A case of consistent discrepancies between urine and blood human chorionic gonadotropin measurements

Abstract

Our laboratory was confronted with two successive urine samples from a single patient which tested positive for human chorionic gonadotropin (hCG) when tested with both qualitative and quantitative assays, combined with no detectable hCG in corresponding plasma samples. Serial dilution and recovery experiments were performed in order to investigate the presence of interfering substances or a high-dose hook effect. The ovarian cysts that were removed from this patient were immunohistochemically stained using polyclonal anti-human hCG antibodies. Furthermore, a urine sample was sent to the USA hCG Reference Service for hCG variant analysis. Dilution and recovery experiments in urine and plasma samples were unremarkable. The biopsy stained negative for human hCG and free β-subunit. hCG isoform analysis in the urine sample revealed that approximately 87.5% of the immunoreactive hCG lacked the β-subunit C-terminal peptide (CTP). We report a rare case in which two successive urine samples test positive for hCG whereas in corresponding plasma samples hCG is undetectable. The majority of the total hCG contained a degraded form of β-subunit that lacks the CTP. This hCG variant, possibly of pituitary origin, is thought to have an extreme fast clearance rate possibly explaining the discordance between the hCG results in urine and plasma samples.