The short answer
If your full sibling has cystic fibrosis, both your parents are carriers, and you do not have CF, your chance of being a carrier is 2 in 3. Being a carrier does not make you ill; it matters only for your children. A test for the two exact CFTR changes found in your sibling can replace that two-in-three with a straight yes or no — and your children's risk then depends on whether their other parent is a carrier too.
Where two in three comes from
Cystic fibrosis is a recessive condition. Someone with CF has a harmful change in both copies of the CFTR gene. Their parents each carry one change, and are healthy.
For every child of two carriers, these are the possibilities at conception:
| What the child inherits | Chance |
|---|---|
| Neither family change | 25% |
| The mother’s change only — healthy carrier | 25% |
| The father’s change only — healthy carrier | 25% |
| Both changes — has CF | 25% |
You know you do not have CF, so the last row is out. Three rows remain, two of them carriers. That is 2 in 3, or about 67%.
This assumes you are a full sibling and that both parents are carriers. Half-siblings, and families with an unusual test result, need a different calculation.
A quick map for the rest of the family
| Relative of the person with CF | Starting chance of being a carrier |
|---|---|
| Each parent | Effectively certain |
| A healthy full brother or sister | 2 in 3 |
| A half-sibling through a carrier parent | 1 in 2 |
| An aunt or uncle on a carrier parent’s side | 1 in 2 |
| A first cousin through that aunt or uncle | 1 in 4, before the aunt or uncle is tested |
These are starting points. A test for the family’s exact changes replaces them with a direct answer.
What being a carrier actually means
A carrier has one working copy of CFTR and one changed copy. Almost all carriers are perfectly healthy and always will be. What a carrier can do is pass the changed copy on.
If you are a carrier, each of your children has a 50% chance of getting your changed copy. But they can only have CF if their other parent passes one on too:
| Your result | Your partner’s result | Chance of CF in each child |
|---|---|---|
| Carrier | Not a carrier on the test used | Very low, though no test is perfect |
| Carrier | Carrier | 25% |
| Has CF | Carrier | 50% |
Which is why, once you know your own status, your partner’s result is the number that matters.
Ask for the family’s own changes first
Your sibling’s laboratory report should name two CFTR changes. A focused test can then look for exactly those two in you.
If one or both were never identified, the family may need updated testing before anyone else can get a definite answer. Standard carrier screens cover many common changes, but no two panels cover the same set and none finds every possible change. A negative panel is worth less than a negative test that definitely included both of your family’s changes.
If you have symptoms of your own
Do not let a carrier calculation settle a medical question. If you have symptoms that raise a question about CF or a related condition — recurring chest problems, digestive trouble, or difficulty conceiving — say so. A doctor may suggest a sweat test and proper diagnostic genetic testing, which is a different thing from carrier screening.
The most useful order to do this in
- Confirm the two exact changes in the person who has CF.
- Test adult relatives for those changes.
- Test the partner of anyone who turns out to be a carrier.
Once a child has CF, both parents are carriers, so each of their own brothers and sisters has a 1 in 2 chance. Adult aunts, uncles and cousins often want to know before starting a family — and when the family’s changes are already known, that is a quick and definite test rather than an open-ended search.
Common questions
- Why is it 2 in 3 and not 1 in 2?
- Children of two carriers have four equally likely outcomes: one who inherits neither change, two who are healthy carriers, and one who has CF. Knowing you do not have CF removes that last outcome, leaving two carriers out of three.
- Can a cystic fibrosis carrier have symptoms?
- Nearly all carriers are completely healthy. Research has found slightly higher rates of a few CFTR-related problems among carriers, but being a carrier is not a mild form of cystic fibrosis.
- Should my partner be tested too?
- Yes, if you are a carrier — or if you are planning a pregnancy before your own result is back. A child can only have CF if both parents pass on a change.
- Is a general carrier screen good enough?
- A test for the exact changes found in your sibling is much clearer. Standard panels cover many common CFTR changes but not all of them, so a negative on a panel leaves more doubt than a negative on the family's own changes.
- I had newborn screening as a baby. Doesn't that answer it?
- No. Newborn screening is designed to find babies who have cystic fibrosis, not to identify carriers. Plenty of carriers pass newborn screening.