Cystic fibrosis (CF) is a genetic condition that can thicken your mucus and sweat. Testing is usually done in newborns, but adults might be screened as carriers, too. Some of these tests are blood tests.

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CF is a genetic condition where the protein that controls your sweat and mucus production doesn’t work correctly. You may not have any symptoms of the condition, depending on what parts of the body are most affected.

In many parts of the United States, newborns are screened for this genetic disease through a blood test. People who weren’t screened as infants can get tested for CF as adults, especially if they are concerned about passing the condition on to their children.

This article will explore how tests are done for CF, who might need testing, and what to expect when you get your results.

Learn more about cystic fibrosis.

CF testing can be done in a few ways. In adults, sweat tests are most common, but newborns usually get a blood test within the first few days of life.

The blood test in newborns to check for CF checks two different elements.

The first is the presence of immunoreactive trypsinogen (IRT). This substance is made in the pancreas and can be high in infants with CF.

If the first test result shows an elevated IRT, a second test will also be reviewed. This test checks for a protein that causes CF, called the cystic fibrosis transmembrane conductance regulator (CFTR).

Newborn screening for CF is required in many states and is recommended even if the birthing parent was already screened for CF during pregnancy.

CFTR testing is also done on adult blood samples, though testing samples of sweat is the more common method of ruling out or diagnosing CF in adults.

Newborn blood testing is usually done with a tiny prick of a needle on a baby’s heel. A few drops of blood are taken and used to screen for several conditions, including CF. The drops of blood are placed onto a testing card and then sent to a lab.

If you are an adult having genetic testing for CF, you will have blood drawn in a lab, and that blood will be checked for the CFTR gene.

No preparation is required for blood testing for CF. Newborns are sometimes swaddled during their heel prick to offer comfort.

Blood testing in adults is done as any other blood testing is, with a sample drawn from your vein with a needle. You do not need to fast or take any special preparations before the test.

Sweat testing for cystic fibrosis

Sweat testing is also a common method for checking for the CF gene. Unlike blood testing, there is some preparation required for this test. Before having a sweat test done, you have to make sure you haven’t used any oils or lotions on your skin. This could alter the results of your test.

A sweat test will generally follow these steps.

  1. A healthcare professional will apply an odorless chemical called pilocarpine to your forearm or leg, and they will place an electrode over it.
  2. A gentle electrical current is sent through the electrode to help the pilocarpine penetrate the skin. You may experience some warmth or tingling.
  3. After about 5 minutes, the electrode is removed, and sweat will be collected using a small piece of paper, gauze, or a plastic coil.
  4. Sweat is collected at this location over the next 30 minutes.
  5. The sweat sample is sent to a lab, and chloride levels are used to make a diagnosis.

The following chloride levels are used to make a diagnosis or rule out CF:

  • Under 30 millimoles per liter (mmol/L): CF is unlikely
  • 30–59 mmol/L: Inconclusive; additional testing is needed
  • 60 mmol/L and higher: A CF diagnosis can be made

Although sweat testing is possible in infants, not all babies make enough sweat for collection. This is why blood testing is the preferred screening tool for newborns.

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If the IRT is elevated and the CFTR gene is present in blood samples, it doesn’t necessarily mean you or your baby have active CF.

There are 23 gene mutations that can cause disease. In most cases, you need to inherit a gene mutation from each parent to have CF. However, the presence of the CFTR gene from one parent alone may also cause the condition.

Positive blood tests will trigger additional testing to confirm that you either carry the CF gene or have active CF. Imaging studies like X-rays, sputum tests, and your doctor’s assessment of any symptoms you are having will also be factored into a diagnosis — or ruling CF out.

Does everyone who carries the cystic fibrosis gene have active disease?

No. Some people who carry genetic mutations common in people with CF are only carriers of the disease and don’t develop any symptoms.

If you and your partner both carry genes for cystic fibrosis, will your baby definitely develop the disease?

If both you and your partner carry the CF gene, your baby has a 25% risk of having CF.

Can your baby have cystic fibrosis without either parent being a carrier?

Testing is not perfect, and while you or your partner may not have been identified as carriers of the genetic mutations that cause CF, it is possible for parents with negative genetic screenings to have a child with the CF gene mutation.

CF is caused by a genetic mutation that is usually passed through families. Most people who develop symptoms of the condition inherit a genetic mutation for CF from each parent.

Genetic screening can be done on adults with sweat or blood testing. Blood testing with a tiny prick on a baby’s heel just days after birth is the most common test for newborns.

Many states require CF testing as part of newborn health screenings, but even the presence of a gene mutation doesn’t mean your baby will ever develop an active form of CF.