CYTOGENETIC DIAGNOSES AFTER CHORIONIC VILLUS SAMPLING ARE LESS RELIABLE IN VERY-HIGH-RISK OR VERY-LOW-RISK PREGNANCIES

An increasing number of cytogenetic prenatal diagnoses are performed o n chorionic villus samplings. The accuracy of this method is influence d by chromosomal mosaicism, mostly confined to direct preparation meth ods. Especially those investigators who have experienced false-negativ e and false-positive findings propagate the combined use of direct and culture methods. Yet large collaborative studies have shown that in a pproximately two-thirds of diagnostic cases only one procedure is appl ied. Moreover, the accuracy of a cytogenetic investigation depends not only on the ontogenetic origin of the tissues investigated, but also on interacting factors such as the 'a priori risk' and the 'predictive value of a cytogenetic finding'. On this basis a differentiated prena tal diagnostic procedure is discussed, including either sole short-ter m culture (STC), combined STC and long-term culture (LTC), primary amn iocentesis (AC), or primary percutaneous umbilical blood sampling (PUB S). The predictive value of the cytogenetic diagnosis from CVS varies significantly dependent on the a priori risk of a chromosome aberratio n and, in the case of an abnormal karyotype, on the specific chromosom e involved. A non-mosaic and 'nonlethal' trisomy detected in STC is hi ghly representative of the embryo/fetus, but there are exceptions of l imited predictive value, e.g., trisomy 18. Guided by the strategy of a n optional follow-up by LTC, AC, or PUBS in 1317 successive CV samplin gs, we are not aware of a false-negative diagnosis, but probably had o ne false-positive diagnosis: 47,XXY after STC; 46,XY after LTC. When r eferring to the rate of fetuses with an unbalanced karyotype expected in the different indication groups, a relative increase of false-posit ive findings in the very-low-risk group (maternal age less-than-or-equ al-to 35 years of age) and of false-negative findings in the very-high -risk group (abnormal ultrasonographic findings) of pregnant women whe n only performing CVS becomes obvious. Because of this dilemma, AC or- especially in the latter group-PUBS might be primarily offered to thes e indication groups instead of CVS.