The Gold-Standard Marker for Viral Infection — Enabling Viral vs. Bacterial Differentiation at the Point of Care
Myxovirus Resistance Protein A (MxA) is a dynamin-like GTPase induced exclusively by Type I and Type III interferons — the body's direct antiviral response proteins. Unlike CRP or PCT, which rise non-specifically with inflammation or bacterial infection, MxA is selectively and rapidly upregulated in response to active viral infection within hours of onset.
Blood MxA is a proven biomarker for distinguishing viral from bacterial infections at the emergency department. At the validated cutoff of 256 μg/L, MxA achieves AUC 0.81, sensitivity 74.4%, and specificity 80.0% — and when combined with CRP in a MxA/CRP ratio, AUC rises to 0.89. MxA levels remain low in healthy individuals, asymptomatic virus carriers, and patients with bacterial-only infection.
Sekbio offers a validated MxA antibody pair for CLIA/FIA platform integration and an MxA rapid test (LFA) incorporating Sekbio's proprietary whole-blood protein release technology — the key technical breakthrough enabling point-of-care MxA testing from a fingerstick sample.
MxA is the most specific blood biomarker for active viral infection — driven by a well-characterized molecular mechanism and validated across large pediatric and adult clinical cohorts.
MxA is transcribed exclusively in response to Type I (IFN-α/β) and Type III (IFN-λ) interferons — the direct antiviral signaling cascade. It does not respond to bacterial endotoxins, IL-1, or TNF-α, making it uniquely specific to viral activation of the innate immune system.
MxA is not merely a marker — it is an effector protein that inhibits influenza, RSV, parainfluenza, Hantavirus, measles, HBV, CCHFV, and dozens of other RNA and DNA viruses. Its elevation reflects genuine host antiviral response, not nonspecific inflammation.
MxA rises within hours of viral infection onset and is significantly elevated during active symptomatic viral illness. Crucially, asymptomatic virus carriers maintain low MxA levels, allowing MxA to distinguish true infection from incidental virus detection by PCR.
Median MxA in bacterial infection: 119 μg/L. Median MxA in viral infection: 467 μg/L (P < 0.001). This 4× difference creates a clear decision boundary — CRP and PCT alone cannot provide this discrimination, making MxA a complementary essential biomarker.
In a prospective multicenter study of 265 hospitalized children, MxA at 256 μg/L cutoff achieved AUC 0.81 (95% CI 0.73–0.90), sensitivity 74.4%, specificity 80.0%. Combined MxA/CRP ratio: AUC 0.89 (95% CI 0.83–0.96) — enabling practical ED antibiotic decision support.
MxA performance validated across pediatric age groups (4 weeks – 16 years). Children <2 years show higher baseline and higher viral-response MxA levels; age-stratified cutoffs (316–524 μg/L for <2 yr) are under investigation for further precision.
Internal sensitivity evaluation on Sekbio's Chemiluminescence Immunoassay (CLIA) platform. Antibody pair: MxA-Ab-02 (coating) + MxA-Ab-01-AE (tracer).
Seven-point standard series from 0 to 0.2 μg/mL. Signal-to-noise (S/N) is calculated relative to the blank (S1). Analytical sensitivity confirmed at 2 pg/mL (S/N = 1.10).
| Standard | MxA Concentration | RLU (Rep 1) | RLU (Rep 2) | Average RLU | S/N |
|---|---|---|---|---|---|
| S1 (Blank) | 0 pg/mL | 4,832 | 4,989 | 4,911 | 1.00 |
| S2 | 0.2 pg/mL | 4,968 | 5,315 | 5,142 | 1.05 |
| S3 (LoD) | 2 pg/mL | 5,476 | 5,339 | 5,408 | 1.10 |
| S4 | 20 pg/mL | 7,776 | 7,742 | 7,759 | 1.58 |
| S5 | 200 pg/mL | 26,713 | 27,374 | 27,044 | 5.51 |
| S6 | 0.2 ng/mL | 210,101 | 211,025 | 210,101 | 42.79 |
| S7 | 0.2 μg/mL | 1,579,458 | 1,561,745 | 1,570,602 | 319.85 |
| Tracer: MxA-Ab-01-AE at 1:2000 dilution. Coating: MxA-Ab-02. Procedure: 15 μL sample + 30 μL Ra + 30 μL Rd, 10 min incubation at 200 rpm, wash, read. Date: 2022-12-20. | |||||
The antibody pair maintains excellent sensitivity even at higher tracer dilution (1:4000), with analytical LoD of 2 pg/mL (S/N = 1.27) — confirming the robustness of the antibody pair across different assay optimization conditions.
