It is difficult to diagnose cardiac-related death only by external examination. However, in the areas where no medical examiner system is established, the diagnosis is made mostly from the findings by external examination. Clinically, cardiac troponin T (cTnT) rapid assay kits are used for the diagnosis of ischemic heart disease. The kits have started being applied to forensic medicine and used for the diagnosis of cardiac-related death without exact evidence. In this study, we collected postmortem blood samples from 110 forensic autopsy cases and evaluated the utility of the kits in the field of forensic medicine. Blood samples collected from the left and the right parts of the hearts showed extremely high positive rates (95.1% each), and peripheral blood samples also showed very high positive rates (71.2%), although cardiac-related deaths were only 7 cases of all 110 cases. Because cTnT concentration of postmortem blood samples is affected by non-cardiac-related natural death, postmortem changes, or cardiopulmonary resuscitation, the results from using the kits are not reliable in determining the cause of death. We conclude that the kits cannot be applied in forensic medicine.