# Cystic Fibrosis: Screening, Diagnosis, and CFTR Modulators

## Overview

Cystic fibrosis is an autosomal recessive disorder caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene on chromosome 7. It is the most common life-limiting genetic disease in Caucasian populations, affecting approximately 1 in 2,500-3,500 live births. The CFTR gene encodes a chloride and bicarbonate channel expressed in epithelial cells throughout the body. Dysfunction leads to thick, dehydrated secretions in the lungs, pancreas, liver, intestines, and reproductive tract. Median predicted survival has improved dramatically to more than 50 years with CFTR modulator therapy, up from approximately 30 years in the pre-modulator era.

## Genetics

Over 2,000 CFTR mutations have been identified, classified into 6 classes based on their mechanism of protein dysfunction. F508del (Class II, protein misfolding and degradation) is the most common mutation worldwide, accounting for approximately 70% of alleles in CF populations. Class I mutations produce no protein (nonsense mutations such as G542X). Class II mutations cause protein misfolding with degradation in the endoplasmic reticulum (F508del). Class III mutations create gating defects where the protein reaches the membrane but the channel does not open properly (G551D). Class IV mutations reduce conductance (R117H). Class V mutations reduce the quantity of normal protein. Class VI mutations cause accelerated surface turnover. Genotype-phenotype correlation is imperfect, with modifier genes and environmental factors playing significant roles.

## Newborn Screening

Universal newborn screening for CF is performed in all 50 US states. The initial test measures immunoreactive trypsinogen (IRT), which is elevated in newborns with CF due to pancreatic duct obstruction. Cut-off values vary by state, with the top 5th percentile typically triggering further testing. Two main screening algorithms exist: IRT/IRT (repeat IRT on a second specimen at 2-4 weeks) and IRT/DNA (elevated IRT followed by a CFTR mutation panel detecting common mutations).

False negatives are possible, especially with rare mutations not included on the panel, so clinical suspicion should prompt sweat testing regardless of screening results. False positives are common, with only 3-5% of positive screens confirming CF; many are carriers. Meconium ileus at birth should prompt CF workup regardless of newborn screen status.

<image>Newborn screening algorithm for cystic fibrosis showing initial IRT measurement, branching into IRT/DNA pathway (with CFTR mutation analysis for 0, 1, or 2 mutations detected) and IRT/IRT pathway (repeat specimen), both leading to diagnostic sweat chloride testing</image>

## Diagnosis: Sweat Chloride Testing

The sweat chloride test is the gold standard diagnostic test for CF. Pilocarpine iontophoresis stimulates sweat production, and a minimum sweat volume is required for an adequate specimen. Results are interpreted as follows: 60 mmol/L or greater is diagnostic of CF, 30-59 mmol/L is intermediate or borderline requiring further evaluation with extended CFTR genetic testing, and less than 30 mmol/L makes CF unlikely. Testing must be performed at a CFF-accredited laboratory with experienced technicians, and repeat testing is recommended for borderline results. CFTR-related metabolic syndrome (CRMS) or CF screen positive with inconclusive diagnosis (CFSPID) describes infants with intermediate sweat chloride and 0-1 CFTR mutations who require longitudinal monitoring.

## Multisystem Manifestations

### Pulmonary

The pulmonary disease in CF involves chronic airway infection and inflammation leading to progressive bronchiectasis and lung destruction. The microbiology follows a characteristic progression: Staphylococcus aureus (including MRSA) and Haemophilus influenzae predominate in infancy and early childhood, while Pseudomonas aeruginosa (mucoid phenotype with biofilm formation) becomes dominant in adolescence and adulthood. Other important organisms include Burkholderia cepacia complex (associated with rapid decline and carrying infection control implications), Stenotrophomonas, Achromobacter, and non-tuberculous mycobacteria.

