Screening.
In the UK all babies are screened for cystic fibrosis at birth using a blood test known as the Guthrie test. The test is carried out sis or seven days after the baby is born. Blood is taken from the baby's heel. The Guthrie test also screens for phenylketonuria, congenital hypothyroidism, MCAD deficiency and sickle cell disease.
Ante-natal testing.
It is possible to test for cystic fibrosis whilst the baby is still in the womb. This is done around the tenth week of pregnancy using the chorionic villus sampling (CVS) technique. This tests a small sample of the placental tissue, looking for the faulty gene that causes cystic fibrosis. The sample is collected by means of a thin needle which is inserted through the stomack wall or via a small tube which is passed into the womb via the cervix.
According to NHS direct this process takes about twenty minutes and the results should be available within 10-14 days.
Estimates of the risk of miscarriage with CVS vary between 1 in 100 and 1 in 400.
The sweat test
The sweat test measures the amount of salt (usually chloride) that is in the child's sweat. Abnormally high levels of chloride in sweat is a good indicator that cystic fibrosis is present. The UK guidelines for sweat testing state that the test can be carried out two weeks after birth providing the baby weighs more than three kg, is normally hydrated and is without significant systemic illness. The guidance says that the test should be delayed for those babies who are dehydrated, systemically unwell or have eczema on the potential stimulation sites.
Sweat is normally collected from either forearm, although the legs can also be used. A lint pad soaked in electrolyte is placed on the collection site, an electrode is then placed upon the pad to generate heat. The electrode is held in place for about 5 minutes which creates a tingling sensation but no pain. The sweat is then collected either by filter paper or a smmall plastic coil. This collection phase lasts for about thirty minutes.
An abnormally high level of chloride in the sweat is a good indicator for cystic fibrosis. Occasionally borderline results are obtained and these require a further test. Most doctors will ask for a further test if the results are abnormally high.
Older children can also be given a sweat test.