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

Abstract

BackgroundQuantitative 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. MethodsWe redetermined hCG in 17 pregnancy urine samples stored at +4, −20 and −80°C for 3–10months and in 74 cancer urine samples stored at −20°C for 5–14years. We further studied the effect of added urea, pH and four preservatives on hCG immunoreactivity during storage at +4, −20 and −80°C. ResultsAt −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 (0.2–1.0mol/L) caused a rapid decrease in hCG immunoreactivity, which glycerol (5–10%) prevented. ConclusionhCG 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.