Incidence of placental mosaicism leading to discrepant results between QF‐PCR and karyotyping in 22,825 chorionic villus samples

Abstract

To review the frequency and analyse the origin of completely discrepant results observed between QF-PCR and karyotyping in chorionic villus samples (CVS) as a result of placental mosaicism. Also, to assess QF-PCR results for biallelic or triallelic patterns and determine their significance. Between May 2002 and December 2009, 22 825 CVS were received at TDL Genetics for processing by QF-PCR and karyotype. The QF-PCR and karyotype data were compared to determine the incidence of discrepant results. Of the 22 825 samples received, 22 779 (99.8%) gave concordant results between the PCR and karyotype, and 46 samples (0.2%) gave discordant results. Of these discrepant cases, 5 displayed triallelic peaks and 41 displayed biallelic peaks. All discordant results are due to the presence of placental mosaicism, a known limitation of using this sample type for prenatal diagnosis. This retrospective study of placental mosaicism in CVS is the largest single centre study to date and provides a figure for the occurrence of completely discrepant results between QF-PCR and karyotype due to placental mosaicism. This study also demonstrates that the presence of triallelic peaks at QF-PCR is not sufficient to exclude the presence of placental mosaicism.