Acute pulmonary exacerbations present with increased cough, sputum production, dyspnea, and FEV1 decline, typically treated with IV antibiotics for 14-21 days. Airway clearance therapy includes chest physiotherapy, positive expiratory pressure devices, high-frequency chest wall oscillation (vest therapy), and exercise. Chronic pulmonary medications include inhaled hypertonic saline (7%), dornase alfa (Pulmozyme), inhaled tobramycin on alternating months, and azithromycin for its anti-inflammatory properties.

### Gastrointestinal/Pancreatic

Pancreatic insufficiency affects 85-90% of CF patients, causing maldigestion, steatorrhea, and failure to thrive. Treatment is pancreatic enzyme replacement therapy (PERT) with all meals and snacks, dosed at 500-2,500 lipase units/kg/meal while avoiding exceeding 10,000 units/kg/day (which risks fibrosing colonopathy). Meconium ileus occurs in 15-20% of CF newborns and represents a surgical emergency or is treated with Gastrografin enema. Distal intestinal obstruction syndrome (DIOS) is the older-patient equivalent. CF-related liver disease (focal biliary cirrhosis, portal hypertension) affects 5-10% of patients. CF-related diabetes (CFRD) develops in approximately 20% of adolescents and 40-50% of adults; it is treated with insulin (not metformin) and screened annually with OGTT from age 10.

### Nutritional

Patients require high caloric intake (120-150% of normal requirements), fat-soluble vitamin supplementation (A, D, E, K), salt supplementation in hot weather and during exercise, and BMI maintenance at or above the 50th percentile for age.

### Other Systems

Near-universal sinusitis with common nasal polyps, male infertility in over 98% (from congenital bilateral absence of the vas deferens), bone disease (osteopenia/osteoporosis from malabsorption, inflammation, and corticosteroid use), and digital clubbing in advanced disease are additional manifestations.

<image>Multisystem manifestations of cystic fibrosis depicted on a body diagram showing sinusitis and nasal polyps (head), bronchiectasis and chronic infection (lungs), pancreatic insufficiency and CFRD (pancreas), DIOS and meconium ileus (intestines), liver cirrhosis (liver), infertility (reproductive), and osteoporosis (bones)</image>

## CFTR Modulator Therapy

### Mechanism-Based Approach

CFTR modulators are classified by mechanism. Potentiators improve channel gating (opening) and target Class III mutations. Correctors improve protein folding and trafficking to the cell surface, targeting Class II mutations. Combination therapy uses correctors plus a potentiator for F508del.

### Available Modulators

| Modulator | Brand Name | Mechanism | Eligible Mutations | Age Approval | Key Efficacy |
|-----------|-----------|-----------|-------------------|-------------|-------------|
| Ivacaftor | Kalydeco | Potentiator | Gating mutations (G551D, others) | ≥4 months | FEV1 +10%, SwCl -50 mmol/L |
| Lumacaftor-ivacaftor | Orkambi | Corrector + potentiator | F508del homozygotes | ≥2 years | Modest FEV1 improvement |
| Tezacaftor-ivacaftor | Symdeko | Corrector + potentiator | F508del homozygotes | ≥6 years | Better tolerated than Orkambi |
| Elexacaftor-tezacaftor-ivacaftor | Trikafta (ETI) | Dual corrector + potentiator | ≥1 F508del allele (~90% of patients) | ≥2 years | FEV1 +14%, SwCl -40-50, exacerbations -60-70% |

Ivacaftor (Kalydeco) is a potentiator approved for gating mutations (G551D and others) in patients 4 months and older. It produces dramatic improvements: approximately 10% FEV1 increase, approximately 50 mmol/L sweat chloride decrease, and weight gain. Lumacaftor-ivacaftor (Orkambi) was the first therapy for F508del homozygotes, approved for ages 2 and older, with modest efficacy. Tezacaftor-ivacaftor (Symdeko) offers improved tolerability over Orkambi for ages 6 and older.

