The MxA/CRP test is a dual biomarker assay that measures myxovirus resistance protein A (MxA) and C-reactive protein (CRP) to provide valuable insights into the immune response. It is mainly used to distinguish between viral and bacterial infections by evaluating the body’s innate defenses and inflammation levels. This combined assessment has proven to be a reliable method for identifying the cause of acute infections, enabling faster and more accurate treatment decisions.
Myxovirus resistance protein A (MxA) is an intracellular protein that belongs to the family of dynamin-like GTPases and is strongly induced by type I interferons, especially interferon-α and interferon-β. Its expression is a hallmark of the antiviral state, as it is triggered when the innate immune system detects the presence of viral pathogens. During viral infections, interferons activate the transcription of MxA, resulting in significantly increased levels. Due to its specific association with viral replication, MxA is regarded as a reliable biomarker for viral infections. Under normal conditions or during bacterial infections, its levels remain very low or undetectable, making it a useful marker in clinical evaluation.
C-reactive protein (CRP), on the other hand, is an acute-phase protein produced by the liver in response to pro-inflammatory cytokines, especially interleukin-6. It plays a key role in systemic inflammation and is one of the most widely studied markers for bacterial infections. CRP binds to phosphocholine on the surface of dead or dying cells and certain bacteria, helping their clearance by activating the complement system. Elevated CRP levels are strongly associated with bacterial infections, but they can also increase in severe viral infections, autoimmune disorders, and tissue injuries. However, when interpreted in conjunction with MxA, CRP provides essential context for identifying the source of inflammation.
The combination of MxA and CRP creates a powerful diagnostic profile. High MxA levels, accompanied by low or moderately elevated CRP levels, are typically observed in viral infections. Elevated CRP levels with little or no MxA indicate bacterial infections. In complex conditions, such as coinfections or severe systemic inflammation, both biomarkers may be increased; however, their relative patterns remain useful for interpretation. When levels are low for both, it suggests a lower likelihood of a significant infection, indicating that non-infectious causes may be contributing to the symptoms.
This dual-marker strategy reflects different pathways of the immune response. MxA indicates activation of the interferon-mediated antiviral defense, while CRP signals the acute-phase inflammatory response. Measuring both provides a more precise classification of infectious states, reducing the uncertainty when either marker is used alone. The MxA/CRP test is a scientifically grounded tool that improves the accuracy of infection assessment and guides decisions that benefit both diagnostic clarity and patient outcomes.
When is the MxA/CRP helpful?
- For differential diagnosis between viral and bacterial infections, since elevated MxA is linked to viral infections, while elevated CRP is associated with bacterial infection.
- When fever or infection-like symptoms are present, a rapid assessment of the underlying cause is needed.
- In cases where CRP is moderately elevated, clarification is required to determine whether the inflammation is viral or bacterial.
- For monitoring the course of ongoing infections, as changes in these biomarkers can reflect disease progression.
- To support the avoidance of unnecessary antibiotic use, the test helps confirm whether symptoms are caused by a viral rather than a bacterial infection.
