Cell free DNA (cfDNA) screening has been established as routine prenatal care to assess the risk of common numerical aneuploidies (trisomy 21, 18, and 13). Clinical laboratories use a variety of methodologies for cfDNA screening, including massively parallel sequencing, fluorescent imaging of single DNA molecules, and single nucleotide polymorphism (SNP)-based platforms. All cfDNA screening has high sensitivity and high specificity, but false-positive and false-negative results still occur. False-negative cfDNA results involving trisomy 13 and trisomy 18 are rare but have significant clinical and emotional impacts on families and healthcare providers. We describe two cases of fetuses with low-risk cfDNA aneuploidy screening results from a SNP-based platform and a subsequent diagnosis of a numerical aneuploidy involving an isochromosome or isodicentric chromosome. Case one presented to maternal fetal medicine care at 30 weeks gestational age with multiple anomalies. Serum screening performed at 13 weeks was low risk for trisomy 21 (1 in 1,000) and trisomy 18 (1 in 10,000). Trisomy 13 risk was not assessed. At a 25-week ultrasound, a two-vessel cord and urinary tract dilation were identified. Follow-up ultrasound at 29 weeks identified multiple anomalies: inferior vermian hypoplasia, small cerebellum, enlarged cisterna magna, left-sided congenital diaphragmatic hernia, pleural effusion, enlarged echogenic kidneys with duplicated collecting systems, bilateral foot polydactyly, and micrognathia. The patient declined amniocentesis. SNP-based cfDNA aneuploidy screening was performed at 30 weeks, and the results returned low risk (<1 in 10,000) for trisomy 21, trisomy 13, trisomy 18, and monosomy X, with a fetal fraction of 24.2%. Fetal echocardiogram at 32 weeks identified an atrial septal defect, ventral septal defects, and minor valve abnormalities. Fetal MRI at 32 weeks identified severe microphthalmia, hypotelorism, incomplete cleft palate, postaxial polydactyly of the right hand, cerebral dysgenesis, corpus callosum hypoplasia, septum pellucidum abnormality, dysmorphic lateral ventricles, prominent extra-axial spaces, misshapen pons, and brachycephaly, and it confirmed the findings seen on prior ultrasounds. The pregnancy was delivered at 34 weeks, and the child passed away on day of life three. Postnatal karyotype results were abnormal: 46,XX,+13,der(13;13)(q10;q10). This result is consistent with an unbalanced Robertsonian translocation resulting in three copies of chromosome 13, demonstrating that isochromosome 13 was not detected by the SNP-based cfDNA aneuploidy screening. Follow-up parental karyotypes were normal, confirming that the translocation occurred de novo. Case two presented to maternal fetal medicine care at 12 weeks gestational age due to an increased nuchal translucency of 5.54 mm. SNP-based cfDNA aneuploidy screening was performed at 10 weeks, and the results returned low risk (<1 in 10,000) for trisomy 21, trisomy 13, trisomy 18, and monosomy X, with a fetal fraction of 11.4%. Chorionic villus sampling was performed at 13 weeks. CVS karyotype results were abnormal: 46,XY,idic(18)(p11.3)[5]/46,XY[15]. CVS microarray was also abnormal with a 4.9Mb deletion of 18p11.32p11.31, 3.9Mb duplication of 18p11.31p11.2, and 69Mb duplication of 18p11.2q23. This result is consistent with an isodicentric chromosome 18, likely to produce a phenotype similar to trisomy 18, and was not detected by the SNP-based cfDNA aneuploidy screening. A follow-up ultrasound at 16 weeks identified a two-vessel cord, marginal cord insertion, umbilical cord cyst, omphalocele, absent nasal bone, strawberry-shaped skull, and a unilateral choroid plexus cyst. The pregnancy ended in an uncomplicated dilation and evacuation at 16 weeks gestational age. Parental follow-up testing was not pursued. We present two cases of false-negative cfDNA aneuploidy screening results due to isochromosome 13 and isodicentric chromosome 18. It is critical that providers and patients understand that cfDNA testing is a screening test where false negative results are rare but possible. While SNP-based cfDNA is a highly effective cfDNA screening methodology, it has unique limitations for trisomies caused by isochromosomes and isodicentric chromosomes.
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