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Best Tests for Septic Arthritis

October 27, 2020

Written by Clay Smith

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The best predictors of septic arthritis in the knee were synovial WBC and Gram stain. Synovial lactate and PCR performed poorly, and no features of the history or exam were helpful.

Why does this matter?
Septic arthritis (SA) is high on the differential diagnosis for monoarticular arthritis. Prior studies have shown history and inflammatory markers to be inaccurate. Synovial WBC >50,000 increased the probability of SA. Synovial lactate and PCR have also been considered as possible aids in the diagnosis, though each has gotten mixed reviews.

If you suspect septic arthritis, tap the joint.
This was a prospective study that included 71 adults with monoarticular knee arthritis. Prevalence of SA was 7% (prior studies had prevalence rates of 27%). Again, history and physical was unable to rule SA in or out. Synovial L-lactate was inaccurate (D-lactate wasn’t detected at all in 8 samples). PCR was also disturbingly inaccurate. I won’t show the table for history and exam, because no aspects were helpful. But here is the table comparing various tests. The best predictors were synovial WBC and Gram stain.

From cited article

This was a small, single center study with predominately older African American adults, and it only considered monoarticular arthritis of the knee. Only 5 patients had SA. Performance of these markers in a larger, different population may be different.

Diagnostic Accuracy of Synovial Lactate, Polymerase Chain Reaction, or Clinical Examination for Suspected Adult Septic Arthritis. J Emerg Med. 2020 Aug 17:S0736-4679(20)30671-5. doi: 10.1016/j.jemermed.2020.06.068. Online ahead of print.

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