Hyperglycosylated human chorionic gonadotropin (invasive trophoblast antigen) immunoassay: A new basis for gestational Down syndrome screening

Background: Serum human chorionic gonadotropin (hCG) and hCG free β- subunit tests are used in combination with unconjugated estriol and α- fetoprotein in the triple screen test, and with the addition of inhibin-A in the quadruple marker test for detecting Down syndrome in the second trimester of pregnancy. These tests have a limited detection rate for Down syndrome: ~40% for hCG or free β-subunit alone, ~60% for the triple screen test, and ~70% for the quadruple marker test, all at 5%, or a relatively high, false- positive rate. New tests are needed with higher detection and lower false rates. Hyperglycosylated hCG (also known as invasive trophoblast antigen or ITA) is a new test. It specifically detects a unique oligosaccharide variant of hCG associated with Down syndrome pregnancies. We evaluated this new Down syndrome-directed test in prenatal diagnosis. Methods: Hyperglycosylated hCG was measured in urine samples from women undergoing amniocentesis for advanced maternal age concerns at 14-22 weeks of gestation, 1448 with normal karyotype and 39 with Down syndrome fetuses. Results: The median hyperglycosylated hCG value was 9.5-fold higher in Down syndrome cases (9.5 multiples of the normal karyotype median). The single test detected 80% of Down syndrome cases at a 5% false-positive rate. Urine hyperglycosylated hCG was combined with urine β-core fragment (urine breakdown product of serum hCG free β-subunit), serum α-fetoprotein, and maternal age-related risk. This urine-serum combination detected 96% of Down syndrome cases at a 5% false-positive rate, 94% of cases at a 3% false-positive rate, and 71% of cases at a 1% false-positive rate. These detection rates exceed those of any previously reported combination of biochemical markers. Conclusions: Hyperglycosylated hCG is a new base marker for Down syndrome screening in the second trimester of pregnancy. The measurement of hyperglycosylated hCG can fundamentally improve the performance of Down syndrome screening protocols.