Free β-hCG Subunit Versus Intact hCG in Down Syndrome Screening

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Abstract
Objective: To assess the ability of second-trimester maternal serum free β-hCG to detect fetal Down syndrome and to compare free β-hCG to intact hCG in the multiple-marker screening test for Down syndrome. Methods: From our bank of stored maternal sera, we selected 40–50 samples from euploid pregnancies at each week of gestation from 14 to 20 weeks and 31 samples from Down syndrome pregnancies. Free β-hCG was measured by enzyme-linked immunosorbent assay, and week-specific multiples of the median (MoM) were derived. The free β-hCG Down syndrome detection and false-positive rates were determined. Free β-hCG was then substituted for intact hCG in the multiple-marker screening test, and the Down syndrome detection and false-positive rates at various risk cutoffs were compared. Results: The mean (± standard deviation) maternal age of all study samples was 35.6 ± 5.3 years. The mean Down syndrome free β-hCG MoM was significantly higher than the mean euploid MoM (2.4 ± 1.1 versus 1.2 ± 1.0; P < .001). A free β-hCG level of at least 1.7 MoM identified 68% of Down syndrome pregnancies at a false-positive rate of 20%. When intact hCG was replaced with free β-hCG in the multiple-marker screening test, a higher Down syndrome detection rate was achieved at a lower false-positive rate at each of several screen positive risk cutoffs. Conclusion: Elevated free β-hCG levels identify Down syndrome pregnancies. Replacing intact hCG with free β-hCG in the multiple-marker screening test results in a higher Down syndrome detection rate at a lower false-positive rate.

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