Objective: The aim of the study was to determine the sensitivity, specificity and accuracy of noninvasive tests for the fetal rhesus CcEc (RHCE) alleles C, c and E in early pregnancy.
Design: A prospective clinical trial was carried out to evaluate diagnostic accuracy.
Setting: Women were recruited at four centres specialising in prenatal diagnosis. Peripheral blood and amniotic fluid samples were obtained and sent to a single laboratory for analysis.
Sample: A total of 233 tests (46 for C, 87 for c and 100 for E) were performed on 181 specimens obtained from pregnant women at weeks 12 to 28 (median week 16) of gestation.
Methods: Following automated extraction of fetal DNA from maternal plasma, two different real-time polymerase chain reaction (PCR) protocols were used for the detection of the C, c and E alleles of RHCE. The results of the PCR were compared with genotyping results for the amniotic fluid.
Main outcome measures: Failure rate, sensitivity, specificity and accuracy were the main outcome measures.
Results: Unequivocal results were obtained for all specimens. With the first PCR protocol, the sensitivity was 100% for C, 38% for c and 59% for E. In contrast, with the second protocol, the sensitivity for C, c and E was 100%. The specificity for all assays was found to be between 99% and 100%.
Conclusions: A highly accurate protocol has been identified for the detection of fetal RHCE alleles in maternal plasma in early pregnancy. This noninvasive approach can be considered as a useful test in the management of pregnancies with anti-c, anti-E or anti-C alloimmunisation.