Nicked free β-subunit of human chorionic gonadotropin: A potential new marker for Down syndrome screening, ,

OBJECTIVE: Our purpose was to investigate maternal serum levels of nicked free β-subunit in normal and Down syndrome pregnancies. STUDY DESIGN: Serum specimens were obtained from 64 karyotypically normal and 6 Down syndrome pregnancies before amniocentesis. Two immunoenzymometric assays for the β-subunit of human chorionic gonadotropin were used to determine the level of free β-subunits with peptide linkage breaks (”nicks”) between residues 43 to 48. RESULTS: The mean level of nicked free β-subunit in Down syndrome was 4.76 multiples of the median, which was significantly elevated compared with normal controls (107 multiples of the median, p< 0.05). In 5 of 6 cases levels were ≥2 multiples of the median, with a range 1.96 to 15.43 multiples of the median, which was 2- to 3-fold higher than the regular free β-subunit assay (mean level 1.53 multiples of the median, range 0.70 to 3.10 multiples of the median). In one case no nicked free-β subunits were detectable. CONCLUSION: Our preliminary data indicate that nicked free β-subunit of human chorionic gonadotropin might be a sensitive marker for Down syndrome screening. (AM J OBSTET GYNECOL 1996;174:609-11.)