Cystic fibrosis testing
Cystic fibrosis (CF) is a genetic disease. It primarily affects the respiratory system and inhibits the body’s ability to clear mucus. It also affects the pancreas and digestive system, and can cause problems with every organ in the body.
Early diagnosis and treatment of CF can improve the health and increase the lifespan of those who have it.
Following are some of the tests doctors use to diagnose and evaluate cystic fibrosis.
|Test||Results that may indicate cystic fibrosis (CF)|
|newborn screening (immunoreactive trypsinogen test)||high levels of immunoreactive trypsinogen|
|sweat test||chloride levels equal to or more than 60 mmol/L|
|stool test||negative for trypsin and chymotrypsin|
|nasal potential difference test (NPDT)||abnormal level of sodium and chloride in the respiratory tract|
Newborn screening (immunoreactive trypsinogen test)
In the United States, newborn babies are tested for various health conditions. This is known as the newborn screening. The screening is typically done within 24-48 hours after a child’s birth. The doctor draws a blood sample and sends it to a laboratory for analysis. The blood is drawn from the heel, and the test is sometimes called a heel stick.
CF testing is included in the newborn screening. This test looks for a chemical in the blood called immunoreactive trypsinogen (IRT). IRT is produced in the pancreas. From there, it travels to the intestine. CF causes so much mucus to build up that it blocks IRT from reaching the intestine. This leads to high levels of IRT in the blood.
A test result that shows high levels of IRT indicates CF. This is usually followed up with a sweat test to confirm the diagnosis.
Sweat test or sweat chloride test
CF can cause people to lose up to five times more salt through sweat than normal. A sweat test measures the amount of salt in a sample of sweat.
If your baby’s newborn screening test indicates CF, it should be followed by a sweat test. Your doctor can perform this as soon as your baby can sweat enough, usually between two to four weeks.
There’s no need for test preparation. You should avoid using skin creams or lotions on your baby’s skin for 24 hours before the test. It’s usually OK to apply diaper rash cream in the diaper area if needed, but check with your doctor first. The test is typically performed on your baby’s arms or legs.
The sweat test is non-invasive. Your doctor will apply a colorless, odorless chemical that helps the body produce sweat to a small area of your baby’s skin. They will then apply electrical stimulation for five minutes, and cover the skin with gauze for about half an hour. Then the sweat is collected and analyzed.
This is what the results mean:
For babies up to six months of age:
|Chloride level||CF diagnosis|
|equal to or less than 29 mmol/L||negative: unlikely|
|30 - 59 mmol/L||intermediate: possible|
|equal to or more than 60 mmol/L||positive: likely|
|After six months of age:||Chloride level|
|CF diagnosis||equal to or less than 39 mmol/L|
|negative: unlikely||40 - 59 mmol/L|
|intermediate: possible||equal to or more than 60 mmol/L|
If your baby has an intermediate or positive result, your doctor will likely repeat the test to confirm the diagnosis.
CF affects the function of the pancreas. If your baby’s pancreas is functioning properly, there should be some trypsin and chymotrypsin in their stool. Your doctor can find out by sending a sample of your baby’s stool for analysis.
If the test is positive for trypsin and chymotrypsin, it means your baby’s pancreas is working well and they don’t have CF. Since this is a screening test, a negative result doesn’t necessarily mean your baby has CF. You will still need more tests to confirm the diagnosis.
This test is also helpful in people already diagnosed with CF to determine if they have problems with the function of their pancreas. The test can be performed on people of all ages.
Nasal potential difference test (NPD)
This test measures the flow of sodium and chloride in the mucous membranes of the nose, which is abnormal in people with CF
To perform the test, your doctor will place an electrode inside your child’s nose. Then liquids will be allowed to flow through. The electrode is used to measure the voltage, or flow, of sodium and chloride across the nasal epithelium. It may also require an IV. The test may be slightly uncomfortable because of the electrode placement. Because of this, young children may not tolerate the test or may require medicine to help them relax while the test is being performed.
This test isn’t available everywhere, and is more invasive than other CF tests. It is usually reserved for when the results of a sweat test or genetic test aren’t clear.
Along with other test results, an abnormal level of sodium and chloride in the respiratory tract can help establish the diagnosis.
There are two ways to test your baby for CF during pregnancy.
- Amniocentesis: During this procedure, your doctor will use a needle to enter your uterus through the abdominal wall. Then a sample of fluid from the baby’s sac is removed and sent to a laboratory for analysis. This test can be taken between the 15th and 20th week of pregnancy.
- Chorionic villus sampling (CVS): For this test, your doctor will insert a thin tube through the vagina and cervix to reach the placenta. They’ll take a small tissue sample from the placenta and send it to a laboratory. This test can take place after the ninth week of pregnancy.
After amniocentesis or CVS, the pathologist will check the genes to determine if your baby is a carrier or has cystic fibrosis. You will receive genetic counseling to discuss these results and what they mean for your baby.
Once a CF diagnosis is confirmed, a few other tests are needed to assess the condition. Some of these include:
- genetic testing to determine the specific genetic defect
- chest X-rays to gather information about lung and heart function
- sinus X-rays to check for complications involving the sinuses
- a sample of sputum to see if the bacteria mucoid Pseudomonas are present, which usually occurs in advanced CF
There are also several different tests that involve breathing into a tube. These are designed to show how well the lungs are functioning.
Treating CF involves a variety of methods to keep the lungs and breathing passages clear. Nutritional supplementation is also crucial.
Specific treatments vary from person to person, but may include several types of medication, physical therapy, and exercise. Complications are treated as they arise. In some cases, surgery, including a lung transplant, is necessary.
There’s no cure for CF. It’s a progressive disease that requires lifelong management. Today’s treatments are helping people with cystic fibrosis enjoy a better quality of life, as well as a longer life. The median survival age today is almost 40.
If you have a family history of CF, you might want to consider genetic testing before starting a family. You can do this even if you’re already pregnant.
With a simple blood test or saliva test, a genetics counselor can check to see if you are a carrier of the abnormal gene.
A child can only have CF if both parents pass on the abnormal gene. If both parents are carriers, there’s a:
- 25 percent chance their child will have CF
- 50 percent chance their child will be a carrier only
- 25 percent chance their child will not have the disease or be a carrier
There are many abnormal defects of the CF gene. These genetic tests can detect the most common ones.
If you inherited one faulty gene and one normal gene, you are a carrier. If one parent is tested and found to be a carrier, the second parent should then be tested.
If both parents test positive and you’re already pregnant, you can consider prenatal testing to see if your baby has CF or is also a carrier.
A genetics counselor can provide you with the information you need to make the right decisions for your family.