| Standard | MxA Concentration | RLU (Rep 1) | RLU (Rep 2) | Average RLU | S/N |
|---|---|---|---|---|---|
| S1 (Blank) | 0 pg/mL | 2,309 | 2,656 | 2,483 | 1.00 |
| S2 | 0.2 pg/mL | 2,632 | 2,600 | 2,616 | 1.05 |
| S3 (LoD) | 2 pg/mL | 3,003 | 3,298 | 3,151 | 1.27 |
| S4 | 20 pg/mL | 4,338 | 3,915 | 4,127 | 1.66 |
| S5 | 200 pg/mL | 13,665 | 13,036 | 13,351 | 5.38 |
| S6 | 0.2 ng/mL | 103,701 | 105,279 | 104,490 | 42.09 |
| S7 | 0.2 μg/mL | 794,445 | 773,721 | 784,083 | 315.84 |
| Tracer: MxA-Ab-01-AE at 1:4000 dilution. Both dilutions confirm sensitivity at 2 pg/mL. Dynamic range spans 6 orders of magnitude (0.2 pg/mL – 0.2 μg/mL), comfortably covering the entire clinically relevant MxA range (26–1,000+ μg/L in patient blood). | |||||
Published clinical data (Piri et al., Microbiology Spectrum, 2022; n=265 hospitalized children) demonstrating the discriminatory power of blood MxA between viral and bacterial infections.
| Patient Group | n | Median MxA (μg/L) | IQR (μg/L) | vs. Bacterial (P) |
|---|---|---|---|---|
| Viral infection only | 39 | 467 | 235 – 812 | <0.001 |
| Viral-bacterial coinfection | 103 | 469 | 178 – 827 | <0.001 |
| Bacterial infection only | 75 | 119 | 68 – 227 | — |
| Bacterial + incidental virus finding | 26 | 150 | 101 – 212 | — |
| Cutoff 256 μg/L: AUC 0.81 (95% CI 0.73–0.90), Sensitivity 74.4%, Specificity 80.0%. MxA/CRP ratio cutoff 18.6: AUC 0.89 (95% CI 0.83–0.96), Sensitivity 92.6%, Specificity 77.3%. Ref: Piri et al., Microbiol Spectrum 2022, e02031-21. | ||||
The critical technical hurdle in MxA rapid test development, and how Sekbio overcame it.
Unlike serum or plasma biomarkers such as CRP or PCT, MxA is an intracellular cytoplasmic protein. It is synthesized inside leukocytes (primarily lymphocytes and monocytes) in response to interferon signaling. In a blood sample, MxA is predominantly trapped inside cells — not freely circulating in plasma.
This creates a fundamental challenge for lateral flow rapid tests: the MxA protein must first be efficiently released from white blood cells and made accessible to the capture and detection antibodies. Insufficient release leads to false-negative results; excessive lysis conditions may damage the antibody-binding epitopes or introduce interfering matrix components.
Achieving efficient, reproducible, and antibody-compatible MxA release in the simple matrix of a lateral flow strip — without external equipment — is the key engineering challenge that has limited commercial MxA rapid test availability.
Through an extensive internal R&D program, Sekbio has developed and validated a proprietary whole-blood MxA release formulation specifically optimized for lateral flow assay conditions. Our release reagent efficiently lyses leukocytes and liberates intracellular MxA while maintaining full antibody pair reactivity — enabling reliable MxA detection directly from a fingerstick whole-blood sample.
