What a genetic result can and cannot tell you: more detail

How are PRNP variants classified?

By enrichment in patients compared with population controls, together with family evidence. Goldman and Vallabh describe E200K, P102L, D178N, A117V, P105L and the five and six repeat octapeptide repeat insertions as the commonest highly penetrant variants, all of which meet the American College of Medical Genetics and Genomics and Association for Molecular Pathology criterion for enrichment in cases over controls. A further 20 variants have been classified as likely highly penetrant using a combination of absence from large population control datasets with either a de novo variant in a patient without a family history or co-segregation with disease in a pedigree with several affected relatives.

A smaller group, including V210I, V180I and M232R, is described as conveying lifetime risk of 0.1 to 10 percent. The practical consequence for counselling is that the word pathogenic on a report covers a range of risk spanning two orders of magnitude, and the variant name is what locates a family within that range. The variants hub gives the individual variant pages.

Variants that are very rare in both patients and population controls cannot be placed either way by this method. These are the reports that come back as variants of uncertain significance.

What should be said about age at onset?

That it cannot be predicted from the result. The 2026 Israeli practice framework for relatives of people with genetic Creutzfeldt-Jakob disease describes penetrance for the variant common in those families as age dependent and nearly complete by the ninth decade, with mean onset in the early sixties and wide variation around it. It states that the triggers for onset are unknown, and records the youngest known symptomatic patient in that population as 34, from unpublished data.

Goldman and Vallabh add two points that bear on what a counsellor can say. Someone who tests positive may differ from other family members in both age at onset and presentation. And duration varies by variant group: roughly two thirds of genetic prion disease cases are rapidly progressive, with half of those patients dying within a year of first symptoms, while the octapeptide repeat insertions and the variants traditionally linked to Gerstmann-Straussler-Scheinker syndrome tend towards a longer course. For P102L, the commonest slowly progressive variant, they describe a disease course that can run from 1 year to 15 years, and state that the variability precludes meaningful individual-level predictions.

Codon 129 genotype influences duration in some genetic prion disease and risk in sporadic and iatrogenic disease, but does not appear to influence age at onset for the commonest genotypes.

What does a negative result establish, and what does it leave open?

If a known familial variant was sought and not found, the person does not carry that variant. Three caveats belong in the disclosure session.

The result addresses that variant only. Sporadic disease, which accounts for the large majority of prion disease, is not excluded by any gene result.

The familial variant must actually be known. Where the affected relative was never tested, a negative result in a relative is uninformative about the family rather than reassuring, which is why Goldman and Vallabh recommend discussing whether testing the affected person is in the interests of that person and the family, and offering DNA banking when testing during life is not wanted.

Laboratory method sets a boundary. Where sequencing alone was performed, an octapeptide repeat insertion may not be covered, since repeat changes need a method directed at them. The issuing laboratory can state what the assay covered.

How are results returned, and who holds them?

Practice differs, and the difference matters when a family is arranging appointments across services.

The Israeli framework recommends that all results, positive or negative, be disclosed in person by the same clinician who gave the initial counselling, with telehealth not usually recommended, and that a positive result come with a written summary, guidance on family and reproductive implications, and support contacts. Its genetics section also states that results go to the referring team rather than directly to the person tested, and the document does not reconcile those two statements.

The National Prion Disease Pathology Surveillance Center in the United States returns results to the ordering health care provider to be shared with the patient or family, requires its PRNP request form with the social and family history section completed, together with a genetic consent form, and states that it does not test patients under 18. Its listed turnaround is 15 business days from receipt of the specimen, with no urgent option. The centre’s pages elsewhere describe turnaround differently, so confirm with the laboratory before a family plans around a date.

In Australia the national registry describes PRNP testing as entirely voluntary, performed at the PathWest Neurogenetics unit in Nedlands on blood or unfixed autopsy tissue, with consultation with a genetic service recommended, and lists no turnaround or price for the gene test.

What does testing cost?

It varies by country and by circumstance, and the figures published are narrow in scope. The United States centre lists its sequencing at 274.83 US dollars under CPT code 81404, charged to the referring institution or the patient when blood is submitted without a confirmed family history of genetic Creutzfeldt-Jakob disease. Goldman and Vallabh record that at the time they wrote, predictive testing at that centre was free of charge for people with a first-degree relative confirmed to have prion disease. The Israeli panel recommends that both pre-test and post-test counselling and the test itself be publicly funded, for example through the health funds, so that access does not depend on means.

A service can confirm what applies before an appointment is booked. This site does not give financial or insurance advice, and insurance and work sets out what is documented about disclosure rules.

Where counselling conversations hit the limits

Three places, in the experience recorded in these documents.

Reproductive questions. The Israeli framework recommends that preimplantation and prenatal testing be decided case by case, lists chorionic villus sampling at about 11 to 12 weeks and amniocentesis at about 17 to 23 weeks as the prenatal options, and notes that exclusion testing before implantation is available at tertiary centres there for first- and second-degree relatives. Goldman and Vallabh note that in vitro fertilisation with preimplantation genetic testing has been used in genetic prion disease, including arrangements that do not disclose the person’s own result. Family planning options covers this for families.

Timing. The same framework recommends that people in acute distress after a relative’s diagnosis or death be encouraged to delay by at least six months, that testing not be offered under 18, and that motives and resilience be reviewed carefully in young adults of roughly 18 to 25.

Uncertain results. The framework does not address variants of uncertain significance at all, and no published prion-specific guidance opened here sets out how to counsel on one. That gap falls to the issuing laboratory and the local genetics service.

Interpretation of any particular report belongs with a genetic counsellor and that laboratory. Counselling comes first describes the appointments that surround a test.