Genetic testing for familial hypercholesterolemia (FH) starts by sequencing specific genes in the person first diagnosed (called the proband or index case). The key genes tested are LDLR, APOB, and PCSK9.

Instead of using the word “mutation,” we now use “variant” to describe any change in a gene. Since we inherit two copies of each gene (one from each parent), a variant can be:

• Heterozygous: one copy of the variant and one normal copy.
• Homozygous: two copies of the variant.

FH is usually caused by heterozygous variants, but homozygous variants lead to much higher cholesterol levels. This pattern is called semi-dominant inheritance. Over 2000 different variants causing FH have been identified so far.

Once a variant is found, it needs to be classified. The lab scientist looks for evidence to decide if the variant causes FH. This includes how rare the variant is, whether it has been seen in other FH index cases or families, and how it affects the protein produced by the gene.

Reported variants are classified as:

• Pathogenic: almost certainly causes FH.
• Likely pathogenic: more than 90% chance it causes FH.
• Uncertain significance: not enough evidence yet to classify as pathogenic or benign.

Classifications can change over time as new evidence appears, such as family studies showing whether the variant is linked to high cholesterol.

For cascade testing (testing family members), only the specific variant found in the index case is checked. This test is faster and easier for the lab.