Immune Blood Markers Offer Rapid Roadmap to Identify Infection Cause and Severity

When a patient arrives at a hospital with a fever, one of the most urgent questions facing clinicians is deceptively simple: what is causing the infection? Bacteria, viruses, and fungi can all produce overlapping symptoms, yet each demands a different therapeutic response. Conventional bacterial culture, the long-standing gold standard for identifying bacterial pathogens, typically requires days to yield results and performs poorly in the early stages of infection, when microbial loads may be too low to detect. That delay forces physicians into a difficult position—either wait for definitive answers while the patient deteriorates, or prescribe broad-spectrum antibiotics empirically, a practice that fuels antimicrobial resistance. A new study published in BMC Infectious Diseases by researchers at Sun Yat-sen Memorial Hospital, Sun Yat-sen University, in Guangzhou, China, proposes a way around this bottleneck: a hierarchical diagnostic strategy built entirely from host immune markers that can already be measured in most hospital laboratories.

The research team, led by corresponding author Ying Xu of the Department of Clinical Laboratory, enrolled febrile patients admitted to the hospital between 2021 and 2023. From this population, 457 eligible patients were classified into three etiological groups—bacterial, viral, or fungal infection—based on final clinical diagnoses corroborated by microbiological results. Rather than hunting for the pathogen itself, the investigators turned the diagnostic lens inward, asking whether the patient’s own immune response could reveal the identity of the invader. The rationale rests on a well-established biological principle: different classes of pathogens trigger distinct patterns of cytokine signaling, the chemical messengers through which immune cells coordinate their defense.

The biomarker panel the team evaluated combined six cytokines—interleukin-2 receptor (IL2R), interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β)—with three routine inflammatory markers: procalcitonin (PCT), C-reactive protein (CRP), and the neutrophil-to-lymphocyte ratio (NLR). Each component carries distinct immunological meaning. IL-6 is an early-response cytokine that drives the acute-phase reaction and is often markedly elevated in bacterial infections. IL-10, an anti-inflammatory cytokine, rises when the immune system attempts to restrain excessive inflammation, and the balance between the two—captured by the IL-6/IL-10 ratio—reflects the net inflammatory tone of the host. IL-8 is a chemokine that recruits neutrophils to sites of infection, while PCT, a prohormone of calcitonin, is released by extra-thyroid tissues under bacterial endotoxin stimulation and has long been regarded as one of the more specific markers of bacterial insult. The NLR, a simple calculation from a standard complete blood count, indexes the relative dominance of innate, neutrophil-driven immunity over adaptive, lymphocyte-driven immunity.

To translate these measurements into clinically usable decision rules, the researchers performed receiver operating characteristic (ROC) curve analysis, a statistical technique that plots diagnostic sensitivity against the false-positive rate across all possible cut-off values. The area under the ROC curve (AUC) quantifies overall discriminative ability, with 0.5 indicating performance no better than chance and 1.0 representing perfect classification. From these analyses, the team derived optimal threshold values for each marker and then combined them into multivariable models tailored to specific diagnostic questions. The result is not a single test but a sequential pathway: first determine whether the infection is bacterial or viral, then, if bacterial, determine whether the pathogen is Gram-negative or Gram-positive, and finally assess whether the bacterial infection has progressed to sepsis.

The first step of this hierarchy addresses the most common and consequential dilemma in febrile patients—separating bacterial from viral disease. A model combining an IL-6/IL-10 ratio greater than 1.13, IL-8 above 28.70 picograms per milliliter, and an NLR above 2.96 achieved an AUC of 0.80 (95% confidence interval: 0.74–0.87) for this distinction. In practical terms, an AUC of 0.80 indicates good discriminative ability, meaning the model correctly ranks a randomly chosen bacterial case above a randomly chosen viral case about four times out of five. The logic behind the chosen variables is coherent: a high IL-6/IL-10 ratio signals unopposed pro-inflammatory activity typical of bacterial stimulation, elevated IL-8 reflects intense neutrophil recruitment, and a high NLR confirms the neutrophil-dominant blood picture that accompanies acute bacterial infection. Viral infections, by contrast, tend to provoke lymphocyte-dominated responses with comparatively restrained IL-6 and IL-8 production.

