Knowledge

Sweat testing is the gold standard for the diagnosis of cystic fibrosis.

Diagnostic Testing

Prenatal Screening

Obstetricians offer prenatal screening for cystic fibrosis (CF). Everyone inherits one CFTR gene from their mother and one from their father. People with CF have mutations in both CFTR genes. The first step in prenatal screening is to test the mother for CFTR gene mutations with a blood test. If she has two normal CFTR genes, then the baby will not have CF (the baby can still inherit an abnormal CFTR gene from his or her father and be a CFTR mutation carrier). If the mother carries a CFTR gene mutation, then the father should also be tested for CFTR gene mutations. If he has CFTR mutation, then the baby has a 25 percent (1 in 4) chance of having CF. If the baby inherits 2 CFTR gene mutations then he or she will have CF. If both parents are found to be carriers of CFTR gene mutations, amniocentesis or chorionic villus sampling can determine if the baby has CF. Amniocentesis is usually done between 15 and 18 weeks of gestation; chorionic villus sampling can be performed even earlier in the pregnancy. If parents elect not to undergo amniocentesis, the baby should be tested for CF as a newborn.

Inheritance pattern of normal or mutated CFTR genes from parents.

Inheritance pattern of normal (N) or mutated (M) CFTR genes from parents who carry an abnormal copy of the CFTR gene.

Newborn Screening

All babies born in the United States and in many parts of the world are screened for CF shortly after birth. Newborn screening is performed on blood samples collected from a heelstick, shortly after birth. These blood samples are collected as blood “spots” on a special filter paper. The dried blood spots are tested for a variety of diseases including CF. Every state performs CF newborn screening testing for infants, but the testing methodology varies from state to state.

Ultrasound

Ultrasound

The Maryland newborn screening program began testing for CF in 2006. Most states that test for CF during a newborn screening test for the presence of an immunoreactive trypsinogen (IRT) level and/or CFTR gene mutations. Immunoreactive trypsinogen is a pancreatic enzyme precursor found in the blood, and the IRT level is elevated in babies with CF. The IRT level is elevated even a infant with CF is pancreatic sufficient. In Maryland, if the IRT is elevated in two tests then the infant will be referred for a sweat test.

Some states test only for elevated IRT levels in the blood while other states also test for the presence of CFTR gene mutations. In these states, if the IRT is high, then the blood will be tested for one or more of the common CFTR gene mutations. If the baby has two CFTR gene mutations, then the infant has CF. If the IRT level is elevated and only one mutation is found, then the newborn screening test is “positive” and the baby may have CF or may be a CFTR gene mutation “carrier”. Any positive newborn screening test must followed by sweat testing, which remains the “gold standard” for diagnosis of CF.

Sweat Test

The sweat test is considered the “gold standard” for the diagnosis of CF. Many hospitals perform screening sweat tests that measure skin conductivity. However, quantitative pilocarpine iontophoresis, which measures the sweat chloride concentration is used for diagnosis. This test is performed at CF Centers throughout the country. There are two sweat test methods approved by the Cystic Fibrosis Foundation. Both methods stimulate sweating on the forearm or thigh using a chemical called pilocarpine. Sweat is then collected on filter paper or gauze (Gibson-Cooke method) or in microbore tubing (Macroduct® collection system). A minimal amount of sweat must be collected for the test to be accurate. The chloride content is the sweat is determined. A sweat chloride concentration of 60 mmol/L or greater is indicative of cystic fibrosis.

Sweat Test Interpretation
Chloride Concentration Interpretation
0 – 29 mmol/L Cystic fibrosis is unlikely
30 – 59 mmol/L Intermediate
≥ 60 mmol/L Indicative of cystic fibrosis

To accurately interpret sweat test results, an appropriate volume of sweat must be obtained. Acceptable volumes of sweat are 75 mg for the Gibson-Cooke procedure and 15 µL for the Macroduct® procedure. Sweat testing cannot be performed accurately in the first two days after birth. If the volume of sweat obtained is inadequate the test should be repeated.

Sweat Test

Elevated chloride levels in the sweat can be used to diagnosis CF.

