Cascade Testing: Telling Your Family

A confirmed monogenic diagnosis is unusual among medical results: it is immediately actionable information for people other than you.

Because most monogenic diabetes is autosomal dominant, each child of an affected parent has a 50% chance of having inherited the same variant. Siblings of an affected person carry the same 50% chance. Parents are worth considering too, since one of them almost certainly passed it on.

Testing relatives is much simpler and cheaper than the original diagnosis was. Once the specific variant is known, a laboratory only needs to check that one place in the gene rather than sequencing a whole panel. This is called cascade testing.

Why it matters practically: a relative currently labelled Type 2 and taking the wrong medication, or labelled Type 1 and injecting insulin unnecessarily, could have their treatment corrected. In some subtypes a relative may need no treatment at all.

How to raise it. Give them the specific information — the gene name, the variant, and the laboratory report — rather than a general warning. A relative arriving at a clinic with a named variant gets a far better response than one saying diabetes runs in the family.

Expect mixed reactions. Some relatives will want testing immediately; others will not want to know, and that is their decision to make.

Genetic counselling helps where family relationships make this difficult.