From the Guidelines
Cystic fibrosis (CF) testing for a 15-year-old male should begin with a sweat chloride test, which is the gold standard diagnostic test for CF, as it measures the concentration of chloride in sweat, with values above 60 mmol/L strongly suggesting CF 1. The sweat chloride test is a non-invasive procedure that can be performed at a CF Foundation-accredited care center by experienced technicians.
- The test involves placing a solution on the forearm or thigh and stimulating with a mild electric current to produce sweating, with only minor discomfort reported 1.
- Additionally, genetic testing for CFTR gene mutations should be conducted, which can identify specific mutations responsible for CF, with the most common panel tests for 23-106 mutations, though comprehensive genetic sequencing may be necessary in some cases 1.
- Other supporting tests include nasal potential difference measurement, which assesses ion transport across airway epithelia, and clinical evaluation of symptoms such as recurrent respiratory infections, pancreatic insufficiency, and poor growth.
- Testing is particularly important if there are symptoms suggestive of CF or a family history of the disease, as early diagnosis allows for prompt initiation of treatments including airway clearance techniques, pancreatic enzyme replacement, and potentially CFTR modulator therapies, which can significantly improve long-term outcomes and quality of life 1. The CFTR gene is located on chromosome seven and to date >2000 CFTR mutations have been identified, with advice from the local clinical genetics department recommended 1.
- Ancillary tests such as nasal potential difference and assessments of pancreatic exocrine function offer additional evidence only and are not diagnostic in themselves, with their use limited to within specialist CF centres 1.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Cystic Fibrosis Test Results
The results of a cystic fibrosis (CF) test for a 15-year-old male can be interpreted based on the sweat chloride test, which is the gold standard for CF diagnosis 2. The test measures the amount of chloride in sweat, with values above 90 mmol/L indicating a high likelihood of CF.
Sweat Chloride Test Results
- A sweat chloride value above 90 mmol/L is considered positive for CF, with a sensitivity of 83.3% and specificity of 99.7% 2.
- A value below 75 mmol/L is considered negative for CF, with a negative predictive value of 99.4% 2.
- However, normal sweat chloride values do not exclude the diagnosis of CF, as some patients with CF may have normal sweat chloride levels 3, 4.
Alternative Diagnostic Tests
- Genetic testing, such as genotyping, can confirm a diagnosis of CF in patients with normal or borderline sweat chloride levels 5, 3, 4.
- Other diagnostic tests, such as nasal transepithelial voltage measurements, may also be used to confirm a diagnosis of CF 3.