The present invention relates to stable compositions useful as primary standards and calibrators and controls comprising a cardiac troponin I (cTnI) such as native, recombinant, addition and deletion forms thereof, whether or not complexed with other troponin subunits such as TnC and/or TnT, in an inactivated human serum. The compositions are obtained by incubating troponin complexes with human serum. The compositions are characterized by an immunodetectability ratio of epitopes on the N-terminal segment to epitopes on the C-terminal segment substantially equivalent to that of pooled, fresh serum from acute myocardial infarction patients.