ACCURACY OF PRENATAL DETERMINATION OF RHD TYPE STATUS BY POLYMERASE CHAIN-REACTION WITH AMNIOTIC CELLS

OBJECTIVE: Our purpose was to determine the accuracy of RhD typing by use of amniocytes obtained at amniocentesis in RhD-negative women. STU DY DESIGN: One hundred thirty-five RhD-negative women undergoing amnio centesis for management of suspected alloimmunization (n = 95) or rout ine second-trimester cytogenetic indications (n = 40) were studied. Am niocytes were then used as template to amplify specific portions of th e Rh D and Rh CcEe genes by polymerase chain reaction. The fetal RhD t ype was confirmed by serologic techniques either after fetal blood sam pling or cord blood samples at birth. RESULTS: Thirty-six fetuses were serologically typed as RhD negative and all 36 were typed RhD negativ e by polymerase chain reaction. Ninety-eight fetuses were serologicall y typed as RhD positive; of these, 96 were correctly typed as RhD posi tive and two were incorrectly typed as RhD negative, with an overall e rror rate of 1.4%. Both of the errors occurred in a single batch of si x samples tested at the same time. In one of these cases the fetus had mild Rh alloimmune disease and required exchange transfusion at birth . In the second case the fetus had severe hydrops fetalis and died in utero at 28 weeks. Deoxyribonucleic acid isolated from fetal blood was tested with the same polymerase chain reaction technique after delive ry, and in both cases the fetus was correctly typed as RhD positive. D eoxyribonucleic acid amplification repeatedly failed in one case. CONC LUSION: Prenatal fetal RhD typing by polymerase chain reaction with am niotic fluid cells is accurate and reliable. In sensitized pregnancies it allows early management of Rh disease and avoids invasive procedur es in RhD-negative fetuses. In nonsensitized pregnancies it avoids the use of anti-RhD immunoglobulin after invasive procedures or during pr egnancy. To eliminate the possibility of genetic and laboratory source s of errors, we suggest using different sets of primers at two differe nt loci (e.g., exon 4 to 5 and exon 10).