The Generative Brain

Predictive testing: what it involves

More detail

A predictive test looks for a known family change in the PRNP gene in someone who has no symptoms. It is a blood test, but the process around it takes weeks: counselling first, time to think, then the sample, then the result given in person. You can stop at any point.

What a predictive testing process can look like
  1. CounsellingTalk through what it could mean.
  2. AssessmentCheck the timing is right for you.
  3. Time to thinkOften a month or more.
  4. Blood sampleOnly if you choose to continue.
  5. Result in personWith support afterwards.

You can stop at any point, including before a result is given. Not testing is a valid choice.

Predictive means the test is done before there are any symptoms. It is looking for a change in PRNP that has already been found in someone else in the family.

What actually happens?

A 2026 Israeli practice framework for relatives of people with CJD sets out four steps, and most countries follow something close to it.

First, a counselling appointment. Then an assessment with a mental health professional, which is there to check that the timing is right for you rather than to judge you. Then a separate appointment for the blood sample. Then a session in which the result is given to you in person, by the same clinician who did the counselling.

The framework recommends at least a month between the first appointment and the blood sample. That gap is deliberate. A pregnancy or another pressing reason can shorten it, with all four steps kept.

The laboratory side is ordinary. A couple of tubes of blood, posted to a specialist laboratory, read over days or weeks. One centre in the United States states 15 working days from the day the sample arrives.

Can I change my mind?

Yes, at any point, including after the blood has been taken and before the result is given. Testing is voluntary. The Australian national registry puts it plainly: genetic testing is entirely voluntary.

Not deciding is also a position you can stay in for years. Among 30 at-risk relatives seen at a French clinic between 2004 and 2017, ten chose not to be tested. The right not to know is about that choice.

What will the result tell me?

Whether you carry the family’s gene change. It cannot say when, or whether, someone who carries it would become ill. For some changes nearly all carriers become ill eventually; others are carried by many people who never develop symptoms. What a result can tell you goes through this.

A result also touches other people. Brothers, sisters and children learn something about themselves from your answer, whether or not they wanted to know. Counsellors spend time on who you would tell. Talking with family covers those conversations.

What about children?

Predictive testing is not offered to people under 18 in the frameworks that cover this. The National Society of Genetic Counselors generally discourages predictive testing of children without symptoms for conditions that begin in adulthood, while allowing that some circumstances may be appropriate, and strongly recommends counselling before and after any test. The 2026 Israeli framework says testing should not be offered under 18, and that young adults should be considered carefully.

Does it cost anything?

It depends where you are. One United States centre lists a charge of about 275 US dollars for sequencing when there is no confirmed family history of genetic CJD. The Australian registry page lists no price for the gene test. Ask the service arranging the appointment what applies to you before you go.

What if I want to help research without a test?

That is possible in some studies. Joining research without testing sets out what exists.

Where next?

Genetic counselling comes first describes the appointments before a test. More detail has the published protocols and figures. Genetics and family risk is the hub.

Nothing here is advice about whether to have a test. That is yours to decide with a genetic counsellor, and either answer is a reasonable one.

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