Objective To evaluate the accuracy of NGS-based quantitative cfDNA analysis for fetal antigen genotyping in alloimmunized pregnancies undergoing clinical testing across US practices. Timely identification of the fetal red blood cell antigen genotype for the antigen to which the pregnant person is alloimmunized is vital for determining fetal risk for HDFN and guiding management. Presently in the US, recommended care is to determine fetal antigen genotype with reproductive partner testing and/or amniocentesis. This approach has many limitations, including availability of reproductive partner testing, risk of nonpaternity, and low uptake of invasive testing such as amniocentesis. These barriers to obtaining fetal antigen genotype information lead to pregnancies not at risk for HDFN undergoing burdensome monitoring and, in some cases, unnecessary intervention. PCR-based qualitative cfDNA analysis for fetal antigen genotyping is available in Europe, however, it is offered at later gestational ages, may require a repeat sample, has a higher frequency of inconclusive results for individuals of non-European ancestry, and entails logistical challenges related to shipping and insurance coverage for patients in the US. The availability of a NGS-based quantitative cfDNA analysis for fetal antigen genotyping in the US that is robust for diverse populations and applicable for singleton and twin pregnancies starting at 10 weeks gestation presents an opportunity to assess performance.

Methods Patients with alloimmunized pregnancies undergoing clinical fetal antigen cfDNA analysis were recruited to the study along with the neonates resulting from the pregnancies. The laboratory issued the results prospectively as a part of clinical care. After delivery, neonatal buccal swabs were sent to an outside laboratory, blinded to the fetal cfDNA results, for antigen genotyping, and the results were compared. Concordance was reported for the fetal antigen cfDNA analysis for antigens to which the pregnant person was alloimmunized as well as for all antigens for which the pregnant person was genotype negative.

Results We observed complete concordance between the fetal antigen cfDNA analysis result and neonatal genotypes for 503 calls, for 100% sensitivity, specificity, PPV, and NPV across a racial and ethnically diverse cohort.

Conclusion This study demonstrates that cfDNA analysis for determining fetal antigen genotype is more accurate than real-life application of the current recommendations, ie., partner testing and amniocentesis, in a diverse US population. In addition, this noninvasive approach reduces barriers to obtaining timely, accurate information about fetal antigen genotype. These results support the routine implementation of fetal antigen cfDNA analysis to guide care of alloimmunized pregnancies in the US.

J.H., and J.W. are employees of BillionToOne, Inc. and have options/equity in BillionToOne, Inc. H.K. and A.W. are paid contractors with BillionToOne, Inc. K.J.M. is a paid consultant of BillionToOne, Inc. S.R. was an employee of BillionToOne, Inc. O.A.A., K.E., and R.O. received research funding from BillionToOne, Inc. J.M.G and G.A.D. have no conflicts of interest.

This study did not receive any funding

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