Genetic testing is used because, in most of the inherited conditions known to cause SADS, mutations of specific genes have been detected and are thought to cause a specific disease.
So in principle, if we could identify these mutations, we would be able to make a diagnosis in any DNA sample including any obtained from SADS victims at their autopsy or from their relatives who have given blood.
Unfortunately, this cannot be done at the moment because we don’t have complete knowledge of all the genes involved in any condition. For example, 7 in every 10 people known to have LQTS have mutations of known identified genes. Also, many variations in the DNA code are found in a large number of people and do not necessarily cause any disease. Many families with LQTS have mutations specific to them (‘private’ mutations) which can also make it difficult to decide whether it is the mutation that is causing the disease or not.
As research progresses, more genes will be identified and there will be better tools to decide whether the impact of a mutation causes a disease.