Cystic Fibrosis

Medical Experts

Contributor

Mao

Professor of Pathology (Clinical), and Co-Director of Laboratory Genetics and Genomics Fellowship, University of Utah

Medical Director, Molecular Genetics and Genomics, ARUP Laboratories

Cystic fibrosis (CF), a serious autosomal recessive disease, affects approximately one in every 4,000 newborns in the United States, but individuals may also be diagnosed later in childhood or adulthood. Symptoms of CF include lung disease, liver disease, and exocrine and endocrine pancreatic insufficiency. Age of onset, severity of symptoms, and manifestations of CF vary widely. CF is caused by pathogenic variants in the CF transmembrane conductance regulator (CFTR) gene. More than 2,000 CFTR variants have been reported but most are very rare and of unknown significance, which presents a diagnostic challenge. Sweat chloride testing is the gold standard diagnostic test for CF. The immunoreactive trypsinogen (IRT) test is a preliminary assay used for newborn screening. Molecular testing (ie, DNA analysis) can be used for carrier screening, newborn screening, and diagnostic testing.

Quick Answers for Clinicians

How does the diagnostic testing strategy for cystic fibrosis differ for adult and pediatric patients?

Testing recommendations for adult and pediatric populations are the same, regardless of whether the suspicion for cystic fibrosis (CF) arises from newborn screening or symptomatic presentation. Both the Cystic Fibrosis Foundation and the European Cystic Fibrosis Society recommend sweat chloride testing.^, Two elevated (positive) sweat chloride test results on two different days are diagnostic for CF; if sweat chloride values are borderline, CFTR genetic testing is recommended.^, The identification of two severe pathogenic CFTR variants on opposite chromosomes is also diagnostic for CF; however, the absence of a positive genetic result does not exclude CF.^,

Which laboratory tests for cystic fibrosis are appropriate to perform before birth?

If carrier screening reveals that both members of a couple are carriers of cystic fibrosis (CF), the couple should be offered the option of preimplantation genetic diagnosis or prenatal diagnosis.

What is the definition of CFTR-related metabolic syndrome/CF screen positive, inconclusive diagnosis?

CFTR-related metabolic syndrome/CF screen positive, inconclusive diagnosis (CRMS/CFSPID) refers to the combination of a positive newborn screen (high immunoreactive trypsinogen [IRT]), inconclusive sweat chloride test, and inconclusive genetic testing results. Newborns with CRMS/CFSPID may never develop any symptoms of CF but are at risk for CFTR-related disorders such as bilateral absence of the vas deferens, lung disease, and pancreatic insufficiency.

Indications for Testing

Molecular testing for CF is indicated for the following^-:

Carrier screening in individuals who are pregnant or planning a pregnancy
Carrier screening in individuals with a family history of CF or CFTR-related conditions
Diagnosis in newborns with a positive newborn screening result
Diagnosis in adult or pediatric patients with signs or symptoms or a family history of CF
Confirmation of the causative CFTR gene variants in a known affected individual to determine eligibility for CFTR variant-specific treatment

Laboratory testing is also used to monitor complications in patients with an established diagnosis of CF.

Criteria for Diagnosis

The Cystic Fibrosis Foundation proposes the following diagnostic criteria for CF in an individual with a positive newborn screening test, signs and/or symptoms of CF, or a family history of CF:

A positive sweat chloride test result ≥60 mmol/L
Identification of two CF-causing variants on separate chromosomes and a sweat chloride value of ≥30 mmol/L
Demonstrated CFTR dysfunction (ie, a sweat chloride test result of ≥60 mmol/L or CF-typical nasal potential difference or intestinal current measurement), regardless of whether or not two molecular pathogenic variants can be identified

Laboratory Testing

Carrier Screening

Carrier screening for CF is recommended for all individuals who are pregnant or planning a pregnancy, individuals with a family history of CF, and individuals with isolated congenital bilateral absence of the vas deferens.^,^, A targeted CFTR variant panel test or a more comprehensive testing method (eg, next generation sequencing) may be used. When a targeted variant approach is used for carrier screening, it is now recommended to test for a minimum of 100 specific known CF-causing variants, an increase from the previously recommended 23 variants (ie, “ACMG-23”). This update aims to enhance variant detection rates across varied biogeographic populations.

Newborn Screening

Screening for CF is recommended for all newborns in the U.S. Screening is performed using a measurement of IRT in blood spots; depending on the results, a reflex test for a panel of common pathogenic CFTR gene variants may be performed.^, False-negative results occur at an increased rate in infants with meconium ileus.

Diagnosis

In individuals with a positive IRT test, positive CFTR genetic test, or suggestive symptoms, sweat chloride testing is recommended. A sweat chloride test result of ≥60 mmol/L is diagnostic for CF, and no further testing is required. A sweat chloride test result of <30 mmol/L suggests that CF is unlikely, and further testing is not indicated unless clinical suspicion persists. An intermediate sweat chloride test result of 30-59 mmol/L should be clarified with CFTR genetic analysis. CFTR gene sequencing and deletion/duplication analysis should be performed if one or no pathogenic variants are identified on a CF panel of selected variants. If CFTR sequencing and deletion/duplication testing fails to identify two causative variants, functional analysis (nasal potential difference and intestinal current measurement) is recommended. If results are inconclusive, a diagnosis of CFTR-related metabolic syndrome/CF screen positive, inconclusive diagnosis (CRMS/CFSPID) is appropriate.

Monitoring

CF has numerous complications that require careful monitoring.

Laboratory Tests To Monitor Complications in Individuals With CF
Complication	Laboratory Tests	Recommended Frequency
Liver disease	Liver enzyme tests (and liver function tests in patients with cirrhosis)	Periodically
Exocrine pancreatic insufficiency	Fecal pancreatic elastase-1	Annually during infancy and childhood; testing should also be performed during periods of failure to thrive, weight loss, or diarrhea
Pancreatitis	Lipase and amylase tests	On presentation with recurrent unexplained pain and associated nausea/vomiting
Diabetes mellitus	Oral glucose tolerance test (confirm with second test)	Annually beginning at 10 yrs of age; more frequently if the individual is symptomatic or in cases of pulmonary exacerbation, initiation of glucocorticoids, enteral tube feeding, planning for pregnancy, pregnancy, and planned organ transplantation
Nutritional deficiency	Essential fatty acid status (if testing is available), fat-soluble vitamin testing	Annually
Allergic bronchopulmonary aspergillosis	Allergy skin testing, serum IgE, Aspergillus-specific IgE, Aspergillus serum precipitins tests	On presentation with symptoms
Airway fungal infections	Airway cultures	Every clinic visit
Bacterial infection (eg, with Pseudomonas aeruginosa or Staphylococcus aureus)	Cultures	On presentation with acute exacerbation of lung function and after treatment to ensure eradication
Nephrolithiasis	Urinalysis	On presentation with signs and symptoms
Medication toxicity	According to standard therapeutic drug monitoring protocols	When taking aminoglycosides
IgE, immunoglobulin E Source: Castellani, 2018