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

Nuruddin Mohammed, Department of Obstetrics and Gynaecology, The Aga Khan University Hospital, Karachi.
Fatima Kakal, Department of Obstetrics and Gynaecology, The Aga Khan University Hospital, Karachi.

Abstract

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.