Non-invasive prenatal determination of fetal RhD genotyping from maternal plasma: a preliminary study in Pakistan.

OBJECTIVE To determine the accuracy of the non-invasive pre-natal real-time polymerase chain reaction based fetal RhD genotyping from maternal plasma. STUDY DESIGN Cross-sectional study. PLACE AND DURATION OF STUDY Juma Health Sciences Research Laboratory, The Aga Khan University Hospital, Karachi, from July to December 2008. METHODOLOGY Cell-free plasma DNA from 21 D-negative women with D-positive spouse between 20-39 weeks of gestation was tested for the presence of exon 5 region of RhD gene using real-time polymerase chain reaction. b-globin was employed as the house-keeping gene. Sensitivity and specificity of the real-time PCR-based non-invasive fetal RhD genotyping was obtained by calculating proportion of the D-positive fetuses that were D-positive at birth as well. RESULTS Of the 21 D-negative women 13 and 8 neonates were determined to be D-positive and D-negative, respectively, by serologic studies on cord blood samples at birth. RhD status was correctly determined in 17 of 21 cases. There were three false-positive and one false-negative results. The sensitivity and specificity of the assay was 92.3% (95% CI: 62.1, 99.6) and 62.5% (95% CI: 25.9, 89.8), respectively. The positive and negative predictive value of the assay was 80% (95% CI: 51.4, 94.7) and 83.3% (36.5, 99.1), respectively. CONCLUSION These preliminary results demonstrate the feasibility of non-invasive pre-natal diagnosis of fetal RhD status of D-negative mothers in Pakistan.