How helpful are the synovial fluid analysis and culture in nongonococcal septic arthritis

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:

ProcedureTechnical AspectsDiagnostic Yield
CulturePlate or inoculate culture bottles immediately70%–90% positive in nongonococcal arthritis
Gram stainMay increase yield by centrifuging synovial fluidSensitivity 30%–50%
WBC countUsually >50,000 cells/mm with neutrophilic predominanceCounts often overlap other inflammatory disease (gout, RA, ReA)
Glucose<50% of serum glucoseHelpful if present
Miscellaneous testsAlpha defensin very sensitive for prosthetic joint infectionsNAAT 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.

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