The disclosure describes immunoassay methods for measurements of concentrations of related analytes in samples of body fluids from different mammalian species. More particularly, the disclosure describes reagents and methods that may be used for assays of a mammalian antigen across a range of different mammalian species, using immunological reagents based on non-mammalian antibodies the mammalian antigen. In addition, a calibrator or a set of calibrators are described that are assigned with concentration values for the antigen for each of the different mammalian species.