How do I order a toxoplasmosis test?

How to Order Toxoplasmosis Testing

For suspected congenital toxoplasmosis or pregnant women with positive IgM results, send specimens to a specialized toxoplasmosis reference laboratory (such as PAMF-TSL) rather than commercial laboratories, as approximately 60% of positive IgM results from commercial labs represent false-positives or chronic infections. 1, 2

Initial Testing Approach by Clinical Context

For Pregnant Women (Screening or Suspected Acute Infection)

• Order both Toxoplasma IgG and IgM simultaneously at initial evaluation 3
• If either test is positive or equivocal from a commercial laboratory, immediately send specimens to a reference laboratory for confirmation with comprehensive serologic panels 1, 2
• Reference laboratories perform the IgG dye test, IgM ISAGA (more sensitive than ELISA), and IgA ELISA 1
• Add IgG avidity testing if IgM is positive to distinguish acute from chronic infection—high avidity (>100%) excludes infection acquired within the past 4 months 4, 5

For Neonates with Suspected Congenital Toxoplasmosis

Send all specimens to a toxoplasmosis reference laboratory and order: 1

• Neonatal IgG tested in parallel with maternal IgG (interpretation requires both) 1, 3
• IgM ISAGA (not ELISA—negative ELISA cannot exclude diagnosis) 1
• IgA ELISA (may be more sensitive than IgM, though still misses 20-30% of infected infants) 1, 3
• Toxoplasma PCR on peripheral blood, urine, and CSF when strong suspicion exists 1

For HIV-Infected or Immunocompromised Patients

• Order IgG and IgM simultaneously for all HIV-infected pregnant women and adolescents without prior infection history 3
• For suspected CNS toxoplasmosis: base diagnosis on clinical symptoms, positive IgG serology, and brain imaging showing space-occupying lesions 3
• Negative serology does not exclude CNS toxoplasmosis—cases occur without detectable IgG 3

Specimen Collection and Shipping Instructions

For PCR Testing (Amniotic Fluid, CSF, Blood, Urine)

Ship to reference laboratory separately from serology specimens: 1

• Amniotic fluid: Perform amniocentesis at ≥18 weeks gestation; send 10 mL on wet ice or frozen with dry ice 1
• CSF/vitreous/aqueous fluid: Freeze immediately after collection; send 1 mL on dry ice by overnight courier 1
• Always include maternal serum for IgG/IgM testing if not recently tested at reference lab 1

For Serology Testing

• Send serum specimens for IgG, IgM, and IgA (if available) 1
• Commercial labs can perform initial screening, but positive or equivocal results require reference lab confirmation 2

Critical Timing Considerations for Neonatal Testing

Avoiding False-Positive Results

• After blood transfusion or IVIg: Wait ≥7 days before testing IgM or IgA 1
• Maternal blood contamination during delivery:
  • Repeat IgM testing ≥5 days after birth (IgM half-life = 5 days) 1
  • Repeat IgA testing ≥10 days after birth (IgA half-life = 10 days) 1

For Late Gestational Maternal Infection

• If maternal infection occurred very late in pregnancy, initially negative neonatal IgM/IgA may reflect delayed antibody production 1
• Repeat testing at 2-4 weeks after birth, then every 4 weeks until 3 months of age 1

When to Order CSF Toxoplasma PCR in Neonates

Order CSF PCR in these high-risk scenarios: 1

1. Positive neonatal IgM ISAGA and/or IgA ELISA 1
2. Maternal confirmed acute infection with positive amniotic fluid PCR or abnormal fetal ultrasound (regardless of treatment) 1
3. Maternal infection in 2nd/3rd trimester without antepartum treatment (even if amniotic fluid PCR and ultrasound were negative) 1

Defer CSF PCR in low-risk scenarios: 1

• Asymptomatic infant whose mother had 1st trimester infection, received spiramycin, had negative amniotic fluid PCR, normal monthly ultrasounds, and infant has negative IgM ISAGA and IgA at birth 1

Common Pitfalls to Avoid

• Never rely on negative IgM ELISA from commercial labs to exclude congenital toxoplasmosis—only IgM ISAGA from reference labs is sufficiently sensitive 1
• Do not interpret neonatal serology without parallel maternal serology—maternal IgG persists in infants for 6-12 months and declines predictably 6
• Low IgG avidity does not confirm acute infection—it only indicates possible recent infection without excluding older infections 4, 5
• Persistent IgM can occur for months to years after primary infection in chronic cases 2, 7