Neutrophil-to-lymphocyte ratio emerges as a practical malaria triage tool in southern Yemen
In conflict-affected southern Yemen, where fever can mean malaria, bacterial infection, or a mixed infectious picture and laboratory confirmation is often delayed or unavailable, routine blood counts may carry more clinical weight than they do in better-resourced settings. A multicenter diagnostic accuracy study from Al-Dhalea and Lahej tested whether neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR) could serve as low-cost surrogates for malaria detection in febrile patients. The answer was not subtle: NLR separated malaria-positive from malaria-negative patients with meaningful accuracy, while PLR added a smaller but still measurable signal.
The study enrolled 700 consecutive patients aged 5 years and older across primary and secondary facilities between January and December 2025. Every participant had a complete blood count ordered for routine clinical reasons such as fever, fatigue, or malaise. Patients with chronic hematologic disorders, active malignancy, recent transfusion, pregnancy, or critical illness were excluded, narrowing the analysis to a group more representative of day-to-day triage in peripheral clinics. Among the cohort, 217 patients, or 31%, had confirmed malaria.
A simple blood count carried a strong inflammatory signal
Median NLR was 4.1 in malaria-infected patients, compared with 1.6 in those without malaria. The gap was large enough to translate into real diagnostic separation rather than a marginal statistical difference. By receiver operating characteristic analysis, NLR achieved an area under the curve of 0.81, with a 95% confidence interval of 0.77 to 0.85. An NLR threshold of at least 3.0 produced 82% sensitivity and 76% specificity.
One sentence captures the practical consequence. A routinely available index on a standard complete blood count identified most malaria cases while keeping false positives at a level that could still be managed in a triage setting.
PLR also rose in infected patients and reached moderate diagnostic performance, with an area under the curve of 0.74 (95% CI 0.70–0.78). It was useful, but less compelling than NLR. In multivariable analysis, NLR remained a strong independent predictor of malaria with an adjusted odds ratio of 5.6 (95% CI 3.9–8.1), and PLR also retained independent predictive value with an adjusted odds ratio of 3.4 (95% CI 2.5–4.7).
Why malaria changes these blood indices
The biologic logic is straightforward. Malaria triggers systemic inflammation, with shifts in circulating leukocyte populations that can raise neutrophil predominance relative to lymphocytes. Platelets may also fall or fluctuate during malaria, which helps explain why platelet-derived ratios can move in the same direction as inflammatory burden. In settings where microscopy, rapid diagnostic tests, or molecular confirmation are inconsistently available, those changes become clinically useful because they are already embedded in a test many clinics can order.
The value of this approach lies in the ratio itself, not in any single cell count. NLR converts two ordinary laboratory values into a more stable inflammatory marker, one that may be less vulnerable to local variation than a stand-alone white cell measure. For frontline clinicians facing a febrile patient in a crowded clinic, that matters because the decision often has to be made before confirmatory testing can be obtained or while supplies are running low.
Conflict settings create a different diagnostic threshold
In high-income hospitals, malaria triage without definitive testing would be an imperfect substitute for laboratory confirmation. In southern Yemen, the threshold for using a surrogate marker is different because access constraints alter the bedside calculus. The study population came from conflict-affected governorates, where diagnostic capacity is constrained and febrile illness is common. Under those conditions, even a moderately accurate marker can improve prioritization: who gets tested first, who needs treatment sooner, and who should be observed more closely while results are pending.
The patient mix also gives the findings clinical credibility. The cohort included both children and adults, with a mean age of 34.5 years and 52% female participants. That broad age spread suggests the marker was not limited to a narrow demographic slice. It was evaluated in consecutive patients, which reduces the risk that the signal was driven by selective enrollment of obvious malaria cases.
Where NLR fits into real-world triage
The most plausible use case is not replacing malaria diagnostics but supplementing them. A threshold of NLR ≥ 3.0 could help flag patients who deserve urgent parasitologic testing or empiric attention when laboratory access is delayed. It may also help peripheral clinicians identify those whose fever is more likely to reflect malaria rather than an alternative inflammatory syndrome, especially where clinical examination alone cannot distinguish among common febrile illnesses.
PLR may serve as a secondary marker, but the performance difference between the two ratios matters. NLR performed better, reached a more compelling discriminatory range, and retained stronger independent association in multivariable models. In practical terms, that makes NLR the more attractive candidate for incorporation into triage algorithms.
The study does not erase the need for validated malaria diagnostics, and it does not claim that NLR can stand alone as a definitive test. It does show that when the clinical system is strained, a routinely generated CBC index can extract useful information from a test already being done. In fragile health systems, that can be enough to change the sequence of care, even if it does not change the diagnostic destination.
The limits are built into the setting
This was a cross-sectional diagnostic study, so it captures a single moment in the clinical course rather than longitudinal changes in NLR or PLR after treatment. Patients with critical illness, pregnancy, active malignancy, recent transfusion, or chronic hematologic disease were excluded, which makes the sample cleaner for diagnostic analysis but also narrows the scope of generalization. The study also reflects two governorates in southern Yemen; performance may differ where malaria species mix, coinfections, treatment access, or baseline inflammatory patterns are not the same.
Even with those limits, the central message is hard to ignore. In a region where malaria remains a major burden and diagnostic infrastructure is strained, NLR offered a low-cost, readily available marker with clinically useful discrimination. The next step is not speculation about laboratory sophistication. It is determining how well this threshold performs inside actual triage workflows, where time, equipment, and staffing are all in short supply.