Topic Overview

When a pregnant woman has toxoplasmosis, a PCR test on amniotic fluid is used to learn whether her fetus is also infected.

Polymerase chain reaction (PCR) is a method of duplicating DNA strands from a tiny sample of blood, hair, or tissue. PCR is used to identify infectious diseases, genetic conditions, and genetic links between people.

PCR is preferred over a fetal blood test for antibodies because:¹

• Collecting amniotic fluid through amniocentesis is considered safer than fetal blood sampling.
• PCR is better than antibody testing to detect toxoplasmosis.
• PCR can be done earlier in a pregnancy than a blood test for antibodies. Because first- and second-trimester fetal infection carry the highest risk of birth defects and intellectual disability, earlier test results are important to parents who are considering ending such a pregnancy.

In rare cases, PCR produces false-positive or false-negative test results. Follow-up testing and fetal ultrasound monitoring for hydrocephalus can help confirm PCR test results.

PCR is seldom used to diagnose toxoplasmosis in adults. Antibody testing of a blood sample is easier and more widely available.