Toxoplasma IgG Avidity Test: Interpretation & Timing

Understanding Toxoplasma IgG Avidity

In my practice, I often see patients who have received a positive Toxoplasma IgG result and are anxious about when their infection occurred. A standard IgG test tells us you've been exposed, but it cannot reliably distinguish a recent infection from one that happened years ago. This is where the Toxoplasma IgG avidity test becomes clinically invaluable.

Avidity measures the overall strength of antibody-antigen binding. Over time, the immune system produces antibodies that bind more tightly to the Toxoplasma antigens. High avidity indicates the infection is likely older than 12-16 weeks; low avidity suggests a more recent infection, typically within the past 3-5 months.

Why This Test Matters

The primary reason clinicians order this test is to manage toxoplasmosis in pregnancy. If a pregnant woman acquires a primary infection during gestation, the parasite can cross the placenta and cause congenital toxoplasmosis, leading to serious fetal complications. Knowing whether the infection is recent or remote helps guide decisions about antenatal treatment and follow-up.

Outside of pregnancy, the test is occasionally used in immunocompromised patients (e.g., after organ transplant or in HIV infection) to assess the risk of reactivation.

What Does Toxoplasma IgG Avidity Tell You?

The avidity index is reported as a ratio or percentage. Laboratories may use slightly different cutoffs, but the general principles are consistent:

• High avidity — IgG binds strongly; infection is likely past (≥ 12 weeks old). This usually excludes recent primary infection.
• Low avidity — IgG binds weakly; infection may be recent (< 12 weeks). However, low avidity alone does not confirm acute infection — serial testing and clinical correlation are needed.
• Borderline (intermediate) avidity — result is equivocal; repeat testing in 2-4 weeks is recommended.

Reference Ranges for Toxoplasma IgG Avidity

Cutoffs vary by manufacturer and laboratory. The table below shows commonly accepted reference ranges (expressed as an avidity index). Values are consistent across all ages and genders — the test is interpreted the same whether you are male or female, young or old.

Note: Some European laboratories express avidity as a percentage (e.g., ≥ 35% = high). Always follow your local lab's reference intervals.

Toxoplasma IgG Avidity in Pregnancy

This is the most common scenario where I request the test. If a pregnant woman has positive IgG and positive or equivocal IgM, the avidity test helps us decide whether the infection happened before conception (low risk) or during gestation (higher risk).

• High avidity in the first or second trimester — very reassuring; infection likely predated pregnancy. No further intervention needed.
• Low avidity in early pregnancy — alarm bell. The infection may have occurred after conception. We then perform amniocentesis to test for fetal infection and consider spiramycin therapy to reduce transmission.
• Low avidity in the third trimester — still a risk, but transmission rates are highest with late acquisition. Antenatal treatment is still beneficial.

How Accurate Is the Avidity Test?

No test is perfect. Avidity testing has a sensitivity of about 90% for detecting old infections when avidity is high. However, low avidity can sometimes persist for many months in a small number of individuals, and rarely, high avidity can develop early. Therefore, the test is best used in combination with other serology (IgM, IgG kinetics) and clinical history.

Can Avidity Change Over Time?

Yes. Avidity maturation is a biological process. After a primary infection, avidity is initially low and gradually increases over weeks to months. Once high avidity is reached, it usually remains high for years. However, in immunocompromised patients, avidity maturation may be delayed or absent.

What If Avidity Is Borderline?

A borderline result is frustrating but manageable. I advise my patients to repeat the avidity test in 2–4 weeks. If the avidity rises into the high range, the infection is likely older than initially thought. If it stays low, a recent infection becomes more probable. In pregnancy, we may proceed with PCR testing of amniotic fluid regardless, based on the clinical context.

Toxoplasma IgG Avidity vs. IgM Testing

A positive Toxoplasma IgM can persist for 12–18 months after an acute infection, leading to false reassurance or unnecessary anxiety. Avidity testing helps clarify. For example, a patient with positive IgM but high avidity almost certainly had an infection in the distant past — the IgM is a leftover from that event. Conversely, low avidity with positive IgM suggests a recent infection.

When Should This Test Be Ordered?

• Pregnant women with positive Toxoplasma IgG and positive or equivocal IgM.
• Immunocompromised patients with suspected acute toxoplasmosis.
• Infants with suspected congenital toxoplasmosis (though PCR and clinical signs are primary).
• Rarely, as a confirmatory test in non-pregnant adults with unexplained lymphadenopathy and positive serology.

In my experience, the test is most clinically useful when performed early in pregnancy. It prevents unnecessary terminations when a high avidity result shows past infection, and it prompts timely intervention when avidity is low.