- Serum hCG reference intervals for various gestational periods in normal pregnancies were determined using three commercial assays--two standardized against the WHO 2nd IS (Amersham Amerlex-M beta HCG RIA (AMX) and Abbott beta-HCG 15/15 (ABB] and one standardized against the WHO 1 IRP (Hybritech Tandem -E HCG (HYB]. Serial samples from patients with accurately determined gestational periods were analyzed. We correlated these assays to determine the validity of the common practice of interchanging values between assays using the same WHO standard and of converting 1st IRP values to 2nd IS values by a fixed factor. The slope of correlation between the two 2nd IS assays (AMX, ABB) was 1.43, r = 0.960; whereas between the 1 IRP assay (HYB) and the two 2nd IS assays the slopes were 1.67, r = 0.963 and 1.22, r = 0.971 for AMX and ABB, respectively. In a prospective study of 52 patients with normal pregnancies, serum beta-hCG values in 46% of samples taken at 28-35 days gestation fell below the lower limit of the reference curves supplied with the AMX kit. Ninety-two percent of samples were within the newly established intervals. These results indicate that supplier's reference limits may not be accurate; in addition, a common factor should not be used to convert values from one commercial kit to another.