In the context of Familial Hypercholesterolaemia (FH), the Index case is the first person in a family branch to be genetically diagnosed with the condition. The genetic test will have revealed the mutation/variant present in one of the four genes that typically cause FH for that person. The Index case’s genetic test is the first step in facilitating the discovery of which gene is causing FH in that family.

Following the Index case’s diagnosis and identification of the specific gene variant, first-degree relatives can be contacted to be genetically tested for the same gene variant. This is called cascade testing. It is much easier to genetically cascade test relatives as only that same variant will be tested for. First-degree relatives include children, siblings and parents.

Second-degree cascade testing can extend genetic testing to other family members at risk of inheriting the same genetic variant. Then first member of a new family branch to be genetically diagnosed becomes the Index case for that family.

In about 90% of cases, the specific gene variant will be detected, and a genetic diagnosis of FH is made. However, in approximately 10% of people who have clinical signs of FH, a gene variant will not be found.