Conference Coverage

ACR: Don’t be fooled by contaminated synovial fluid




SAN FRANCISCO – Hold off on surgery in patients with presumed septic arthritis if they’re not otherwise too sick and their cultures don’t grow out a pathogenic organism within 48 hours.

The reason is because those patients are likely to have synovial fluid that was contaminated during collection, not a true joint infection.

The advice comes from investigators at Beth Israel Deaconess Medical Center, Boston, who compared 425 monoarticular septic arthritis cases with 25 cases that turned out to be false positives due to synovial fluid contamination; most of the false positives got antibiotics, and three (12%) had joint operations that they did not need.

Dr. Robert Shmerling

Dr. Robert Shmerling

“Rushing off to the operating room isn’t” always warranted. “You can suspect contamination if patients have milder disease manifestations and cultures grow late,” said investigator Dr. Robert H. Shmerling, clinical chief of Beth Israel’s division of rheumatology.

The findings help determine when – and when not – to be aggressive with patients who present with what looks to be septic arthritis. “No one’s ever really looked at this before,” he said at the annual meeting of the American College of Rheumatology.

“These are very different sorts of patients. Look at the full range of clinical characteristics and lab values, not just the synovial fluid tap. If contamination is suspected, you can wait until the cultures come back or possibly do serial taps before going to the operating room,” said coinvestigator Clara Zhu, a medical student at Boston University.

Patients with true joint infections had higher mean peripheral polymorphonuclear neutrophil percentages (78% vs. 68% in false positives) and synovial fluid polymorphonuclear cell percentages (88% vs. 74% in false positives). True cases also had substantially higher mean synovial fluid white blood cell counts (88,000 vs. 29,000).

Clara Zhu

Clara Zhu

Unlike true cases, contaminated synovial fluid took about 4 days to grow out a positive culture, and the most common organisms by far were coagulase-negative staphylococci, typically normal skin bacteria.

Patients with contaminated fluid also tended to be older (71 vs. 59 years), with fewer prior admissions. They were far less likely to have had recent joint procedures and histories of septic arthritis but were more likely to have synovial fluid crystals, as in gout. False positives also left the hospital sooner (7 vs. 11 days) and were less likely to be readmitted within 2 months. They were also less likely to present with fever (19% vs. 37%) but not significantly so.

This “study suggests that contaminated synovial fluid is found in up to 6% of patients with suspected septic arthritis and positive synovial fluid or synovial biopsy cultures. We recommend a conservative approach for patients with ... mild disease manifestations and no growth of pathogenic organisms within the first 48 hours,” the investigators concluded.

The authors have no disclosures, and there was no outside funding for the work.

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