Diagnosing cystic fibrosis (CF) is a multi-step process. The tests include newborn screening with blood tests, genetic tests, a sweat test, a carrier test, and clinical evaluation at an accredited center.
Sweat chloride test
The sweat test is the most preferred test since it was introduced in 1959. The accuracy of the test is ranked at 98%. One of the primary symptoms of CF is salty tasting skin as sweat is not reabsorbed into the cells. The chloride content of the accumulated salt is measured and diagnosed to confirm CF. The test takes about one hour and is painless and non-invasive. It is generally done on the forearm but may be done on the thigh of babies and children. The test procedures are:
- An electrode which contains a disc with a sweat-inducing gel is placed on the skin.
- An electrical current is delivered through the electrodes for around 10 minutes to enhance drug activity. Once done, the skin is wiped and a filter paper is placed on the area.
- When a sufficient amount of sweat has been collected, the patch is removed and sent for evaluation to a lab. The results, measured in mmol/L, are given in a day or two.
- A result of 60 mmol/L or greater is considered as CF positive.
If the result is positive a second sweat test and genetic testing are recommended. If the results are on the borderline too, a second sweat test should be done.
Genetic testing is used to detect specific genetic mutations associated with the disease. Around 2000 mutations have been identified, which may cause CF by producing defective forms of the CFTR protein. The standard test is designed in such a way that only 23 most common CFTR mutations are detected. The test is performed on a blood sample. Sometimes swabs of cells from the inside of the cheeks are also taken. Results are given within three to five days from testing. This test can also help to analyze the extent of the seriousness of the problem, based on the type of mutation.
Genetic testing is done for couples planning a baby or when a woman is pregnant to determine the risks of having a baby with CF. The gene test is referred to as carrier screening when used for screening the parents or fetus. The test can confirm if one or both of the partners have a CFTR mutation. Testing is initially done on the woman if she is pregnant. Since it takes two copies of the mutation for CF to occur, a negative result means the baby will not have CF. If the woman is positive then the man is tested. If both are tested positive, the baby has 25% chances of having CF and a 50% chance of being a carrier.
Prenatal diagnostic testing
If both the parents are positive for CFTR mutation, and the mother is pregnant, tests are done to determine whether the fetus has the condition. This is done either by obtaining a small amount of amniotic fluid or samples from the cells in the placenta.
Blood tests are done on a newborn to determine IRT levels. The test is done on the first or second day from blood drawn with a needle prick on the baby’s heel. If the IRT level is high, genetic tests are done to confirm the condition.