Noninvasive fetal blood group genotyping of rhesus D, c, E and of K in alloimmunised pregnant women: evaluation of a 7-year clinical experience.

OBJECTIVE To evaluate the diagnostic performance of noninvasive fetal blood group genotyping. DESIGN Descriptive analysis. SETTING Dutch national reference laboratory for pregnancies complicated by alloimmunisation. POPULATION All consecutive alloimmunised pregnant women for whom fetal blood group genotyping of rhesus D, c, E or of K in maternal plasma was performed from 2003 up to 2010. METHODS The test results of each individual assay were collected. Real-time polymerase chain reaction was performed for RHD exon 5 and RHD exon 7, or the specific allele of the RHCE or KEL gene. A stringent diagnostic algorithm was applied. In the case of a negative result, the presence of fetal DNA was ascertained by the analysis of the Y chromosome-specific SRY gene or other paternal genetic markers. Results were compared with available serology after birth or genotyping results of amniotic fluid cells. MAIN OUTCOME MEASURES Percentage of conclusive test results and diagnostic accuracy. RESULTS A total of 362 tests was performed (D: n = 168; c: n = 49; E: n = 85; K: n = 60). The median gestational age was 17 weeks (range 7-38 weeks). In 351 women (97%), a test result was issued: in seven samples, the presence of fetal DNA could not be confirmed; in two samples, non-specific amplification in the K assay led to an inconclusive result; in two samples, a maternal silent RHD gene prevented fetal RHD genotyping. No false-positive or false-negative results were found among those women for whom cord blood serology or genotyping results of amniotic fluid cells were available (n = 212). CONCLUSIONS Noninvasive fetal blood group genotyping is accurate and applicable in a clinical diagnostic setting.