IgM
What it means
- Marker of recent immune activation
- Appears early after primary infection
What it does NOT reliably mean
It does not always mean recent infection
IgM can:
- Persist for months
- Reappear with reactivation
- Be falsely positive
IgG
What it means
- Evidence of exposure at some point in life
- Appears later than IgM
- Persists lifelong
What it does NOT tell you
- When the infection occurred
- Whether it is active or remote
IgG
What it means
- Evidence of exposure at some point in life
- Appears later than IgM
- Persists lifelong
What it does NOT tell you
- When the infection occurred
- Whether it is active or remote
IgG avidity (this is the key discriminator)
What avidity measures
- The binding strength of IgG antibodies
- Increases over time as immune response matures
Interpreting IgG avidity
Low avidity
- IgG is immature
- Infection occurred recently
- Typically within:
- Last 8–12 weeks (depends on pathogen)
- → Strong evidence of primary infection
High avidity
- IgG is mature
- Infection occurred well before pregnancy or early in first trimester
- → Effectively rules out recent primary infection
This is the single most powerful test for dating infection in pregnancy.
Pattern 1: IgM negative, IgG negative
- No prior exposure
- Susceptible
- No infection yet
- → Repeat only if clinical suspicion or exposure
Pattern 2: IgM positive, IgG negative
- Very early primary infection
- Or false-positive IgM
- → Repeat in 1–2 weeks to document IgG seroconversion
Pattern 3: IgM positive, IgG positive, low avidity
This is the classic primary infection pattern.
Meaning:
- Recent primary infection
- High fetal transmission risk (for CMV, toxo, rubella)
- → Amniocentesis is indicated (timed appropriately)
Pattern 4: IgM positive, IgG positive, high avidity
This is where most confusion happens.
Meaning:
- Past infection
- IgM persistence or reactivation
- Not a recent primary infection
- → Amniocentesis usually NOT indicated, unless ultrasound abnormal
Pattern 5: IgM negative, IgG positive, high avidity
- Remote infection
- Immune
- No fetal risk
- → No further action
Role of serial titres (very important, but limited)
What serial titres can do
- Demonstrate seroconversion
- Demonstrate rising IgG titres
What serial titres cannot reliably do
- Distinguish primary infection from reactivation once IgG is already present
- Date infection precisely without avidity
Seroconversion
Definition:
Change from IgG negative → IgG positive
This is proof of primary infection.
If this happens during pregnancy:
→ High fetal risk
→ Amniocentesis indicated
Rising IgG titres
A fourfold rise suggests recent infection
But:
- Titres vary between labs
- Reactivation can also cause rises
- Less reliable than avidity
Amniocentesis is justified when ALL apply:
Evidence of primary maternal infection
Low IgG avidity
OR documented seroconversion
Appropriate gestational timing
Usually = 21–22 weeks
And = 6–7 weeks after presumed infection
Testable pathogen in amniotic fluid:
CMV PCR
Toxo PCR
Rubella PCR
Amniocentesis is NOT usually justified when:
- IgG avidity is high
- Only IgM is positive
- No seroconversion
- Ultrasound is normal