Evaluation of high-fluorescence lymphocyte count as an early diagnostic marker of dengue infection using the Sysmex XN-350 hematology analyzer

Background: Dengue is a major mosquito-borne viral infection prevalent in tropical and subtropical regions. Early diagnosis is essential for appropriate patient management. High-Fluorescence Lymphocyte Count (HFLC), generated by Sysmex XN-Series hematology analyzers, may provide additional information about lymphocyte activation during viral infections. The aim is to evaluate HFLC as an early diagnostic marker of dengue infection using the Sysmex XN-350 hematology analyzer. Methods: This prospective observational case-control study was conducted in the Department of Pathology, GMERS Medical College and Hospital, Gandhinagar, from 1st August 2026 to 31st August 2026. The study included 100 participants, comprising 50 laboratory-confirmed dengue-positive patients and 50 dengue-negative controls. Complete blood count parameters, including hemoglobin, total leukocyte count, platelet count, lymphocyte percentage, and HFLC percentage, were analyzed. Diagnostic performance was evaluated using receiver operating characteristic (ROC) curve analysis. Results: Dengue-positive patients showed significantly lower mean WBC counts (5140±2033.70 vs. 6522±1843.28 cells/µl; p<0.001) and platelet counts (208,340±143,548 vs. 287,680±93,384 cells/µl; p=0.002) than controls. Lymphocyte percentage was significantly higher in dengue-positive patients (40.16±16.93% vs. 29.66±6.06%; p<0.001). Mean HFLC percentage was significantly lower in dengue-positive patients (9.16±7.73% vs. 24.50±14.58%; p<0.001). ROC analysis yielded an AUC of 0.807. At a cutoff of ≤20%, HFLC demonstrated 96.0% sensitivity, 58.0% specificity, 69.6% positive predictive value, and 93.5% negative predictive value. Conclusions: HFLC may serve as a useful adjunctive screening parameter for suspected dengue infection. However, its moderate specificity necessitates interpretation alongside clinical findings, routine hematological parameters, and established dengue-specific diagnostic tests.