Neutrophil Traps in the Blood Show Strong Promise for Diagnosing Pneumonia and Spotting Mixed Infections

When bacteria or viruses invade the lungs, the immune system responds with a weapon that is as dramatic as it is deadly to pathogens: neutrophil extracellular traps, or NETs. These are web-like lattices of DNA studded with antimicrobial proteins, expelled by neutrophils in a programmed form of cellular self-sacrifice called NETosis. The neutrophil unspools its own chromatin, decorates it with enzymes such as neutrophil elastase and myeloperoxidase, and casts the resulting mesh into the surrounding tissue to ensnare and kill microbes. A new study published in BMC Infectious Diseases by Hui Zhang, Xiaoshu Zhang, Jing Li and colleagues in Lanzhou, China, now provides some of the most detailed evidence yet that the abundance of these traps in a patient’s blood can serve as a diagnostic fingerprint for community-acquired pneumonia, and, strikingly, may help clinicians identify cases in which more than one pathogen is attacking at once.

Community-acquired pneumonia, or CAP, remains one of the most common and consequential infectious diseases worldwide. It is defined as pneumonia acquired outside of hospitals or long-term care facilities, and it presents clinicians with a persistent diagnostic dilemma: the symptoms, fever, cough, sputum production and breathlessness, overlap heavily with other respiratory conditions, and the causative pathogen is often never identified. Standard laboratory markers such as C-reactive protein (CRP), procalcitonin and the erythrocyte sedimentation rate are informative but imperfect, offering only coarse indications of inflammation rather than specific insight into the nature of the infection. The Chinese team set out to test whether NETs, as a direct readout of neutrophil activation by pathogens, could fill part of that gap.

The study enrolled 222 patients with community-acquired pneumonia admitted to a tertiary hospital in Lanzhou between January 2021 and December 2022. From each patient the researchers collected paired samples: peripheral blood serum and bronchoalveolar lavage fluid, or BALF, the liquid retrieved from the deep airways after it has been washed across the alveolar surfaces where the infection resides. As a baseline, they also gathered serum from 100 age- and gender-matched healthy volunteers. NETs levels in every specimen were quantified using a double-antibody sandwich enzyme-linked immunosorbent assay, an ELISA format in which capture antibodies immobilize NET-associated antigens from the sample and detection antibodies generate a measurable signal proportional to the amount of trap material present. Statistical analysis was performed with SPSS 25.0, with Pearson correlation used to relate NETs concentrations to routine laboratory indicators and receiver operating characteristic (ROC) curves used to evaluate diagnostic performance.

The first and most fundamental finding was that NETs are markedly elevated in pneumonia. Serum NETs levels in CAP patients were significantly higher than in healthy controls, and within the patients themselves, serum concentrations were substantially greater than the paired BALF concentrations, a difference that held at the stringent threshold of P < 0.001. This pattern is biologically interesting: it suggests that NETosis in pneumonia is not confined to the local lung environment but spills over into systemic circulation, where trap components or their degradation products become detectable in the blood. That spillover is precisely what makes serum NETs attractive as a clinical biomarker, because a blood draw is far simpler, faster and less invasive than bronchoscopy with lavage.

The researchers then stratified patients by whether a pathogen had been detected. Patients in whom a pathogen was identified showed elevated NETs levels in both serum and BALF compared with patients in whom no pathogen was found, with P values below 0.05. This supports the idea that NETs release is not merely a nonspecific consequence of lung inflammation but is actively driven by microbial invasion. When the analysis turned to the type of pathogen, a nuanced picture emerged. NETs levels did not differ significantly between patients with single bacterial infections and those with single viral infections, indicating that both major classes of respiratory pathogens provoke comparable NETotic responses as measured by this assay. However, the mixed-infection group, patients harboring both bacterial and viral pathogens simultaneously, exhibited significantly higher NETs levels in both serum and BALF than either single-infection group.

