There are various risk factors for Alzheimer’s, but the role of genetics is becoming much more clear. The apolipoprotein E (APOE) test detects the presence of the APOE E4 gene, which is the strongest genetic risk factor for late-onset (after the age of 65) Alzheimer's disease.
There are three types of the APOE gene, called alleles: APOE E2, E3, and E4. Each person inherits two APOE alleles, one from each biological parent, and the combination determines their genotype. People with E2/E2 have the lowest overall risk for Alzheimer's and those with E4/E4 have the highest risk. The other combinations of APOE (E2/E3, E2/E4, E3/E3, and E3/E4) fall in between.
- APOE E2 is the least common gene and may provide some protection against Alzheimer’s.
- APOE E3 is the most common gene and is neutral, so doesn’t affect the risk of Alzheimer's.
- APOE E4 is the risk-factor gene and increases the risk of Alzheimer's. It is also associated with getting the disease at an earlier age.* Approximately 25% of the general population carry one copy of the APOE E4 allele, while 2-3% carry two copies.
* Inheriting an APOE E4 allele does not mean that a person will definitely develop Alzheimer's. Some people with an APOE E4 allele never get the disease. Others who develop Alzheimer's do not have any APOE E4 alleles, suggesting that the APOE E4 gene affects risk but is not a cause.