The role of molecular microsatellite identity testing to detect sampling errors in prenatal diagnosis.

Objective: The objective was to determine the risk of sampling error in amniocentesis and chorionic villus sampling (CVS) in singleton and multiple pregnancies. Data from this and other published studies were used to discuss current practice guidelines for molecular identity testing.

Method: Clinical and laboratory records of all patients undergoing molecular-based identity testing in our clinical laboratory from July 2002 until March 2008 were reviewed. DNA microsatellite testing was performed to determine zygosity in multiple pregnancies and maternal cell contamination (MCC) in both singleton and multiple pregnancies.

Results: MCC was detected in 6/148 (4%) CVS and 1/87 (1%) amniotic fluids from singleton pregnancies. In two of the CVS, only maternal cells were found. In 2/24 (8%) twin pregnancies, the same fetus was tested twice. In a total of 285 pregnancies (235 singleton, 24 twin, 26 with >or= 3 fetuses), without molecular identity testing, four women would have received erroneous results.

Conclusion: Current guidelines recommend molecular identity testing for MCC in conjunction with molecular diagnostic testing, but not for cytogenetic testing. No published guidelines were found for zygosity testing in multiple pregnancies. We suggest that identity testing be considered for all prenatal testing of multiple pregnancies, especially if CVS is performed.