This last result may prove to be the study’s most clinically valuable contribution. Mixed infections in pneumonia are notoriously difficult to recognize at the bedside, yet they carry important therapeutic implications: a patient co-infected with influenza and Streptococcus pneumoniae, for example, may need both antiviral and antibacterial coverage, and missing one component can be fatal. If a simple serum measurement could flag the likelihood of a mixed infection, it could guide empiric therapy decisions, prompt more aggressive microbiological workup, and help steward antibiotics more rationally. The finding that NETs are amplified specifically in mixed infections, rather than merely in bacterial ones, suggests the biomarker reflects the total burden of pathogen-driven neutrophil activation rather than the identity of any single pathogen.

The correlation analysis reinforced the mechanistic coherence of the findings. Serum and BALF NETs levels were strongly correlated with the peripheral neutrophil count, which is expected since neutrophils are the source of the traps, and with fibrin degradation products (FDP), a marker of fibrin turnover. The link to FDP is notable because NETosis and coagulation are deeply intertwined: NETs provide scaffolding for thrombus formation, activate platelets, and interact with the coagulation cascade, while fibrinolytic products may in turn reflect the inflammatory meshwork that NETs help create. NETs levels were moderately correlated with monocyte count and only weakly correlated with CRP, suggesting that NETs capture a dimension of the immune response, granulocyte-driven and coagulation-linked, that conventional inflammatory markers only partially overlap.

For diagnostic performance, the ROC analysis delivered the headline numbers. Serum NETs yielded an area under the curve (AUC) of 0.8293 for distinguishing CAP patients from healthy controls, with an optimal cut-off value of 362.2. At that threshold, the assay achieved a sensitivity of 63.96 percent but a specificity of 98.00 percent. In practical terms, this profile describes a rule-in test: a positive result above the cut-off makes pneumonia highly likely, even though a negative result does not exclude it. High specificity is a prized property in emergency departments, where distinguishing true bacterial or viral pneumonia from heart failure, pulmonary embolism or other mimics can change management immediately. Used alongside clinical assessment and existing markers, a highly specific NETs measurement could add genuine discriminative value.

The authors conclude that community-acquired pneumonia induces significant NETs release, that pathogen invasion further upregulates NETs expression, and that this upregulation is prominently amplified in mixed-infection conditions. They position serum NETs as reliable biomarkers for the auxiliary diagnosis of CAP and for the identification of mixed infection, with favorable prospects for clinical application. The study was approved by the Ethics Committee of Gansu Provincial Center for Disease Control and Prevention under approval number 013 of 2019, and all participants provided informed consent. The work received support from the National Natural Science Foundation of China and several Gansu provincial funding programs, and it was conducted with technical assistance from the Virus Laboratory of the Gansu Provincial Center for Disease Control and Prevention and the Department of Clinical Laboratory Center of Lanzhou University Second Hospital.

As with any biomarker study, the path from these findings to routine clinical use will require further validation in independent and more diverse cohorts, standardization of the ELISA platform across laboratories, and clarification of how NETs measurements perform in patients with chronic inflammatory diseases, autoimmune conditions or other states of neutrophil activation that could confound specificity. The study was published as an open-access article under a Creative Commons Attribution 4.0 license, with the accepted manuscript released early for rapid access and citable under its permanent DOI. Even so, the conceptual advance is clear: a century-old view of neutrophils as simple phagocytes has given way to an appreciation of them as engineers of extracellular scaffolds, and those scaffolds, it turns out, leave measurable traces in a vial of blood that speak volumes about what is happening deep in the lungs. For a disease as common, costly and diagnostically slippery as community-acquired pneumonia, that is a signal worth pursuing.

Subject of Research: Neutrophil extracellular traps as diagnostic biomarkers for community-acquired pneumonia and mixed respiratory infections

Article Title: Diagnostic value of neutrophil extracellular traps in community-acquired pneumonia and their potential in identifying mixed infection

Article References: Zhang, H., Yang, X., Zhou, J., Wang, B., Wang, M., Zhang, X., & Li, J. (2026). Diagnostic value of neutrophil extracellular traps in community-acquired pneumonia and their potential in identifying mixed infection. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14390-4

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14390-4

Keywords: neutrophil extracellular traps, community-acquired pneumonia, biomarkers, mixed infection, ELISA, serum diagnostics, neutrophils, bronchoalveolar lavage fluid, ROC analysis, respiratory pathogens, C-reactive protein, fibrin degradation products