A test on PRNP, the prion protein gene, reads one short stretch of your DNA. It answers a narrow question, and it is worth knowing how narrow before you decide anything.
What are the possible answers?
There are three.
The change that runs in your family is there. The change is not there. Or something was found whose meaning is not yet known, which laboratories call a variant of uncertain significance.
A genetic counsellor explains all three before a sample is taken, because each one leads somewhere different.
What does a positive result not tell me?
It does not tell you when. Age at onset varies a great deal, including between relatives who carry the same change. One Israeli framework for families with the commonest change describes an average onset in the early sixties with wide variation around it, and says the triggers are unknown.
It does not always tell you whether. For some changes nearly everyone who carries them becomes ill eventually. For others most carriers stay well all their lives. Which group a particular change falls into is something the counsellor and the laboratory can explain for your family.
What does a negative result mean?
If the laboratory looked for a known family change and did not find it, you do not carry that change. That is a clear answer about that one thing.
It is not a promise of good health in general. Prion disease also happens in people with no family history and no gene change at all, which is far more common than the inherited forms.
Why does a result involve other people?
Because close relatives share the same family history. An answer about you says something about brothers, sisters, parents and children, whether or not they wanted to know. Counsellors spend time on who you would tell, and when.
There are also ways to stop short of a full answer. DNA from a relative can be stored for possible use later, and a result can be held by the service rather than given to you. These middle options exist because the decision is rarely all or nothing.
Do I have to decide now?
No. Testing is voluntary, and nothing happens on the day you first ask about it. Counselling comes first, there is usually a month or more before any blood is taken, and you can stop at any point. People who have recently had a diagnosis or a death in the family are often encouraged to wait.
Choosing not to be tested is a reasonable choice that many people make and keep. The right not to know is about that.
Where next
Counselling comes first describes the appointments. The variants hub explains how a named change is written and what is known about the common ones. More detail covers result classes, penetrance and reporting for counsellors and clinicians. Genetics and family risk is the hub for this section.
Your own result can only be interpreted by a genetic counsellor with your family history in front of them.
Sources for this page
- Goldman JS, Vallabh SM. Genetic counseling for prion disease: updates and best practices. Genetics in Medicine 2022;24(10):1993-2003
- Shir D and others. A practice framework for genetic testing in asymptomatic relatives of patients with Creutzfeldt-Jakob disease. European Journal of Neurology 2026
- Australian National CJD Registry (The Florey): testing specifications