Diagnostic accuracy of noninvasive polymerase chain reaction testing for the determination of fetal rhesus C, c and E status in early pregnancy

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.