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Genetic Testing

Genetic Testing

In 1993, scientists discovered exactly where the gene which causes Huntington's disease is located in our DNA and figured out what it was doing. Because of this people are now able to have what is called a 'predictive test'. It is known as a predictive test because the test can be taken by people at risk of Huntington's disease who currently show no symptoms and want to learn whether they will develop the condition in the future. The result of this test will usually be able to tell a person whether they have the expanded gene or not.

It is important to note that if a person already has symptoms but has not been diagnosed, then a different approach would be taken which is known as 'symptomatic testing'. This would involve that person going through a series of assessments with a doctor to confirm a diagnosis of Huntington's disease and is quite different to 'Predictive Testing', which is what we are focusing on in this section.

The Science Behind Testing

​You may hear people say that they 'have the gene' when referring to themselves as having Huntington's disease. But the truth is all humans have the gene that may cause Huntington's disease - even those who never develop the condition. So if we all have the gene, why do some develop Huntington's disease when others do not? The difference is people who develop symptoms of Huntington's disease have a larger expansion of the gene than normal.


What is Gene Expansion?

Without getting too scientific (which is hard as we are discussing DNA!), there is a section, in the gene, which repeats itself over and over again. This is known as a 'CAG repeat' and each individual has a different amount of repeats. However, people with Huntington's disease tend to have more repeats than usual, which means that the gene expands more than it should. This expansion can lead to Huntington's disease.

The reason we are explaining CAG repeats is because of their relevance to predictive testing for Huntington's disease. When people at-risk for Huntington's disease have a predictive test, that person's DNA is sent to a lab, and the clinical scientists will use special machines and lab techniques that will count how many CAG repeats that person has in the gene specific to Huntington's disease. Once they know how many repeats a person has, they can tell if they will get the disease or not.


How Many CAG Repeats Will Cause Huntington's Disease?

CAG repeats are split into four categories. If a person has less than 27 CAG repeats then they are what is called, 'negative' — meaning they will not get Huntington's disease and they are not at risk of passing the disease onto any children that person has. Someone who has 40 CAG repeats or more will definitely develop symptoms of Huntington's disease in their lifetime (although it is not possible to say when symptoms will start) and the risk to any children is 50%.

If a person has between 27-35 CAG repeats, they will not develop Huntington's disease, but there will still be a very small risk of that person passing on the disease to any children (the risk is about 5%). Whereas somebody who has a CAG repeat between 36-39, would be in what is sometimes called the 'grey area', because it is uncertain whether they will develop symptoms in their lifetime or not.

Cag Repeats Status Prognosis
Less than 27 CAG repeats Negative Will not develop Huntington's disease and children not at risk.
27-35 CAG repeats Negative Will not develop the disease but small risk to children.
36-39 CAG repeats Positive May develop symptoms at an older age. 50% risk to children.
40+ CAG repeats Positive Will develop symptoms in lifetime. 50% risk to children.


Genetic Counselling and the Testing Process

As you have read above, testing is ultimately about finding out your CAG repeat - and this can be done with a blood test. But taking a predictive test in order to find out whether you will get Huntington's disease or not is a huge decision to make in one's life, and an enormous thing to deal with emotionally. What if a person gets a positive result? Would they be able to cope? Can 
they live positively with this news? These are questions that have to be asked, and that is why, before anybody can have a predictive test for Huntington's disease, they should undergo genetic counselling. How long the counselling takes depends on you and your counsellor, and your particular personal circumstances but generally it can take anything from 2-6 months.
To begin the testing process you usually need to speak with your doctor, who will then put you in contact with your local genetics department. In order to be tested you normally have to be at least 18 years old - although if you are under 18, you can still speak with a genetic counsellor about what a test involves and any other issues you may have with regards to Huntington's disease.

If at any stage during the counselling process you change your mind and do not want to be tested, you can stop the process straight away. Some people even get all the way to having their blood taken and decide now is not the right time for them to hear their results, so they ask for their results to be kept under lock and key until they feel ready to collect them. The doctors and genetic counsellors will not read your results until you've decided to receive the news.

It Is Your Choice

It is very important to remember that testing is a personal choice — a choice only you have the right to make. Some people in your life may tell you to test or not to test. But it is not up to them at all - it is entirely your decision and you should only get tested if you are sure this is what you want.
There is no right or wrong answer when it comes to making a decision about testing. People are different! We all live different lives and we all make different decisions. Some will choose to test (& will vary at what stage in their life they choose to do so), others will not - there are perfectly good reasons for deciding one way or the other. Although it is your decision, you may also wish to discuss your feelings with those closest to you (like a partner) for whom the result will also have a bearing. But ultimately it is your choice.


