Why sweat test if a genetic test has already come back positve?

[anonymous]

I am due with a baby girl in about a week that has been already diagnosed (via CVS) as having CF. My husband and I have talked to two doctors at two different hospitals, and both said that after the baby is born, they would schedule a sweat test for her. Both also said that because of the mutations that we carry, it is likely that the sweat test will come back borderline or even negative.

If the baby has already been diagnosed through genetic screening (we know the mutations and everything), what in the world is the point of a sweat test?

And another sweat test/genetic test question ... I had dinner a couple of nights ago with someone who mentioned that her friend was just told that her baby MIGHT have CF. Apparently the blood work taken during the newborn screening came back positive for CF, but they are scheduling a sweat test for the baby, which the doctor said will be the definitive answer.

I know you can have a negative sweat test but still have CF, but does it make any sense that the CF genes would be identified in a blood test, but that a negative sweat test would override that? I'm wondering if the friend has possibly misheard the information the doctor has given her, and I just feel awful thinking that she may have a very false sense of hope right now, letting it all ride on the results of the sweat test.

Jenica

 

[anonymous]

I don't understand the doctor's wanting a sweat test either. When my daughter was diagnosed via a sweat test we were scheduled for genetic testing. We were told that genetic testing gives the definite diagnoses due to false positive and/or negative sweat test readings. Try to get in touch with an accredited CF center to get some information to give your doctor. Many doctor's do not have a clue when it comes to CF.

 

[anonymous]

Jenica,

To answer your question about the positive screening:

(This is from the NY newborn screening site)

CF newborn screening is a two-tiered process performed on the standard Guthrie card: 1) analysis of immuno-reactive trypsin (IRT); 2) specimens with IRT levels in the top 5% are tested for a panel of CF-causing gene mutations. When two mutations are identified, it is consistent with a diagnosis of CF and the baby should be referred to an accredited CF Care Center. Over 1000 mutations have been identified in the CF gene, and this screening includes only a fraction of them. Therefore, babies who have one mutation identified, or whose IRT is in the top 0.2% of results without identifiable mutations, are called "screen positive."


Kind of complicated, but basically your friend's baby probably had a high IRT level, and the blood was tested for a panel of the most frequent gene mutations. They probably found at least one mutation and therefore it resulted in a positive screen. But, unless the second mutation was identified, the baby could result to only be a carrier of CF. So, a sweat test is needed to confirm CF. She could probably ask which/ and how many mutations were identified on the screening.

The NY site also stated that among 1,000 positive newborn screens only about 5% of the babies actually have CF.

Hope this helps, I learned about all of this screening stuff on a thread earlier this year.

Maria (mother of three daughters, the youngest, Samantha w/CF)

 

[anonymous]

Thanks, Maria for all of the info. For some reason I had thought that they did a full genetic test during the newborn screening. I have one more question though ... when you said my friend could probably ask which/how many mutations were identified, wouldn't they have only identified one? Wouldn't the identification of two mutations definitively indicate CF?

The 5% in 1000 stat is very reassuring though, and I'm hoping they're one of that 5%.

Thanks again!

-- Jenica

 

[anonymous]

Actually, you're hoping that they are not in the 5% (but I know what you meant<img src="i/expressions/face-icon-small-smile.gif" border="0">.

You're right that if they identified two mutations on the screening, the baby would have CF. They could identify two mutations if the baby has two very common mutations that are among those screened. However, if your friend said that the screening indicated the baby "might" have CF, it leads me to believe that the IRT level was high and EITHER they identified just one mutation/ OR they didn't identify any mutations but the IRT was extremely high (in the top 2%).

What I don't know is how they report the results to parents. Do they just indicate "positive screening for CF"? Do they indicate the IRT level? Do they list any mutations identified? Anyone have experience with this?