The second step of the pathway tackles a subtler question: once a bacterial infection is confirmed, is the culprit Gram-negative or Gram-positive? This distinction matters because the two bacterial classes differ in cell-wall structure, antibiotic susceptibility profiles, and clinical behavior. The study found that no single universal model sufficed; instead, site-specific models were required, reflecting the fact that the immune signature of a bacterial infection is shaped by both the pathogen’s Gram status and the anatomical location of the infection. Notably, in bloodstream infections, a model incorporating IL-6 above 44.91 picograms per milliliter and PCT above 2.30 nanograms per milliliter yielded an AUC of 0.82 (95% confidence interval: 0.70–0.93). The pairing is immunologically sensible—Gram-negative organisms release endotoxin, a potent inducer of both IL-6 and PCT, so elevated values of these two markers together point toward a Gram-negative etiology.

The third and final step addresses severity stratification: among patients with confirmed bacterial infection, which ones have developed sepsis, the life-threatening dysregulated response that demands immediate aggressive intervention? Here the combination of IL-8 above 72.45 picograms per milliliter and PCT above 1.60 nanograms per milliliter achieved an AUC of 0.80 (95% confidence interval: 0.72–0.89). Very high IL-8 concentrations likely reflect the massive neutrophil activation and endothelial inflammation that characterize septic physiology, while elevated PCT tracks the systemic bacterial burden. Together, the three steps of the pathway achieved AUC values of 0.80 to 0.82 across the key diagnostic decisions—a consistent level of performance that, while not replacing microbiological confirmation, is sufficient to guide early therapeutic choices in the critical first hours of illness.

The practical appeal of this strategy lies in its accessibility. Every marker in the panel—cytokines, PCT, CRP, and the NLR—can be measured with routine laboratory instrumentation, and many are already part of standard workups for febrile patients. The study’s authors emphasize that the approach constitutes a cost-effective, rapid decision-support tool designed to facilitate early clinical decisions and antibiotic stewardship, with particular relevance for medical institutions that lack access to expensive molecular diagnostics such as multiplex PCR panels or next-generation sequencing. In resource-limited settings, where blood cultures may take days or be unavailable altogether and where antibiotic misuse is often most severe, a tiered pathway built from existing assays could meaningfully shorten the time to targeted therapy. The work was supported by the National Natural Science Foundation of China and by Science and Technology Projects funding from Guangzhou.

As with any biomarker study, the findings come with caveats that will shape future validation. The analysis was retrospective, drawing on existing medical records from a single center, and the classification of patients depended on final clinical diagnoses, which themselves carry inherent uncertainty. Cut-off values derived from one population may require recalibration in others, and the confidence intervals around some models—particularly the Gram-negative versus Gram-positive bloodstream infection model—indicate a degree of statistical uncertainty that larger cohorts could narrow. Nonetheless, the study demonstrates a compelling concept: that the immune system’s own chemical conversation, read through a carefully chosen panel of cytokines and routine markers, can be decoded into a structured diagnostic pathway. By sequencing the questions—etiology, pathogen subtype, severity—the strategy converts a scatter of laboratory values into an actionable clinical narrative, offering a template for how host-response diagnostics might complement, and in some cases temporarily stand in for, the slower tools of classical microbiology.

Subject of Research: Host immune response biomarkers for rapid differentiation of bacterial, viral, and fungal infections

Article Title: A rapid diagnostic strategy of pathogen infection using host markers of immune response

Article References: A rapid diagnostic strategy of pathogen infection using host markers of immune response. (n.d.). https://doi.org/10.1186/s12879-026-14518-6

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14518-6

Keywords: biomarkers, cytokines, interleukin-6, interleukin-8, procalcitonin, neutrophil-to-lymphocyte ratio, bacterial infection, viral infection, sepsis, antibiotic stewardship, diagnostic markers, ROC analysis