January 2023

Authors: Rob Hirst, Andy Neill, Dave McCreary, Chris Connolly, Becky Maxwell / Codes: / Published: 09/01/2023


– Andy Neill
– Dave McCreary

Clinical Question

– can the ratio of neuts to lymphs in the synovial fluid clinich the diagnosis of septic arthritis

Paper title

– Synovial Fluid and Serum Neutrophil-to-Lymphocyte Ratio


– Varady, 2022, JBJS


– Septic arthritis is a fairly common clinical question. Knees being the classic example where you dont know if its OA or gout (more common) or something nasty like septic arthritis. Growing a bug or seeing it on gram stain is our gold standard but this is not a test compatible with an ED time line. As a surrogate we often use the total WCC in the fluid. If >50k then its probably septic. As you can imagine this is somewhat arbitrary and will fail both in sensitivity and specificity.
– From an ED point of view we are generally interested in sensitivity – ie we dont want to miss any cases but are less bothered if we admit someone who turns out to have gout. The authors here are orthos and more interested in making the diagnosis which generally requires a wash out from them.
– this paper looks at the ratio of neuts to lymphs as an indicator of infection. Bacterial infection tends to cause a predominance in neuts and has been looked at in serum for lots of other things to predict bacterial infection. Here these guys look at its use in synovial fluid to predict infection


– this was a chart review of 5 US hospitals with less than stellar methods i would say
– included all patients who had synovial fluid taken for potential septic arthritis. I suspect this might be quite tricky to identify as lots of fluid is taken for other reasons. So it’s not entirely clear how they made the distinction
– in addition in a sub set with confirmed septic arthtitis they looked at the blood NLR
– reference standard was a positive bacterial culture as expected
– 50K whites and 90% neuts was the comparator
– NLR cut off was decided on by a Youden index which is effectively looking at the best point on the ROC to estabilish the optimal combo of sens and specificity


– 600 pts
– AUC 0.85 vs 0.8 for the WCC
– translates to 80/80 sens/spec which isn’t awful (though certainly not a rule out)
– the ideal synovial NLR was 25.
– 230 pts had septic arthtitis; of these the serum NLR had an AUC of 0.83. Here the cut off for NLR was ~10
– they have nice little tables showing how you can trade sens for spec by changing the cut off


– worth doing routinely but I don’t think the numbers sufficient to rule out completely. Will need addition of gram stain and risk factors and clinical exam.
– often it will be worth treating till culture

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