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A Simple New Biomarker for Septic Arthritis

August 9, 2022

Written by Vivian Lei

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Synovial fluid and peripheral blood neutrophil-to-lymphocyte ratios (NLR) are promising novel diagnostic and prognostic biomarkers for septic arthritis (SA).

Why does this matter?
Traditional diagnostic markers of septic arthritis based on synovial fluid white blood cell count (SF-WBC) and percent polymorphonuclear cells (SF-%PMN) have relatively low sensitivity and specificity. Perhaps the ratio of neutrophils to lymphocytes in synovial fluid offers a better diagnostic test?

Diagnosis of septic arthritis by dividing some white cells?
This was a retrospective study of 598 adult patients diagnosed with septic arthritis of the native shoulder, hip, or knee by positive bacterial culture. Synovial fluid neutrophil-to-lymphocyte ratio (SF-NLR) was calculated by dividing the absolute neutrophil count by the absolute lymphocyte count, and researchers compared the diagnostic performance of SF-NLR with established criteria: SF-WBC>50,000 cells/mm3 and SF-%PMN>90%. Diagnostic accuracy for diagnosis of SA was assessed using the area under the ROC curve (AUC). SF-NLR had an AUC of 0.85, which was significantly more accurate than SF-WBC (AUC 0.80) and SF-%PMN (AUC 0.81). The optimal threshold of SF-NLR was found to be 25, with a 78% sensitivity and 81% specificity for SA compared with 56% sensitivity and 80% specificity for SF-WBC of >50,000 cells/mm3, and 65% sensitivity and 78% specificity for SF-%PMN of >90%.

A secondary subgroup analysis of 235 patients with confirmed SA and CBC with differential was performed to assess the prognostic value of peripheral blood NLR, which was found to be independently associated with 90-day treatment failure (OR 7.04) and death (OR 7.33); optimal cut point ≥9.49. In their analysis, elevated peripheral blood WBC (OR 5.76) and serum CRP (OR 2.12) were also associated with 90-day treatment failure.

This study supports the use of this new biomarker in diagnosing septic arthritis, and it can be calculated from data that should already be available following joint arthrocentesis. However, SF-NLR and peripheral NLR require prospective validation before becoming a standard part of our work-up for a hot, swollen joint.

Editor’s note: The title and text refers to “serum NLR.” But serum is, by definition, clotted blood that has been centrifuged, with no cellular elements. So, we have referred to it as peripheral blood CBC with differential and peripheral blood NLR. ~Clay Smith

Source
Synovial Fluid and Serum Neutrophil-to-Lymphocyte Ratio: Novel Biomarkers for the Diagnosis and Prognosis of Native Septic Arthritis in Adults. J Bone Joint Surg Am. 2022 Jun 20. doi: 10.2106/JBJS.21.01279. Online ahead of print.

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