Maria

 

[julie]

I do understand why the doctors want to confirm CF upon the babys birth and test the child, what I dont understand is why they are saying it has to be a sweat test????? There are many times that because of hte mutations a person has, they show up negative or borderline for a sweat test, but have been identified as carrying 2 CF mutatons and therefore DO have CF. I think your doctor and your friends doctor are both very confused people. it would make sense for them to do a GENETIC test on the new baby, but not a sweat test. Maybe they are saying sweat test because that is a cheaper route, but it really makes no sense at all to me.

The someone who you had dinner with who has a friend whos baby might have CF.... they should go to an accredited CF doctor immediately. If the newborn screen came back positive for CF (does it differentiate between 1 mutation or 2 mutations?????), vice positive for a carrier, then the baby has CF and needs to be treated accordingly. A sweat test isn't going to do anything for the baby except waste time, frustrate the parents and delay proper treatment of the CF (provided that the newborn screening found 2 cf mutations).

aahhhhhhhhhhhhhhh doctors who don't know what they are talking about, but insist on risking peoples lives because they think they are right and refuse to admit they don't know it all and aren't interested in learning anything are REALLY starting to get on my nerves!!!!!!!!!

 

[anonymous]

Something definitely could have gotten lost in the translation in regard to my friend's friend, but I did suggest her going to an accredited CF center just to make sure she is getting all of the right information. My hope is that the doctor that was explaining the testing procedure isn't raising any false hopes for the poor mom.

And as far as sweat testing our daughter (who has already been diagnosed via genetic testing), neither doctor was saying that the results would have any impact, just that it is part of the standard procedure. If there were some sort of information they could gather from those results (such as mutations and their effect on sweat tests), I would completely understand. But it just seems to me like a waste of time (ours and the doctor's) and money (for the insurance company covering the costs) in this case.

Am I wrong in assuming that there isn't anything they can learn from the results?

 

[ReneeP]

My older daughter with CF has had 3 sweat tests (all very much positive, extremely high #'s) and my younger daughter has had 2 with the same results. They are both double Delta F508. Each time we moved and switched CF Centers they did new sweat tests despite having the medical records and even the CF Registry info with the genetic mutations already determined. I have no clue why they always insisted on sweat tests. Since the girls were on Medicaid at the time I really never questioned it, although I don't see the necessity either.

 

[anonymous]

I am from NY and I never even heard of cf until my daughters screening came back. When her ped. called to tell me that she had to go to Albany for a cf sweat test I asked why and they said the test showed one most common mutation. So we went and 2 hours later it came back pos. Then we started the blood tests for mutations after 2 more tests we did genetics through quest genetics and found she has a rare mutation only recorded once and a deletion that has never been seen. We got in contact with Ambry genetics and they have never seen either one. Now they say she is pancreatic insufficient but she gained all her weight up to the day she was diagnosed on her own which was like a pound a month and grew regular. They put her on enzymes and hasn't gained anymore then what she was on her own. She is on the lowest enzyme and a lower dose then they reccomend for her feedings so I take it it isn't a big insufficiancy. They said her stool showed a touch of fat and something else I cann't remmember. But she is 9 months and weighs 17.9 and is 26.5 inches she is on the 50% for both. She was only 6.11 when she came home from the hospital.She did have infection in stom. and lungs when born but was a csection baby and the doctors said it was common. Other than that no lung infections. Even had RSV virus at 2.5 months and lungs stayed clear of infection. The doctors couldn't believe how well she did. She pulled through on her own except Pulmacort and albuteral breathing treatments every 4hours for 4 days.
Katina(mom of 3 youngest Saydee with cf)

 

[julie]

Ok, thinking a little more about this over the weekend.... the ONLY thing I think the doctors might be reaching to learn from this would be an association between certain mutations and the levels of the sweat test (i.e. very high level with Delta F508, borderline with X mutation...) If that is their purpose, to gather some more information then so be it (at least in my opinion) but if I were you, I would question them as to what exacty their reasoning is (studying, trying to find relationships between certain mutations and positive/negative/borderline sweat tests) and make them give you an honest answer.