Human chorionic gonadotropin discriminatory zone in ectopic pregnancy: does assay harmonization matter?

OBJECTIVE: To determine the effect that lack of hCG assay harmonization has on the interpretation of a serum hCG concentration with regards to the hCG discriminatory zone. DESIGN: A multisite method comparison study. SETTING: Clinical laboratories. PATIENT(S): Eighty serum samples containing various concentrations of hCG. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Concentrations of hCG obtained from seven hCG reagent platforms. RESULT(S): The hCG concentrations were significantly different across hCG reagent platforms. Seventy-one percent of assay pairs showed significant differences with samples selected based on hCG concentrations between 1,500 and 3,500 IU/L as determined by a comparative method. Relative to the comparative method, the calculated hCG discriminatory zones for five assays were within 9%, and one assay was within 40% of the target concentrations of 1,500 and 3,500 IU/L. CONCLUSION(S): Despite significant differences in hCG concentrations across hCG immunoassays, an hCG concentration within a discriminatory zone of 1,500-3,500 IU/L can be used for all but one commonly used hCG reagent platform.