Joint sampling,cytology and septic arthritis

For suspected polyarthritis, sample at least 4 joints. Keep going until you have 3 good cytology samples from different joints. Submit synovial fluid in a blood culture bottle if infection is suspected. A synovial biopsy may help when fluid volume is too small.

OA usually has fewer cells, with mononuclear cells predominating. Rheumatoid arthritis and nonerosive immune-mediated polyarthritis (IMPA) are more inflammatory. Septic fluid often has many neutrophils, but these patterns overlap. Cytology alone cannot separate IMPA from infection.

The historical 4.2% lateral-suture and 8.4% TPLO figures describe infection–inflammation within 6 months: 21/496 and 34/406 procedures in 808 dogs. They are broader than confirmed joint sepsis, and are not current universal risks.

Reported canine isolates include S. intermedius (a historical species label), S. aureus and beta-haemolytic streptococci. Cat bites commonly introduce Pasteurella multocida or Bacteroides.

A total cell count >50×10⁹/L with >40% neutrophils raises suspicion. Intracellular bacteria confirm infection, but a negative culture does not exclude it. Interpret the clinical picture, cytology and culture together.

Systemic antibiotics are central to treatment. The cited textbook recommendation is at least 4 weeks, then repeat arthrocentesis. Continue treatment if cytology has not returned to normal or an OA pattern. The note’s <3% neutrophil cue is a reassessment target, not a stop rule by itself. Individualise lavage or surgery, especially with a penetrating wound, gross contamination, foreign material or an infected implant. Routine surgery in their absence remains debated.

References

Johnston & Tobias, Veterinary Surgery: Small Animal, 2nd ed., ch. 68. Frey et al., JAVMA 2010; 236: 88–94, doi: 10.2460/javma.236.1.88. Clements et al., JSAP 2005; 46: 171–176, doi: 10.1111/j.1748-5827.2005.tb00307.x.