IRT neonatal screening

Question

Dear expert team,

our 12-month-old daughter is a little bit underweight (8 kg, 74 cm). She was born in the 36th week of pregnancy (2.150 grams). Neonatal screening was done and the IRT [immunoreactive trypsinogen] was examined which was inconspicuous. One week after birth the test was repeated and these values were inconspicuous as well.

Since our daughter is such a lightweight I am worried and ask myself if cystic fibrosis can be excluded with this test. Until now, she has no bronchitis or diarrhea.

Many thanks for your answer.
Best regards.

Answer

Hello,

You are saying that your daughter was born in the 36th week of pregancy and weighed 2.150 grams. Neonatal screening was done and the IRT [immunoreactive trypsinogen] was examined which was normal. One week after birth the test was repeated and these values were normal as well. You are asking if cystic fibrosis can be excluded by the measurement of the IRT since you are worried about the underweight of your daughter. You are also saying that there are no suspicious symptoms like diarrhea or cough.
What is the informative value of the IRT examination? IRT values are increased in newborn babies if there were troubles in the abdomen of the unborn baby before birth. Generally speaking, about 80% of all CF patients have a disturbance of their pancreatic function at birth; in most cases there is a complete deficiency of the exocrine function already, i. e. that almost no functioning enzymes of the pancreas are released to the intestine. In reverse, this also means that about 20% have a regular pancreatic function although these children have CF. That means that an increased IRT value is only found in 80% of the CF babies. 20% are not covered by this test. On the other hand, the value can also be increased if a child without CF has an obstacle in the intestine (stenosis or atresia or twisted intestine). This is called a false-positive result.
Especially if there are no problems with the lungs, cystic fibrosis cannot be the reason for the underweight or insufficient weight gain in children with a regular IRT value. Therefore, it is pretty unlikely that your child has CF. If this information does not ease your worries sufficiently you should contact a CF centre close to where you live and consult with them if furher testing could be done to exclude CF. In my personal opinion this is currently not necessary.

I wish you and your daughter all the best,
Dr. H.-G. Posselt

17.09.2012