A 24-year-old pregnant woman at 12 weeks gestation is found to have positive CMV IgM. To distinguish primary infection from past infection with IgM persistence, IgG avidity testing is performed and shows LOW avidity anti-CMV IgG. This indicates:

• A Past infection acquired more than six months ago
• B Reactivation of latent infection
• C False-positive IgM due to rheumatoid factor interference
• D Recent primary infection, likely within the preceding three months ✓

Explanation

IgG avidity measures the binding strength of antigen-antibody complexes. Antibodies produced early in primary infection bind weakly, so low avidity IgG indicates recent primary CMV infection within roughly the previous three to four months, while high avidity excludes primary infection in the preceding months. Reactivation and past infection produce maturing, high avidity antibody, so this result carries major implications for counselling about congenital transmission risk.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.