Predictive testing

A predictive test can provide information about whether or not someone will develop or is likely to develop a specific condition, usually at a later stage in life.

The test is usually performed on a blood sample. The blood is analysed in a genetics laboratory to test if they have inherited the faulty gene associated with the condition.

Why might I consider having predictive testing?

If there is a known genetic condition in your family, and the faulty gene that causes that condition is known, then you might be able to have a predictive test to see if you have inherited the faulty gene. You might want to have a test if:

  1. the condition can be prevented or its symptoms effectively treated, or if
  2. the condition can neither be prevented, nor its symptoms effectively treated, but:
  • you want the information to help with decisions about having children, or to tell you more about your own children’s risks.
  • you believe that knowing more about your chance of getting the disease will help you to make important life decisions including decisions about your health care.
  • you are the type of person that prefers to know more about your own future and prefers to live with certainty than with uncertainty.

What do we mean by ‘risk’?

In most cases (apart from cancer), having the faulty gene usually shows that you will go on to develop the condition.

However it is not usually possible, at least at present, to predict at what age you will develop the disease, how you will be affected, or how rapidly the symptoms will progress.

If you feel you may be at risk and want to consider having a predictive test for one of these conditions, you should make an appointment to see a genetic specialist.

What is the procedure for genetic testing?

Before you can proceed with predictive testing, it must be confirmed that you are at risk. First, a detailed family history will be done to see if you are at risk of the genetic condition. In most cases, if genetic testing is possible, a close relative of yours who is affected by the condition will be offered genetic testing first in order to identify the faulty gene that runs in the family. If a specific genetic fault is found in their blood sample then it is possible to take a blood sample from you to see if you have also inherited the same genetic fault. If the result of this test shows that you have inherited the same genetic fault then you would also be at risk of developing the condition during your lifetime. If you have not inherited the fault then you would not be at increased risk. For some conditions such as Huntington disease, it will not be necessary to test an affected family member first because scientists already know what changed gene to look for.

The decision to have a predictive test can be difficult, we discuss some of the most important issues to think about here.