Elexacaftor-tezacaftor-ivacaftor (Trikafta/ETI) is a triple combination therapy that has been transformative. It is approved for ages 2 and older with at least one F508del mutation (homozygous or heterozygous with a minimal function mutation), covering approximately 90% of CF patients. Clinical results include FEV1 improvement of approximately 14% in both F508del homozygotes and heterozygotes, sweat chloride decrease of 40-50 mmol/L, pulmonary exacerbation reduction of 60-70%, and dramatic improvements in nutritional status and quality of life.

### Monitoring on CFTR Modulators

Monitoring includes liver function tests (hepatotoxicity risk, especially with lumacaftor), cataract screening in children on ivacaftor (lens opacities reported in juvenile animal studies), and awareness of CYP3A4 drug interactions requiring dose adjustment with strong inhibitors or inducers. Standard CF therapies (airway clearance, antibiotics) are still needed, though regimen simplification is being studied for patients with excellent modulator responses.

### Patients NOT Eligible for Current Modulators

Approximately 10% of CF patients have nonsense/stop mutations (Class I) that produce no functional protein. Read-through agents (ataluren) and gene therapy/editing are under investigation. Antisense oligonucleotides and mRNA therapy represent emerging approaches for these patients.

<image>Timeline of CFTR modulator development from 2012 (ivacaftor approval for G551D) to present, showing progressive expansion of eligible mutations and age groups, with key clinical trial results including FEV1 improvements and sweat chloride reductions for each modulator therapy</image>

## Clinical Pearls

Any infant with meconium ileus should be tested for CF (sweat chloride plus genetics) regardless of newborn screen results. A positive newborn screen does not diagnose CF; it identifies infants who need confirmatory sweat chloride testing. Sweat chloride testing is unreliable in the first 48 hours of life and in dehydrated or edematous infants; it is optimally performed after 2 weeks of age and 2 kg body weight. Trikafta has been so transformative that some CF centers are rethinking lung transplant waiting lists as patients improve dramatically. CFRD is unique in being primarily insulin deficiency without the autoimmune component or insulin resistance; HbA1c underestimates glycemia due to increased RBC turnover. CF should always be considered in any child with recurrent sinopulmonary infections, failure to thrive, and steatorrhea, even in non-Caucasian populations.

## Key Controversy: Expanding CFTR Modulators to Younger Ages

ETI approval has been expanded from ages 12 and older down to ages 2 and older, with clinical trials ongoing for infants. The rationale is that early treatment may prevent irreversible lung damage, pancreatic destruction, and nutritional consequences. Concerns include unknown long-term safety of lifelong CFTR modulator use starting in infancy, cataract risk identified in animal models with ivacaftor, impact on the CF transplant landscape as patients live longer with native lungs, and cost (approximately $300,000 per year per patient) with significant access and insurance coverage disparities globally.

## References
- Middleton PG, et al. Elexacaftor-Tezacaftor-Ivacaftor for Cystic Fibrosis with a Single Phe508del Allele. N Engl J Med. 2019;381(19):1809-1819.
- Ramsey BW, et al. A CFTR Potentiator in Patients with Cystic Fibrosis and the G551D Mutation (STRIVE). N Engl J Med. 2011;365(18):1663-1672.
- Borowitz D, et al. Cystic Fibrosis Foundation Practice Guidelines for the Management of Infants with Cystic Fibrosis. J Pediatr. 2009;155(6 Suppl):S73-S93.
- Farrell PM, et al. Diagnosis of Cystic Fibrosis: Consensus Guidelines from the Cystic Fibrosis Foundation. J Pediatr. 2017;181S:S4-S15.
- Moen IE, et al. Long-Term Safety and Efficacy of Elexacaftor/Tezacaftor/Ivacaftor in People with CF. Lancet Respir Med. 2023;11(4):345-357.
- Cystic Fibrosis Foundation Patient Registry Annual Data Report. 2022.