Under weight 18 month old

My 18 month old daughter had her 18 month check up today and based on her height (31.5 in) is approx. 7 lbs under weight.  She weighs 18 lbs 12 ozs.  She eats 4 meals a day plus 3-4 snacks, she also gets 13-18 oz of pediasure a day.  She is very active and does not eat many proteins.  I offer them but all she likes is peanut butter (my pediatrician OK'd this) and turkey bacon.

She had a full fecal screening at 12 months and everything came back normal, she also had bloodwork done at this time and it came back normal.  My Dr. is recommending a sweat test.  I was tested for cystic fibrosis when I was pregnant and I don't carry the gene.  Also my daughter has been very healthy (she does have a heart murmer which is tested by a specialist and we were told it was fine) and does not cough.  She has never needed to be on antibiotics, she has had a bit of a runny nose once with teething but it was clear and that is it.

I really feel that the sweat test is invasive and I would have to drive her 2 hours 1 way for the test.  Should I have the test anyway or go with my feeling that she is a healthy small person?

Also is it possible for her stool to not show malabsorption and for her to have c.f?

Developmentally she is doing exceptionally.  She already counts to 10 (just memorization, she doesn't know what the numbers mean) and loves books, besides being able to do the gross and fine motor skills expected at her age.  

The NP called me noncompliant today and as a former social worker I was highly offended.  Any advice would be great.

Mandy

[Renee Radenberg]

Has she been steadily losing weight? What percentile is her height and weight? I know it is a hardship to do the sweat test, however, if your MD recommended it, it should be done. Malabsorption may do show at an early stage of the disease. The sweat test is the only way to test CF at this age.

[Mandy5406]

She has not lost any weight, in fact she keeps gaining.  But she is in the less than 5% for her weight and in the 48% for her height.  She exhibits none of the sytems of CF.  Her stool is normal and her lungs have always been clear and were today.  I just hate to do a test that may not be needed.  She was breast fed for the first 5 months of her life.

[Renee Radenberg]

It is good that your daughter has been gaining weight, however, she is not at the weight she should be for her age. That is probably why your MD wants the sweat test done to rule out CF. Children with CF gain weight very slowly. A sweat test is done on the surface of your child's skin and they measure the chemicals from her sweat. It takes about 30-60 minutes. I suggest you get a second opinion from another MD about the sweat test. Best of luck.