A 24-year-old primigravida at 10 weeks gestation is found to have rubella-specific IgM positive and IgG positive. Rubella IgG avidity testing shows low avidity antibodies. The most appropriate interpretation is:

• A Past infection with immunity acquired before pregnancy, fetus not at risk
• B Reinfection with wild virus, negligible fetal risk
• C False positive IgM due to rheumatoid factor, no infection
• D Recent primary rubella infection, high risk of congenital rubella syndrome ✓

Explanation

Antibody avidity matures over weeks to months after primary infection. Low avidity IgG indicates a recently acquired primary infection, and rubella in the first trimester carries a very high risk of congenital rubella syndrome. High avidity antibodies would indicate remote past infection, and reinfection typically produces a rapid, high avidity anamnestic response with minimal fetal risk, which is exactly what the low avidity result excludes here.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.