Noninvasive prenatal screening for aneuploidy: positive predictive values based on cytogenetic findings

T21, which has the highest prevalence of all aneuploidies, demonstrated a high true-positive rate, resulting in a high PPV. However, the other aneuploidies, with their lower prevalence, displayed relatively high false-positive rates and, therefore, lower PPV. Patients and physicians must fully understand the limitations of this screening test and the need in many cases to follow up with appropriate diagnostic testing to obtain an accurate diagnosis.