplification test; ReA , reactive arthritis.
Pearl: Only 40% to 50% of all patients with septic arthritis have synovial fluid WBC counts of over 100,000 cells/mm 3 . So even if the synovial fluid WBC is not “classic” for septic arthritis, there can still be an infection even with WBC counts less than 50,000 cells/mm 3 especially in immunocompromised and neutropenic patients. Notably, crystal-induced arthritis (gout, pseudogout) can coexist with septic arthritis.
