Noninvasive Determination of Fetal Rh Blood Group, D Antigen Status by Cell-Free DNA Analysis in Maternal Plasma: Experience in a Brazilian Population

ABSTRACT

We evaluated the diagnostic accuracy of Rh blood group, D antigen (RHD) fetal genotyping, using real-time polymerase chain reaction in maternal blood samples, in a racially mixed population. We performed a prospective study conducted between January 2006 and December 2007, analyzing fetal RHD genotype in the plasma of 102 D− pregnant women by real-time polymerase chain reaction, targeting exons 7 and 10 of the RHD gene. Genotype results were compared with cord blood phenotype obtained after delivery or before the first intrauterine transfusion when necessary. Most of the participants (75.5%) were under 28 weeks of pregnancy, and 87.5% had at least one relative of black ancestry. By combining amplification of two exons, the accuracy of genotyping was 98%, sensitivity was 100%, and specificity was 92%. The positive likelihood ratio was 12.5, and the negative likelihood ratio was 0. The two false-positive cases were confirmed to be pseudogene RHD by real-time polymerase chain reaction. There were no differences between the patients with positive or negative Coombs test (p = 0.479). Determination of fetal RHD status in maternal peripheral blood was highly sensitive in this racially mixed population and was not influenced by the presence of antierythrocyte antibodies.

KEYWORDS

Prenatal diagnosis - cell-free DNA - fetal RHD status - real-time polymerase chain

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