Interaction between heparin and cardiac troponin T and troponin I from patients after coronary bypass surgery

OBJECTIVES: Heparin is thought to play a crucial role in the clinical monitoring of patients with acute coronary syndrome as well as after coronary bypass surgery in that it interferes with different commercial immunoassay test systems for cardiac troponin T (cTnT) and troponin I (cTnI). The mechanism, however, by which heparin apparently affects the cTnT and cTnI levels in plasma is not yet resolved. DESIGN AND METHODS: We analyzed the effect of heparin by simultaneously collecting serum and heparin plasma samples from 32 patients after coronary bypass surgery. The cTnT and cTnI levels were determined using the Roche/Elecsys, the Dade-Behring/Opus and the Bayer/ACS:Centaur immunoassay systems in the absence or in the presence of either heparinase or protamine. Association between the cardiac troponins and the anticoagulant has been demonstrated by affinity chromatography using heparin as the ligand. RESULTS: The data obtained indicate that heparin produces an apparent decrease in cTnT as well as of cTnI levels, analyzed either by the Elecsys, the Opus or the ACS:Centaur immunoassay systems. Individual patients show a wide variation in the discrepancies between serum and heparin plasma troponin especially in the cTnT immunoassay. Pretreatment of the heparin plasma samples either with heparinase or protamine cannot completely reverse the heparin-induced decrease in cTnT and cTnI levels and therefore addition of these reagents to the commercial test systems could not significantly improve the performance of the assay. When serum is supplemented with increasing concentrations of heparin, and cardiac troponin levels were reanalysed, significantly lower recoveries for the cTnT than for the cTnI immunoassays were detectable. Affinity chromatography with heparin Sepharose demonstrates that cTnT and cTnI interact differentially with the negatively charged ligand. Whereas cTnI shows minor affinity to the immobilized heparin and is eluted at near physiological conditions, cTnT is bound and can be quantitatively recovered only by solutions of high ionic strength. CONCLUSIONS: We conclude, therefore, that the apparent decrease in cTnT values by addition of heparin is a result of direct molecular interaction between the negatively charged glycosaminoglycan and clusters of basic residues within the sequence of the cardiac protein. In contrast, the effect of heparin on the cTnI immunoassay systems, is primarily indirect, most possibly induced by changes within the sample matrix itself.