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Background: The interpretation of changes in serum human chorionic gonadotropin (hCG) concentrations in early pregnancy requires a knowledge of the day-to-day imprecision of the method at the concentrations measured. We therefore attempted to determine the analytical imprecision of our total hCG method over a 4-week period at concentrations up to 20 000 IU/L.    Methods: Serum specimens with hCG concentrations between 100 and 20 000 IU/L were collected during early pregnancy and analysed using the Bayer Centaur Total hCG method at weekly intervals after storage at +4°C and -20° C.    Results: The reproducibility of hCG results over a 4-week period in refrigerated specimens with an initial hCG concentration below 1000 IU/L was poor (mean coefficient of variation = 17.4%) and there was an apparent increase in serum hCG concentrations of up to 68%. Similar changes occurred in some specimens stored at -20°C. Further experiments confirmed that significant increases can occur during the first week following specimen collection.    Conclusions: Total hCG concentrations in serum specimens collected during early pregnancy increase significantly during storage when measured by the Bayer Centaur Total hCG method, possibly due to a conformational change in hyperglycosylated hCG. When this method is used for monitoring early pregnancy, specimens should ideally be analysed on the day of collection.

Increasing hCG concentrations during storage at +4°C with the Bayer Centaur Total hCG method