Measuring quantitative serum human chorionic gonadotropin. Variations in levels between kits.

Human chorionic gonadotropin (hCG) may be used in the monitoring of early pregnancy. It may also be used as a tumor marker in the diagnosis and follow-up of gestational trophoblastic disease, choriocarcinoma and testicular carcinoma. The combination of maternal serum unconjugated estriol, alpha-fetoprotein and quantitative hCG has shown promise as an antepartum screen for Down syndrome. In the quantitative assessment of hCG, the calibrators used by various kits are standardized to one of two different standards, either the Second International Standard or the First International Reference Preparation (IRP), established by the World Health Organization in 1968 and 1975, respectively. The IRP is now considered the Third International Standard, and both terms may be used interchangeably. Confusion may exist in clinical situations if quantitative hCG levels determined by assays in different laboratories using different standards are compared or used simultaneously. Practitioners are advised to be aware of which calibration standard is utilized in their laboratory and to interpret the results accordingly.