The stability of hCG and free beta-subunit in serum samples

Human chorionic gonadotropin (hCG) free beta-subunit measurement is used as a screening test for Down syndrome pregnancies. Use, shipping, and storage conditions have, however, to some extent been limited by a stability problem; the swamping of serum free beta-subunit levels by new molecules coming from the dissociation of hCG. We examined the stability of free beta-subunit levels in six fresh serum samples from the first trimester of pregnancy. The mean hCG level in the fresh serum samples was 3710 +/- 886 micrograms/l; this included a 3.0 +/- 0.39 per cent nicked hCG component. The mean free beta-subunit level was 0.27 +/- 0.04 per cent and the nicked free beta-subunit level was 0.10 +/- 0.04 per cent of the hCG concentration. Samples were incubated for 0, 0.5, 1, 2, and 4 weeks at 21 degrees C, with no additives. After half a week, the free beta-subunit level rose to 137 +/- 17 per cent, and after 4 weeks to 360 +/- 53 per cent of the starting level (ANOVA P < or = 0.05). Parallel, but greater, increases were observed in nicked hCG and nicked free beta-subunit levels. The experiment was repeated with the addition of penicillin-streptomycin-fungizone to fresh serum. After half a week, the free beta-subunit level increased to only 101 +/- 3.0 per cent (t-test, with/without additive, P < or = 0.05), and after 4 weeks to only 136 +/- 7.8 per cent (P < or = 0.05) of the starting level. At 4 weeks, nicked hCG production was reduced from 499 +/- 83 to 158 +/- 15 per cent of the starting level. We infer that nicking of hCG and dissociation of unstable nicked hCG may be pathways supplementing free beta-subunit in serum samples. We further infer that hCG nicking activity comes from microbes, and that free beta-subunit levels can be stabilized for shipping and longer storage by antibiotic/antimycotic additives.