CFTR Gene Mutation Analysis

CFTR channel mutations

CFTR channel mutations

Everyone has two copies of the CFTR gene, one inherited from their father and one inherited from their mother. To have CF, an individual must have two abnormal, or mutated, CFTR gene. If the individual has only one CFTR gene mutation, he or she will not have CF, but will be a “carrier”. Because carriers have one abnormal CFTR gene, they can pass this mutation to their children, placing them at risk for CF.

More than 2,500 CFTR gene mutations have been identified. In white populations, the most common mutation is called F508del, which occurs in approximately 70 percent of those CF. Half of the people with CF have two F508del CFTR gene mutations.

Most people with CF can be diagnosed with commercially available CFTR gene mutation screening panels, which test for the most common CFTR gene mutations. The accuracy of the test will depend on how many CFTR gene mutations are tested and the ethnic origin of the patient (TABLE). However, many rare CFTR gene mutations are not part of the typical test panels. Therefore, some people with CF cannot be diagnosed using the typical CFTR gene mutation panels. More extensive testing can be performed by sequencing the entire CFTR gene. Some people may require additional testing supportive testing such as nasal potential difference measurement.

Detection of Cystic Fibrosis by Mutation Analysis

 

25 Mutation Test
Ethnic Group Detection Rate
Caucasian 90%
African American 69%
Hispanic 57%
Ashkenazi Jewish 97%
Other Unknown

Nasal Potential Difference (NPD)

Cystic fibrosis is caused by abnormalities in salt transport into and out of cells due to a defective CFTR protein. This chloride channel regulates the salt content in the fluid that covers cell surfaces in the nose and lungs. Transport of ions such as sodium and chloride creates an electrical potential difference across the airway lining. This potential difference can be measured by placing an electrode on the surface of the nose. After the electrode is positioned, the surface of the nose is bathed in a series of solutions that contain different salts. These solutions are designed to change the flow of ions across the cell surface in a predictable way. These solutions contain (1) a Ringer’s saline solution (a special salt solution used to obtain the baseline NPD), (2) amiloride which blocks sodium channels, (3) a chloride-free solution and (4) isoproterenol, which stimulates CFTR. The solutions are always administered in the same order during the NPD testing.

Because people with CF either don’t make CFTR chloride channels or have mutated forms of CFTR, the potential across their nasal epithelium responds differentially to the administration of the various solutions. Three specific features of the NPD test distinguish people with CF:

  1. A more negative baseline potential difference.
  2. A larger inhibition of NPD after addition of amiloride.
  3. Little or no change in NPD after addition of the chloride-free and isoproterenol solutions.

Nasal potential difference testing is a research test, which is not commercially available. Because the test is technically very difficult to perform, only a few CF centers around the country perform NPD measurements. Clinical features of the individual as well as sweat test and mutation analysis results must be taken into consideration when interpreting the NPD to diagnose CF.

Select from the menu above to compare the normal to Nasal Potential Difference (NPD) with CF over time.bNPD is a test that measures the salt (sodium and chloride) transport in and out of the cells in the nose in response to different salt solutions. The way that cells respond to the changing salt solutions can be used to make a diagnosis of CF.

Additional Testing

Chest CT

A chest CT shows the lung in more detail than a regular chest X-ray. Individuals with CF who have mild lung disease and normal chest x-rays may have changes that can only be discovered by a chest CT. Bronchiectasis typically first appears in the upper lobes of the lung, and as CF lung disease progresses, it becomes more extensive and can be viewed on chest x-rays.

Sinus CT

Sinus CT scans in individuals with CF often show complete filling of the sinuses with mucus. Normally sinuses are filled with air, making them appear black on CT scans. In people with CF, the sinuses are often completely filled with mucus, creating a white appearance on the CT scan. Nasal polyps and damage to the bone surrounding the sinuses may also be seen.

Airway Culture

Sputum cultures from individuals with CF grow characteristic types of bacteria. Approximately, 80 percent of these cultures grow Pseudomonas aeruginosa or Staphylococcus aureus. Other bacteria that can infect the airways include: Burkholderia cepacia and Stenotrophomonas maitophilia. With the exception of S. aureus, these infections are very unusual in people without CF.

Fecal Elastase

The amount of pancreatic elastase-1 in the stool is an indicator of pancreatic function. The absence of elastase in the stool is suggestive of pancreatic insufficiency, which is present in over 80 percent of people with CF.