If you decide that you want to be tested and feel now is the right time for you, then it is important to be prepared. One of the main practical things you may wish to do before you get your test result, is to get life insurance, as you may experience higher premiums if you seek insurance after you have tested (positive). Whereas if you already have insurance before you test, then you do not need to disclose your test results to insurers and your results will not affect your existing insurance. Issues with insurance vary depending on where you are and which country you are in. If you do not have insurance and want to get some before you test - then you can talk with your genetic counsellor or your national Huntington's disease organisation about this issue.

Young people tend not to think about things like insurance, it is usually something people look into when they are buying a house or starting a family etc. But insurance is actually very important and should not be overlooked.

It is your choice, you do not need to be insured in order to test — it is not a requirement, but it would benefit you in the long run to look into it.

Emotional Aspects of the Testing Process

Aside from the practical ways to be prepared, testing is also a very emotional process from start to finish. From making the decision to test, going to your genetic counselling sessions, having your blood taken and receiving your result — each stage provides its own emotional hurdles to overcome.

Dealing with the emotions of testing is incredibly challenging for anyone. When you take into account that the whole process can take months, it can become very difficult for a person to maintain their calm throughout. You may have many ups and downs during the process, don't be alarmed by this, it is quite normal and you should keep discussing your emotions with the genetic counsellor (that is what they are there for). You could also seek advice from others who have gone through the testing process for Huntington's disease already and have faced the same emotions and difficulties.

The Waiting Period

There is a part of the testing process that has not been highlighted yet, 'the waiting period'. This period covers the time between having your blood sample taken and receiving your test results. People have to wait for weeks in-between their blood being taken and their test results appointment. Just how long that wait is depends on where you live and the clinic you are using. But this can be an incredibly difficult time for a person in the testing process; those weeks can often feel like months! People can become very anxious during the waiting period and it is important to try and stay busy during this time.

If you feel it would be helpful to speak to your genetic counsellor during this period do call them (they will not know your result at this stage). Or again, seek advice and support from those who have experienced the waiting period themselves — this can be quite comforting.

Preparing for Your Results

Preparing for results is another challenging aspect of the testing process. Many people tend to develop a 'feeling' one way or the other about whether they are going to test positive or negative. Obviously you can't guess or 'feel' what result you are going to get, and these guesses can end up causing more shock when you receive your results (should you get the result you weren't expecting). It can often be good just to remind yourself during the process that until you test you are 'at-risk', rather than trying to guess one way or the other.

However, preparing for your results can be useful and during your counselling you may be asked to think about what you will do if you test positive or negative. This is a good exercise because it allows you to really think and plan how you will react to your results, whatever they may be.


Support during the testing process is not only important but can be very comforting. You are entitled and encouraged to bring along a family member or a friend to both your counselling sessions and your results appointment — and it is highly advised that you do so. Some people, when they decide to get tested, are hesitant to tell their family/friends, and decide to face the process alone. People's reasons for not telling their family about such a huge life decision are usually because they do not want to put their loved ones through any extra stress, worry or pain - people want to protect their family. It is an understandable reason for keeping the process quiet; after all, many people testing have families that are already dealing with Huntington's disease and people often feel like they are burdening their family by adding the testing process to the list of things to cope with. But it is unlikely to help you in the long run to keep this to yourself. As has been highlighted, testing is an incredibly emotional journey — you will need support during this process. Do not face it alone if there is no reason to - this is hopefully a time for the family to all get behind the person testing and support each other.

For people testing who have siblings either at-risk, already tested or symptomatic, testing can be an even tougher experience. Imagine testing negative if one of your siblings had already tested positive - how do you tell them your news? This is often called 'sibling guilt' and it can be uncomfortable and difficult to handle. But it does not have to be, as one family highlights:
Support is the key, whatever the results as siblings you should use this as an opportunity to become closer and even more supportive. Talk about the worries you have with your siblings, express your concerns about how the family will react to your results before you get them — be prepared as a family.

However, you shouldn't feel that you need your family's permission or support to get tested — remember, it is your choice.
For a listing of HDSA resources and support groups as well as professional services in your area CLICK HERE or contact HDSA at 1-800-345-HDSA.