Loss of human chorionic gonadotropin in urine during storage at -20°C

Abstract

Background: Quantitative determination of human chorionic gonadotropin (hCG) in urine is used in population studies of pregnancy disorders and in doping control. For these purposes samples are often stored at -20°C, which in our experience may cause loss of hCG.

Methods: We redetermined hCG in 17 pregnancy urine samples stored at +4, -20 and -80°C for 3-10 months and in 74 cancer urine samples stored at -20°C for 5-14 years. We further studied the effect of added urea, pH and four preservatives on hCG immunoreactivity during storage at +4, -20 and -80°C.

Results: At -20°C, twenty to 100% of hCG immunoreactivity was lost in 15 of 17 pregnancy urine samples and in 43 of 74 cancer urine samples. At -20°C, added urea (0.2-1.0 mol/L) caused a rapid decrease in hCG immunoreactivity, which glycerol (5-10%) prevented.

Conclusion: hCG immunoreactivity is lost in many urine samples during storage at -20°C. Urea most likely plays a role in this process, but other mechanisms contribute to the loss. Urine should not be stored at -20°C before analysis of hCG.