To identify a blood test that can differentiate cardiac from skeletal muscle injury in animals, we compared tissue reactivities for various species with the use of an immunoassay for human cardiac troponin I (cTnI). Tissue reactivity varied as a function of the homology of tissue troponin with human cTnI. Cardiac reactivity in large mammals was equivalent to cTnI, 9.8 +/- 0.6 mg/g, and was 2-fold, 10-fold, and 100-fold greater than in small mammals, birds, and fish, respectively. Skeletal muscle reactivity was equivalent to cTnI, 5.1 +/- 0.6 micrograms/g, in all species except fish, in which it was 50% lower. The ratio of reactivities of cardiac and skeletal muscle was: 1800 in large mammals, 1100 in small mammals, 230 in birds, and 43 in fish. We conclude that cTnI is a powerful candidate in mammals, a possible candidate in birds, but unlikely to be of use in fish as a sensitive and tissue-selective diagnostic test for cardiac injury.
