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

Objective: 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.

Methods: 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.

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.

Conclusion: 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.