Urinary Levels of Intact, Free Beta, and Beta Core Fragment of hCG in Early Pregnancy [35

Abstract

INTRODUCTION: Reference ranges have been published for intact urinary human chorionic gonadotropin (hCG) but not for other forms. Free β and β core fragment have different profiles of daily rise of hCG compared with intact hCG. Very high levels of β core can cause false-negative point-of-care pregnancy test results, and ratio of intact to free β hCG has been related to pregnancy viability. Therefore, we sought to improve the understanding of the levels of free β and β core hCG in viable pregnancies. METHODS: Early morning urine samples were collected from 37 women with viable pregnancies throughout early pregnancy. Intact, free β, and β core hCG were measured using DELFIA immunoassays. Median levels and centile ranges by day of pregnancy were derived. RESULTS: As expected, intact hCG was present 8 days after ovulation; however, free β hCG was not measurable until day 21. Free β hCG occurred at a constant 1/100th ratio of intact hCG. Beta core hCG had a different profile, appearing in urine later than intact hCG (day 19) yet becoming the predominant form by day 35. CONCLUSION: The ranges of free β and β core hCG in viable pregnancies provide a valuable reference tool. Although levels of β core hCG in early pregnancy are negligible, they can reach 500,000 pmol/L by day 28 postovulation, a level shown to cause false-negative results in some tests. Therefore, only assays that demonstrate they are unaffected by β core hCG interference should be used in later pregnancy.