This breakthrough allows MxA to be measured at the point of care — in an emergency department, clinic, or primary care setting — without laboratory centrifugation or complex sample processing steps.
MxA antibody pair and rapid test validated for clinical infection differentiation, antibiotic stewardship, and POCT kit development.
Children and adults presenting with fever of uncertain origin are the core target. MxA rapidly differentiates viral from bacterial illness at the ED — enabling safe antibiotic withholding in viral cases and appropriate escalation in bacterial cases, without waiting for culture results.
Currently available biomarkers (CRP, PCT) only estimate bacterial infection risk. MxA provides the complementary viral infection signal. Combined MxA/CRP ratio achieves AUC 0.89 — a clinically actionable tool for antibiotic stewardship protocols in both hospital and community settings.
MxA is induced by all clinically significant respiratory viruses — influenza A/B, RSV, SARS-CoV-2, parainfluenza, adenovirus, rhinovirus, and hMPV. A single MxA test can indicate active viral respiratory illness regardless of specific pathogen, guiding antiviral vs. antibiotic therapy decisions.
The MxA-Ab-02 / MxA-Ab-01-AE antibody pair is available as OEM raw material for CLIA analyzer manufacturers and fluorescent immunoassay (FIA) kit developers. 2 pg/mL sensitivity ensures detection well below the 119 μg/L bacterial infection baseline — enabling full clinical range quantification.
Understanding the molecular basis of MxA as both an antiviral effector and a reliable clinical biomarker.
Human MxA, encoded by the MX1 gene, is a cytoplasmic dynamin-like GTPase with the broadest antiviral spectrum of any known interferon-stimulated gene. Its clinical value as a biomarker is directly tied to this breadth: elevation of MxA reliably signals active Type I/III interferon signaling regardless of the causative virus.
| Virus Family | Representative Viruses Inhibited by MxA | Genome Type |
|---|---|---|
| Orthomyxoviridae | Influenza A (all subtypes), Influenza B, Thogoto virus | ssRNA (−) |
| Paramyxoviridae | Measles virus, hMPV, parainfluenza viruses | ssRNA (−) |
| Bunyaviridae | Hantaan virus, La Crosse virus, Rift Valley fever virus, CCHFV, Puumala virus | ssRNA (−) |
| Rhabdoviridae | Vesicular stomatitis virus (VSV), Rabies virus | ssRNA (−) |
| Togaviridae | Semliki Forest virus | ssRNA (+) |
| Hepadnaviridae | Hepatitis B virus (HBV) | dsDNA |
| Clinical respiratory panel | Influenza, RSV, adenovirus, coronavirus, rhinovirus, enterovirus | Multiple |
| MxA achieves antiviral activity by oligomerizing around viral ribonucleoprotein (vRNP) complexes, disrupting their function through GTPase-driven mechano-chemical constriction. This conserved mechanism underlies MxA's broad antiviral specificity. (Ref: Verhelst et al., MMBR 2013, 77:551–566.) | ||
The MxA gene promoter contains an Interferon-Stimulated Response Element (ISRE) that responds exclusively to Type I (IFN-α/β) and Type III (IFN-λ) interferons. These interferons are produced in direct response to viral pathogen recognition — the body's antiviral alarm signal.
| Stimulus | MxA Induced? | Clinical Implication |
|---|---|---|
| Viral infection (active, symptomatic) | Yes — strongly | MxA elevated to diagnostic range (>256 μg/L) |
| Viral carriage (asymptomatic) | Minimal | MxA remains at baseline — no false positive |
| Bacterial infection (no virus) | No | MxA stays low (median 119 μg/L) |
| IFN-γ (Type II interferon) | No | MxA promoter does not respond to IFN-γ |
| IL-1, TNF-α (inflammatory cytokines) | No | Nonspecific inflammation does not elevate MxA |
| This selectivity is the molecular basis of MxA's diagnostic specificity for viral infection. No other routine blood biomarker shares this property. | ||
Request the MxA antibody pair technical datasheet, CLIA performance data, or a rapid test evaluation sample from our team.