VHOSPITAL.CLINIC · Serology Test
The IgG avidity test resolves the timeline of toxoplasma infection by measuring antibody binding strength — high avidity confirms infection more than 4 months ago, low avidity indicates recent primary infection.
Measures binding strength of toxoplasma-specific IgG to antigens — high avidity indicates past infection (>4 months); low avidity indicates recent primary infection.
After primary toxoplasma infection, IgG antibodies initially bind antigen weakly (low avidity). Over 3–4 months, the immune response matures and IgG binds antigen with high affinity (high avidity). This maturation is irreversible — high avidity reliably excludes primary infection in the preceding 12–16 weeks.
The same blood sample as the standard ELISA. The avidity test adds a urea wash step that disrupts low-affinity (low avidity) IgG-antigen bonds. Avidity Index = (OD with urea / OD without urea) × 100. High avidity: >60%; Low avidity: <40%; Intermediate: 40–60%.
High avidity (>60%): primary infection occurred >4 months ago — fetal risk in pregnancy is very low. Low avidity (<40%): infection within the past 3–4 months — cannot confirm timing, continued monitoring needed. Intermediate avidity (40–60%): indeterminate — clinical context and serial testing required.
High avidity sensitivity for ruling out recent infection: >97%. Low avidity is not diagnostic of recent infection (can persist in some individuals). Specificity of high avidity for excluding first-trimester infection: >95% in most validated assays.
A high avidity result (>60%) means the toxoplasma infection occurred more than 4 months before testing. If testing occurs in the first trimester and avidity is high, it rules out first-trimester primary infection — the period of highest fetal risk. No treatment is needed for a high avidity result in an immunocompetent pregnant woman.
Yes. If a blood sample was collected earlier and stored frozen, avidity can be tested retrospectively. This is particularly useful when a pregnant woman has an early-pregnancy IgM+ result discovered late — avidity on the original sample can clarify infection timing.
Low avidity (<40%) suggests infection in the past 3–4 months but cannot confirm exact timing. Management depends on gestational age and clinical context. Spiramycin is typically started immediately to reduce placental transmission. Amniocentesis for toxoplasma PCR may be offered after 18 weeks' gestation.
Toxoplasmosis is caused by the parasite Toxoplasma gondii, transmitted through cat feces, undercooked meat, or vertically to the fetus. It is usually asymptomatic in healthy individuals but can cause severe disease in immunocompromised patients and congenital infection.
Get a structured AI clinical assessment — possible parasitic causes, recommended diagnostic tests, and next steps.
Start Free AI Analysis →Content on this page is informed by evidence-based clinical sources including: