APOE genotype is sometimes used as part of follow-up testing if high cholesterol and triglyceride concentrations are found, to check if a particular lipid abnormality has been genetically inherited. It is not widely used, but when it is requested, it may be in combination with other tests, such as lipoprotein electrophoresis.

APOE genotyping is sometimes requested when a patient has significantly elevated cholesterol and triglyceride levels that do not respond to changes in the patient's lifestyle (dietary and exercise patterns); when a patient presents with xanthomas (yellowish raised patches) on their skin and the doctor suspects Type III hyperlipoproteinaemia; or when family members have APOE e2/e2 and a doctor wants to see if the patient may be at a higher risk for early heart disease.
 

Patients with APOE e2/e2 genotype are at a higher risk of premature vascular disease, but they may never develop disease. Likewise, they may have the disease and not have e2/e2 alleles because it is only one of the factors involved. In fact only 1 in 50 people with e2/e2 will ever manifest the disease. Apo E genotyping adds additional information and, if symptoms are present, e2/e2 is diagnostic of Type III hyperlipoproteinaemia (although the diagnosis must be made in conjunction with other test results and the patient’s clinical history). Therefore, this test is limited and used solely by lipid specialists on rare occasions.

Although APOE genotyping is being used clinically by Alzheimer's experts, the most it can provide at this time is additional information about a patient with dementia. A definite diagnosis of Alzheimer's disease can only be made by examining a patient's brain tissue after their death.

APOE genotyping is not available in many laboratories. If your doctor recommends this test, your specimen may need to be sent to a reference laboratory and results may take longer to return than they would from local laboratories.

Alterations in lipid concentrations are only one risk factor for, and do not lead directly to, vascular disease or atherosclerosis. Other factors, such as obesity, diabetes, smoking, high blood pressure and hypothyroidism, may also play a role in whether a person actually develops disease.

No, not at this time. Forty percent of those who do have late onset AD are negative for APOE e4 alleles.

Currently there are only three unknown genes that associated with AD.
Mutations of these genes (PSEN1, PSEN2 and APP) are associated with AD in a very small number of specific family lines, they tend to be associated with early onset AD, rather than late onset. If your father did not show signs of AD until after the age of 65, then these other genetic tests are not indicated. (If you have a very strong family history of AD, several family members over several generations have had AD, you may want to talk to your father's doctor about family risk factors).

Not necessarily. Most people who have Down syndrome will eventually have some degree of Alzheimer's disease symptoms. Down syndrome is associated with a lifelong overproduction of amyloid precursor protein; a portion of this protein, called amyloid beta 42 peptide (Aß42), is associated with the formation of senile plaques (areas of dead nerve cells and protein deposits in the brain) that are characteristic of AD.

The inheritance of APOE e2/2 in a person does not mean that they will develop type III hyperlipoproteinaemia. Only 1 in 50 people with APOE e2/2 will develop it; basically of 50 people with that combination of genes, 49 will be fine and 1 person will develop the condition. For your child to also be APOE e2/2 then they would have to inherit an e2 from the other parent. 
Pre-natal genetics and testing is not done, however as one percent of the population has the genetic pattern (APOE e2/e2) but the vast majority will never see ill effects from it. Other factors appear to determine whether the genes ever cause a problem including medical conditions such as hypothyroidism or diabetes mellitus. Therefore, the fact that you have have type III hyperlipoproteinaemia migh raise the risk to your child very slightly above that of a randomly selected member of the general population but it is only slight and depends on too many unpredictable factors making routine testing of family members unnecessary.

No, the test is not intended to be used to screen the general population. It is intended to be used in very specific situations to give a doctor additional information. The majority of people have APOE e3 and so will tend to have normal lipid metabolism. Most of the people with other APOE combinations will either; never develop significant problems associated with their lipid metabolism, have type III hyperlipoproteinaemia, or develop APOE associated late onset AD.

No, not unless your doctor suspects that the first test was in error. A person inherits one copy of the gene from each parent and genotype does not change.