Should IgG avidity be checked in a pregnant patient with a positive TORCH (Toxoplasma gondii, Rubella, Cytomegalovirus, Herpes Simplex Virus) IgG result but negative IgM result?

IgG Avidity Testing in TORCH-Positive IgG, Negative IgM Cases

No, IgG avidity testing is not indicated when TORCH IgG is positive but IgM is negative, as this serologic pattern indicates past infection with no evidence of recent or acute infection. 1, 2

Rationale for Not Testing

When both IgG is positive and IgM is negative, the patient has immunity from prior infection acquired before pregnancy, and avidity testing adds no clinical value. 3, 1 The absence of IgM antibodies effectively rules out acute infection in most clinical scenarios, making avidity testing unnecessary and not cost-effective. 4, 5

When Avidity Testing IS Indicated

IgG avidity testing should only be performed in specific scenarios where acute infection timing needs clarification:

Primary Indication: IgG Positive AND IgM Positive

• Avidity testing is specifically designed for pregnant women who have BOTH positive IgG and positive IgM antibodies, where distinguishing acute from chronic infection is critical. 3, 1, 4
• High avidity (>85-100% specificity) confirms past/chronic infection and excludes recent infection acquired within the past 4 months. 4, 5
• Low avidity suggests possible recent infection but cannot definitively confirm it, as low avidity can persist for months to years after primary infection. 3, 1, 6

Secondary Indication: Clinical Suspicion Despite Negative IgM

• In rare cases with strong clinical suspicion (e.g., fetal ultrasound abnormalities like echogenic bowel) and negative IgM, avidity testing may be considered as part of a comprehensive evaluation. 3
• However, this scenario specifically requires sending specimens to a reference laboratory for comprehensive serologic panels, not just avidity testing alone. 3, 1, 2

Critical Limitations of Avidity Testing

The IgG avidity test is insufficient when used alone and cannot distinguish between acute and chronic infection without additional testing. 3, 1 Key limitations include:

• Low avidity can persist for years after primary infection in some patients. 3, 6
• Approximately 60% of positive IgM results from commercial laboratories represent false-positives or chronic infections. 2
• Medium avidity results are particularly problematic and require comprehensive serologic panels at reference laboratories. 1

Recommended Approach for Your Scenario

For a pregnant patient with positive TORCH IgG and negative IgM:

1. No further serologic testing is needed - this represents immunity from past infection. 1, 2
2. Document immune status and reassure the patient that no risk of congenital infection exists from this pathogen. 7
3. Only pursue additional workup if there are specific clinical findings (ultrasound abnormalities, maternal symptoms) or known exposure history that raises concern despite negative IgM. 3

Special Considerations

If clinical suspicion remains high despite negative IgM (such as with fetal echogenic bowel), send specimens to a specialized Toxoplasma reference laboratory rather than ordering isolated avidity testing. 3, 1, 2 Reference laboratories can perform comprehensive panels including IgG dye test, IgM ISAGA, IgA ELISA, and avidity testing together, which provide much more accurate timing estimates than avidity alone. 3, 2

For CMV specifically, when IgG is positive and IgM is negative, avidity testing is only applicable if IgM becomes positive or if there is IgG seroconversion documented. 3