Cell-free fetal dna testing has a great false-positive in detection of trisomy 18 but not in trisomy 21 and sex chromosomes

Milad Babasalari,1,* Mohammadreza bazrafshani,2 Zahra miri,3 Farida hassanzade,4

1. Department of Medical Genetics, Kerman University of Medical Science
2. Assistant Professor of Kerman University of Medical Science
3. Department of Medical Genetics, Kerman University of Medical Science
4. Dr. Bazrafshani Medical Genetic Laboratory

Abstract

Introduction

After first-trimester scan, cell-free fetal dna (cfdna) testing could be used in women with high-risk, as a primary screening option. although this test is safe and preventing the 0.5-1.0% risk of miscarriage related to amniocentesis/cvs, but the false-positive results of its may be troublesome. based on this suppose, this study was conducted with the aim of examining this issue.

Methods

In this study, cfdna testing was performed on 126 patients with high-risk of trisomy 18, 21 in screening tests that were referred to dr. bazrafshani medical genetic laboratory in 2018. the pregnant women were tested in this study was at 10-14 weeks of gestations. the analysis of cfdna was performed by massively parallel sequencing. the threshold of fetal fraction for test evaluation was about 4-17%. then, invasive prenatal screening (amniocentesis) were performed on patients who the results of their cfdna testing were positive for trisomy 18, 21 and sex chromosomes.

Results

Cfdna testing was performed after first trimester scan in 126 patients. among them 14 patients were high-risk for trisomy of 18, 21 and sex chromosomes. amniocentesis (qf-pcr or karyotype or both of them) were performed these patients to confirm cfdna results. the results of cfdna in detection of trisomy 21 and sex chromosomes were so exact. also, all patients who had positive results for trisomy 21 and sex chromosomes detection had same results in amniocentesis. but we were surprised when the results of cfdna testing were not compatible with the results of amniocentesis. 75% (3 out of 4) patients with high-risk of trisomy 18 in cfdna testing had not positive results in amniocentesis. although in two patients who their cfdna results were positive for trisomy 18, their amniocentesis results were normal. in another patient, although the cfdna testing report was positive for trisomy 18, not only this result did not obtain through amniocentesis but also amniocentesis resulted in 46, xx, t (2;4)(q12;13.2). in this case screening markers candidated her as high-risk for trisomy 21 but the cfdna analysis demonstrated a trisomy for chromosome 18 in this patient.

Conclusion

Our study showed that cfdna testing was successful in detection of aneuploidies of 21 and sex chromosomes but this test has limitation in trisomy 18 detection. paying attention to this results may be informative for better screening and decrease the mental and financial pressure on patients. due to our low sample size we suggest more studies need to be performed.

Keywords

Cell-free dna, cfdna, nipt, false positive rate, amniocentesis