Apolipoprotein E genotyping

ApoE genotyping is sometimes used as part of follow-up testing if high cholesterol and triglyceride levels 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 also sometimes used to help confirm a diagnosis of probable late onset Alzheimer's disease (AD) in adults with symptoms. It is called 'susceptibility' or 'risk factor' testing because it indicates whether there is an increased risk of developing AD but is not diagnostic of AD. There are no clear-cut tests to diagnose Alzheimer's disease; the best a doctor can do right now is to rule out anything else that may be causing dementia and check for a genetic predisposition to AD (with ApoE genotyping, perhaps in conjunction with Tau/Aß42 testing).

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.

ApoE genotyping is also sometimes requested when patients have symptoms of progressive dementia, such as decreasing intellectual ability, language and speech skills, memory loss, and personality and behavioural changes that are starting to interfere with daily living. After non-AD causes, such as overmedication, vascular dementia (caused by strokes), and thyroid disease, have been ruled out, ApoE genotyping may help establish a diagnosis of probable Alzheimer's disease.

Patients with ApoE e2/e2 alleles 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. ApoE genotyping adds additional information and, if symptoms are present, e2/e2 is diagnostic of Type III hyperlipoproteinaemia (although diagnosis must be made in conjunction with other test results and the patient's clinical history).

Patients who have ApoE e4/e4 are more likely to develop atherosclerosis. Patients who have symptoms of late onset Alzheimer's disease AND have one or more e4 copies of the Apo E gene are more likely to have AD. It is not diagnostic of AD, though, and should NOT be used to screen patients without symptoms or their family members. Many people will have e4 alleles and never develop AD. Even in symptomatic patients, only about 60% of those with late onset AD will have ApoE e4 alleles.

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 will need to be sent to a reference laboratory and results may take longer to return than they would from other laboratories.