A 28-year-old pregnant woman in her first trimester is found to have IgG antibodies to Toxoplasma gondii. Her physician orders an IgG avidity test, which shows LOW avidity antibodies. The correct interpretation is:

• A Infection acquired in the remote past, fetus is safe
• B Reactivation of latent infection from a previous chronic focus
• C Test result is invalid because avidity cannot be interpreted during pregnancy
• D Infection acquired recently, usually within the preceding 3 to 4 months, raising concern for fetal transmission ✓

Explanation

Antibody avidity matures as the immune response evolves, so low-avidity IgG indicates recent primary infection, typically within the preceding three to four months. In early pregnancy this raises real concern for transplacental transmission and justifies counselling and treatment. High avidity would point to past infection and effectively exclude recent acquisition, making option A the strongest distractor, but it contradicts the stated low avidity result. Reactivation does not produce low-avidity antibodies, removing option B.

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

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Written and medically reviewed by the StethoPrep medical team.