Cytomegalovirus Reviewed
Xchange newsletter - Winter 2006/2007
It takes three
Cytomegalovirus (CMV) is the most common virus globally to be transmitted in utero, affecting up to 2.5% of all live births (WHO). It is a member of the herpesvirus family, a group characterised by their persistent latent infections. As many as 40-90% of adults carry the virus and usually without symptoms, however, infection during pregnancy can be extremely dangerous for the unborn child and screening of maternal blood is important to assess serological status. Here we examine CMV testing in further detail.
Unnoticed
In most cases, CMV passes unnoticed but when symptoms do occur, they resemble a mild glandular fever-like illness and are often attributed to other causes. The virus remains in the body for life and if the immune system is later weakened, CMV may become reactivated (non-primary infection). Transmission of the virus may be by aerosol, sexual intercourse, organ transplantation or blood transfusion.
Pregnant danger
CMV screening may be recommended for:
• pregnant women - to determine immunity or diagnose primary CMV infection
• transplant donors/recipients - to determine CMV serostatus, as CMV disease is a significant cause of post-transplant morbidity and mortality
• blood donors - to identify CMV antibody negative units for transfusion
If a woman becomes infected with CMV for the first time while she is pregnant, the virus may cross the placenta and infect the foetus. However, the risk of CMV-related complications is low if the mother has been infected for at least six months prior to conception.
Should primary infection occur early in pregnancy, the chances of miscarriage or congenital malformations are increased. Primary intrauterine CMV infections rank second behind Down’s syndrome as a cause of mental retardation. Infection later in pregnancy increases the likelihood of premature labour or stillbirth.
Schematic overview of CMV Infection
Figure 1
Ask the panel
• The ARCHITECT CMV panel will include three assays: CMV IgG, IgM and IgG avidity
• ARCHITECT offers the first fully automated complete CMV panel including avidity, with an editable reflex test option available
• When ARCHITECT CMV Avidity was tested in combination with IgG and IgM assays, the resulting sensitivity of this retest rule was 97.2% with 99.8% specificity (Maine et al)
• Avidity features AVIcomp technology – a novel approach in avidity testing>
There is no single test for CMV. Instead, a panel of assays should be used – CMV IgG, CMV IgM and CMV IgG avidity (figure 1). CMV IgM is a sensitive indicator of ongoing or recent infection. Since CMV IgM antibodies can be produced during primary and non-primary CMV infections, CMV IgG avidity tests are useful to discriminate between primary and non-primary infections in pregnant women with CMV IgM positive serology. Positive IgG of low avidity confirms primary infection while IgG of high avidity confirms non-primary.
It is recommended that avidity testing should be performed in pregnant women within the first trimester as a high avidity result later on cannot rule out a primary infection during the early stages of gestation.
A combination of three CMV assays makes screening for primary infections more reliable and also enables the time of CMV infection to be estimated, which helps determine suitable treatment.
For further information:
Maine GT, Hsu S, Smith D, Hausmann M, Curdt I, Herzogenrath J. Development of novel cytomegalovirus and toxoplasma IgG Avidity assays using an antigen competitive format ‘AVIcomp’ on the Abbott ARCHITECT instrument. Poster available here <www.abbottdiagnostics.com/science>
