Monocyte Distribution Width (MDW) as an Early Biomarker of Blood Culture Positivity and Clinically Significant Bloodstream Infection in Emergency Department: A Real-World Study

Background: Monocyte Distribution Width (MDW) is a hematological parameter automatically generated during routine blood count analysis and currently represents the only blood count-derived biomarker for early sepsis risk assessment in adult emergency department (ED) patients. Evidence regarding its ability to identify blood culture (BC) positivity and bloodstream infection (BSI) remains limited. This cohort study aimed to evaluate its diagnostic accuracy. Methods: Adult patients presenting to the ED of a tertiary hospital with suspected infection and BC request were evaluated. Between March 2022 and December 2024, MDW, C-reactive protein (CRP), and BC results were retrospectively collected from the laboratory information system. Samples were categorized as BC-negative or BC-positive and further stratified into microbiological categories. Diagnostic performance was assessed using ROC curve analysis and decision curve analysis (DCA). Analyses were performed both including and excluding CoNS. Results: A total of 2,748 samples were available. MDW values were significantly higher in BC-positive than in BC-negative samples (25.8±6.7 vs. 23.4±4.7) and were highest in Gram-negative infections (27.7±7.5). When CoNS were excluded, MDW achieved an AUC of 0.65 at the optimal cut-off of 24.5, showing a sensitivity of 58% and specificity of 66.2%. Rule-out thresholds of 17.7 and 18.7 achieved sensitivities of 95% and 92.9%, respectively, whereas rule-in thresholds of 29.4 and 32 achieved specificities of 90.0% and 95.2%. MDW consistently outperformed CRP and demonstrated positive clinical net benefit in DCA. The addition of CRP did not provide improvement in diagnostic accuracy (AUC 0.65). Conclusions: MDW is associated with BC positivity and clinically significant BSI and outperforms CRP in identifying patients with BC-positive in the ED supporting its integration into diagnostic stewardship strategies and early risk stratification pathways.