In most of the channelopathies and cardiomyopathies mutations of specific genes have been detected that 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.
Unfortunately this cannot be done at the moment because we don’t have complete knowledge of all the genes involved in every condition. For example, 7 in every 10 people known to have long QT syndrome (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.