PCR vs karyotype for CVS and amniocentesis-the experience at one tertiary fetal medicine unit

Despite the rise of non-invasive screening tests for fetal aneuploidy, invasive testing during pregnancy remains the definitive diagnostic tool for fetal genetic anomalies. Results are rapidly available with polymerase chain reaction (PCR) tests, but cases have been reported whereby initial results were not confirmed after pregnancy termination and the fetal karyotype was ultimately normal. We sought to examine the potential discordance between PCR and karyotype for fetal aneuploidy.

The results from all amniocentesis and CVS tests performed over a 6-year period in a large tertiary level fetal medicine unit were reviewed. The results of PCR and karyotype were recorded and discrepancies examined. Pregnancy outcomes were also recorded.

A total of 1222 invasive tests were performed (716 amniocentesis and 506 CVS). Within the cohort having amniocentesis, 11 had discrepant results (normal QF-PCR result but with a subsequent abnormal karyotype). There was 1 case among this group which QF-PCR should have identified. Within the CVS group, 7 patients had discrepant results. All had a diploid QF-PCR and would not have been identified as abnormal by it.

PCR can be reliably used to determine aneuploidy of chromosomes 13, 18, and 21. However, in cases of sex chromosome aneuploidy, its performance is less reliable and warrants waiting for a complete karyotype. Given such discordance, we advise waiting for karyotype for all invasive tests performed in the presence of a normal ultrasound before advising a patient of a diploid QF-PCR result or potentially terminating a normal pregnancy.