Simple Blood Ratio Could Flag Severe Influenza Before Treatment Begins

Every winter, hospitals face the same diagnostic dilemma: which of the many patients arriving with fever, cough, and muscle aches is actually harboring the influenza virus, and which of those infected will go on to develop severe, potentially life-threatening disease? Antiviral drugs like oseltamivir work best when started early, yet severe cases can be hard to spot at the front door. A new study from China suggests that the answer may already be sitting in a routine complete blood count, in the form of a simple ratio between two cell types that are not usually discussed together: neutrophils and platelets.

The research, published in BMC Infectious Diseases, analyzed real-world clinical data from 1,091 adults diagnosed with influenza at Guangdong Second Provincial General Hospital between March 2016 and December 2023. Led by Seyin Zou of the hospital’s Department of Laboratory Medicine, in collaboration with researchers at Universiti Selangor in Malaysia and Southern Medical University in Guangzhou, the retrospective study set out to determine whether the pre-treatment neutrophil-to-platelet ratio, or NPR, could differentiate severe from non-severe influenza in adults. The answer, according to the team’s statistical models, was a qualified but striking yes.

Neutrophils are the immune system’s first responders, white blood cells that flood sites of infection and tissue damage within hours. Platelets, by contrast, are best known for clotting, but they are also active participants in inflammation, carrying receptors that modulate immune signaling and interacting directly with neutrophils in the bloodstream. When the body mounts a strong inflammatory response, neutrophil numbers surge while platelet counts often fall, consumed by activation, aggregation, and sequestration in the inflamed vasculature. The ratio between the two therefore rises as inflammation intensifies, making it a candidate biomarker for conditions ranging from coronary artery disease to chronic obstructive pulmonary disease and acute respiratory distress syndrome. What has been missing, the authors note, is evidence for its value specifically in severe influenza in Chinese adult populations.

The study’s design was deliberately pragmatic. Rather than enrolling patients in a controlled trial, the team mined eight years of hospital records, applying predefined inclusion and exclusion criteria to identify adults with laboratory-confirmed influenza A or B. Baseline characteristics and laboratory parameters were collected before any treatment began, a critical detail because antiviral therapy and corticosteroids can themselves alter blood cell dynamics. Scatter-dot plots were used to visually compare severe and non-severe groups, Spearman correlation analysis evaluated associations between NPR and other inflammatory indicators, and logistic regression models identified independent predictors of severe disease.

The results revealed meaningful differences between patients with influenza A and influenza B, both in clinical characteristics and in indicator levels. But the most important finding concerned severity. Compared with patients who had non-severe illness, those with severe influenza A or B had significantly higher pre-treatment NPR values. After treatment, NPR decreased markedly in both groups, with statistical significance at P = 0.012 for influenza A and P = 0.028 for influenza B. This trajectory, high before treatment and falling afterward, suggests that NPR is not merely a static snapshot of patient health but a dynamic marker that tracks the inflammatory response as it waxes and wanes.

Perhaps the most mechanistically revealing result came from the correlation analysis. Pre-treatment NPR showed a positive correlation with interleukin-6, or IL-6, levels, with a P value below 0.0001. IL-6 is a central cytokine in the inflammatory cascade, driving fever, acute-phase protein production, and, when dysregulated, the cytokine storm associated with severe viral illness. The link between NPR and IL-6 suggests that the ratio is not an arbitrary hematological curiosity but a proxy for the underlying inflammatory state, one that can be calculated from a standard complete blood count without the cost and delay of cytokine assays.

The logistic regression models went further, identifying pre-treatment NPR as an independent predictor of severe influenza even after accounting for other variables, with P = 0.001 for influenza A and P below 0.0001 for influenza B. A high pre-treatment NPR value was associated with an increased risk of severe disease. In practical terms, this means that a clinician looking at a newly admitted influenza patient’s blood count could calculate the ratio and use it as one piece of evidence in deciding how aggressively to monitor and treat. The study also found that combining pre-treatment NPR with lymphocyte count distinguished healthy individuals from adults with influenza better than either indicator alone, for both influenza A and B, with statistical significance at P below 0.05.

The implications for clinical practice are considerable. Influenza remains a major cause of hospitalization and death worldwide, and the window for effective antiviral intervention is narrow. Biomarkers that can be calculated instantly from routine blood work, without additional cost, could help emergency departments and fever clinics triage patients more effectively. The authors suggest that pre-treatment NPR may serve as a screening and prognostic indicator for severe influenza in adult patients, potentially guiding decisions about hospital admission, antiviral timing, and the intensity of monitoring. Because the ratio is derived from two parameters already measured in every complete blood count, implementation would require no new equipment, reagents, or laboratory workflows.

Still, the study has limitations that temper enthusiasm. It was retrospective, based on a single hospital in Guangdong Province, and relied on real-world data rather than a controlled trial design. The need for informed consent was waived because of the retrospective nature of the study, and the analysis depended on predefined inclusion and exclusion criteria applied to historical records. Whether the findings generalize to other populations, other healthcare systems, or other viral respiratory infections remains to be established. The authors also note that clinical characteristics and indicator levels differed between influenza A and B patients, suggesting that subtype-specific considerations may be necessary when interpreting the ratio.

Nevertheless, the study adds to a growing body of evidence that simple hematological ratios can carry outsized prognostic weight. The neutrophil-to-lymphocyte ratio, or NLR, has already been studied extensively in infections and cancers, and NPR joins a family of derived markers that include plateletcrit, mean platelet volume, and platelet distribution width. What distinguishes the current work is its focus on pre-treatment values, a detail that matters because treatment itself alters blood cell dynamics and can obscure the baseline inflammatory state. By capturing NPR before any intervention, the researchers aimed to isolate the body’s initial response to the virus, unconfounded by therapy. The finding that this baseline value independently predicts severity, and that it correlates with IL-6, offers a mechanistically coherent picture of severe influenza as an inflammatory disease whose trajectory can be glimpsed in routine blood work. As influenza seasons grow more unpredictable and hospital resources remain stretched, the appeal of a zero-cost, instantly calculable severity marker is hard to ignore. The next step, the authors and the field will need to determine, is whether this ratio can be validated prospectively across diverse populations and integrated into clinical decision-making algorithms that improve outcomes for the patients who need it most.

Subject of Research: Pre-treatment neutrophil-to-platelet ratio as a potential indicator for adult severe influenza in China: a retrospective study based on real-world data

Article Title: Pre-treatment neutrophil-to-platelet ratio as a potential indicator for adult severe influenza in China: a retrospective study based on real-world data

Article References: Zou, S., Mohtar, S. H., Othman, R., Hassan, R. M., Chen, C., Yan, J., Xu, S., & Hua, W. (2026). Pre-treatment neutrophil-to-platelet ratio as a potential indicator for adult severe influenza in China: a retrospective study based on real-world data. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14465-2

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14465-2

Keywords: Pre-treatment, neutrophil-to-platelet, ratio, potential, indicator, adult, severe, influenza, China, retrospective, based, real-world