Neutrophil-to-lymphocyte ratio as a low-cost triage marker for malaria in febrile patients in conflict-affected Southern Yemen

In many fragile and conflict-affected settings, access to reliable laboratory diagnostics for febrile illnesses remains limited. Malaria and other infectious diseases continue to impose a major burden in Yemen. This study evaluated the diagnostic performance of routinely available complete blood count (CBC)–derived indices, particularly the neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR), as potential low-cost surrogate markers for infection in resource-constrained healthcare settings. A multicenter cross-sectional diagnostic accuracy study was conducted in primary and secondary healthcare facilities in two conflict-affected governorates in southern Yemen (Al-Dhalea and Lahej) between January and December 2025. A total of 700 consecutive patients aged ≥ 5 years for whom a complete blood count was clinically indicated (including those presenting with fever, fatigue, or general malaise) were enrolled. Patients with chronic hematological disorders, active malignancy, recent blood transfusion, pregnancy, or critical illness were excluded. Diagnostic performance of NLR and PLR was evaluated using receiver operating characteristic (ROC) analysis. Multivariable logistic regression was used to assess their independent predictive value. Among 700 participants (mean age 34.5 ± 14.2 years; 52% female), 217 (31%) had confirmed malaria. Median NLR was significantly higher in malaria infected compared with non-infected individuals (4.1 [IQR 3.2–5.3] vs. 1.6 [IQR 1.2–2.1], p < 0.001). NLR showed good diagnostic accuracy (AUC = 0.81, 95% CI 0.77–0.85). An optimal cut-off value of NLR ≥ 3.0 yielded 82% sensitivity and 76% specificity. PLR also demonstrated moderate diagnostic performance (AUC = 0.74, 95% CI 0.70–0.78). In multivariable analysis, NLR remained a strong independent predictor of malaria (adjusted OR = 5.6, 95% CI 3.9–8.1), followed by PLR (adjusted OR = 3.4, 95% CI 2.5–4.7). CBC-derived inflammatory indices, particularly NLR and to a lesser extent PLR, may serve as practical and low-cost triage markers for detecting malaria in conflict-affected and resource-limited settings. The NLR threshold of ≥ 3.0 showed good diagnostic accuracy and could be incorporated into clinical triage algorithms to support early identification of malaria and strengthen diagnostic capacity in peripheral health systems in fragile settings.