Predictive genetic diagnostics are understood to mean the examination of a phenotypically healthy person for mutations that create a predisposition to diseases in later life. On the basis of predictive testing it is therefore possible to diagnose the existence of a genetic predisposition long before onset of the disease.
However, since what are involved here are merely probability statements about the occurrence of a hereditary disease, the informational value varies from case to case. The higher the risk of becoming ill (penetrance) with a hereditary disease, the higher the positive predictive value that onset of the disease will really occur.
If, for example, a predictive genetic test determines that a person has a mutation on the fourth chromosome that is typical of Huntington's disease there is a near 100 % probability that the still healthy carrier of the mutation will become ill with Huntington's disease in the course of his or her life. The time of onset and the precise course of the disease remain, however, uncertain.
Yet the positive predictive value of a mutation, i.e. the probability with which a test result points to the actual existence or non-existence of a disease, can also be low. It is entirely possible that a genetic test may indicate the existence of a predisposition to a certain disease (positive predictive value), without onset of the disease ever occurring. Haemochromatosis (an autosomal recessive hereditary iron overload disorder) illustrates the problems associated with predictive diagnostics in low-penetrant genotypes: if haemochromatosis goes untreated, severe life-shortening organ damage will result sooner or later. Onset of the disorder can, however, be prevented by regular blood removal. The problem is that only 1–2 % of those persons in whom a predisposition to haemochromatosis has been diagnosed actually develop the disorder in the course of their lives.