Cytomegalovirus (CMV) | Antibodies IgG avidity

General Information

Cytomegalovirus (CMV) is the most common cause of congenital retardation and deafness in infants.

The frequency of intrauterine infection with cytomegalovirus is 1:100 live births.

15% of children born with congenital infection have symptoms at birth (microcephaly, intracranial calcifications, chorioretinitis, jaundice, hepatosplenomegaly, purpura). The remaining 85% are asymptomatic at birth, although 15% of them develop neurological disorders before the age of 4 years.

Congenital cytomegalovirus infection of the newborn is caused by infection of the mother during pregnancy. Reinfection or reactivation of the causative agent very rarely leads to intrauterine infection of the fetus.

Cytomegalovirus infection is asymptomatic in >90% of cases in adults with normal immune status, including pregnant women. Detection of specific antibodies is the most common method of disease diagnostics. IgM antibodies develop within a few weeks after infection, followed by the development of IgG antibodies.

Cytomegalovirus IgM antibodies increase only in a few weeks and after 6 months it decreases significantly. Despite the high sensitivity of IgM antibodies in the diagnostics of primary infection, the specificity is quite low, because IgM may develop during re-infections and reactivation of the latent infectious process.

Also, there are false positive results of cytomegalovirus IgM antibodies, which are caused by cross-reactivity of different pathogens.

After the primary infection, the avidity (avidity, affinity for the antigen) of IgG antibodies increases progressively.

Determination of IgG antibody avidity is an important study for differentiating primary infectious process from secondary infection. A low degree of avidity indicates the presence of a primary infection. However, a low degree of antibody avidity detected in the first trimester of pregnancy does not indicate that infection occurred after conception.

Avidity of IgG antibodies is an important indicator for differentiation of primary and recurrent (repeated) infection in infants older than 3 months.

Research in infants less than 3 months of age is not relevant, because during this period the antibodies that have entered the blood of the fetus through the placenta are still active.

Patient preparation and study sample  - For the correct interpretation of the results, it is recommended to perform an antibody avidity test from the same serum from which the titer of IgM and IgG antibodies was determined.

Interpretation of results

Avidity index <0.4  (low avidity) Indicates the presence of a primary, recently acquired infection, not more than 3 months. It is necessary to inform pregnant women about the transplacental transmission of the infection to the fetus and the development of further complications of the fetus, and it is recommended to conduct a cytomegalovirus DNA quantitative study through amniocentesis. Amniocentesis must be performed no earlier than 7 weeks after the suspected infection of the mother, after 21 weeks of pregnancy. It is necessary to take into account this period in the research, because after the infection of the mother, it takes an average of 6-7 weeks for the fetus to be infected and for the virus to separate and multiply in the amniotic fluid.

Avidity index 0.4 ≤ 0.65 (moderate average avidity) It does not exclude a newly started infectious process, although such an indicator characterizes an older infectious process without active activity.

Avidity index ≥ 0.65 (high avidity) Indicates that the infectious process started more than 3 months ago.

The criteria for diagnosing the mother's primary infection are:

• Detection of IgG antibodies in previously seronegative patients
• Detection of low avidity of IgG antibodies in IgM-positive patients.

The criteria for diagnosing secondary (recurrent) infection of the mother are:

• An increase in the titer of IgG antibodies - with an increase in the IgG avidity index (against the presence or absence of IgM antibodies).

The results of the study are inconclusive in those patients who have undergone blood or blood component transfusions, as well as immunosuppressive therapy in recent months.