Primary infection or reactivation
Class M antibodies to cytomegalovirus are produced during primary infection, but, unlike many other infections, they can also appear when the virus is reactivated from a latent state - against the background of another illness, stress, pregnancy or immunosuppression. This makes interpretation of the result difficult.
Primary infection in an adult with normal immunity most often occurs as a protracted condition with low fever, weakness and enlarged lymph nodes, reminiscent of infectious mononucleosis. It poses a danger primarily during pregnancy: it is the primary infection of the mother that is associated with the greatest risk of congenital cytomegalovirus infection in the child.
To differentiate situations, the same approach is used as for toxoplasmosis: simultaneous determination of IgG, assessment of dynamics in paired sera and determination of IgG avidity. High-avidity IgG indicates long-standing infection and therefore reactivation rather than primary infection. If it is necessary to confirm active replication, PCR is performed on viral DNA in the blood and urine.
What do IgM and IgG mean?
The immune system produces different classes of antibodies at different times. Immunoglobulins M appear first, usually in the first or second week after infection, last for several weeks or months and then disappear. Their detection indicates an acute or recent process.
Immunoglobulins G appear later, after a few weeks, and persist for years, and often for life. They mean that a meeting with the pathogen took place in the past, and in many cases indicate formed immunity. Therefore, positive IgG in itself is not a reason for treatment.
Why dynamics are important, not just a number
A single result is often ambiguous. Antibody levels vary from person to person, with borderline levels falling into what is known as a gray area. A reliable sign of active infection is a fourfold increase in IgG titer in paired sera taken at intervals of 10–14 days.
This is why serology is rarely prescribed as a single test. The doctor either combines IgM and IgG, or repeats the test, or confirms the result with a direct method - PCR, which detects the DNA or RNA of the pathogen itself.
Limitations of the method
Antibodies do not appear immediately: in the first days of the disease, serology can be negative if the infection has already begun - this is the period of the serological window. In case of immunodeficiency and against the background of immunosuppressive therapy, antibodies may not be produced at all.
Cross-reactions with related pathogens are also possible, giving a false-positive result. Therefore, treatment is never prescribed “based on analysis” - only on the basis of a combination of clinical data and research results.