Diagnostic efficacy of monocyte distribution width to high-sensitivity C-reactive protein ratio in differentiating pathogen types in community-acquired pneumonia: a retrospective cohort study

Objectives Community-acquired pneumonia (CAP) remains a diagnostic challenge in emergency departments, where rapid and accurate pathogen differentiation is essential for appropriate antimicrobial therapy and stewardship. Current biomarkers demonstrate limited accuracy in distinguishing bacterial from viral and atypical pathogens, necessitating novel diagnostic approaches. This study aimed to assess the diagnostic accuracy of the monocyte distribution width to high-sensitivity C-reactive protein ratio (MDW/hs-CRP) for differentiating bacterial, viral, atypical, and mixed community-acquired pneumonia (CAP) pathogens. Methods This retrospective cohort study enrolled 486 adult patients with radiologically confirmed CAP in an emergency department (Jan 2023–Dec 2024). Pathogens were identified using comprehensive microbiological testing. MDW was measured via automated hematology analyzer (Beckman Coulter DxH900); hs-CRP via standard assays. The primary outcome compared the MDW/hs-CRP ratio’s diagnostic accuracy to individual biomarkers using ROC analysis (DeLong test), Youden index (cutoff), decision curve analysis (DCA), and subgroup analyses. Results Pathogen distribution: 198 (40.7 %) bacterial, 142 (29.2 %) viral, 89 (18.3 %) atypical, 57 (11.7 %) mixed. For distinguishing bacterial from viral CAP, the MDW/hs-CRP ratio significantly outperformed MDW alone (AUC 0.752) and hs-CRP alone (AUC 0.798), achieving an AUC of 0.893 (95 % CI: 0.865–0.921; p<0.001). The optimal cutoff (0.185) yielded 85.4 % sensitivity and 82.7 % specificity. Performance remained consistent in subgroups. DCA showed clinical utility across threshold probabilities (15–65 % net benefit). Conclusions The MDW/hs-CRP ratio shows promise as a rapid tool for pathogen differentiation in CAP, potentially aiding early antimicrobial decisions in emergency care. Multicenter validation is needed.