Cystic fibrosis - Diagnosis and Screening Strategies

Diagnosis of Cystic Fibrosis Introduction Cystic fibrosis (CF) diagnosis relies on a combination of clinical and laboratory tests rather than a single definitive test. The diagnostic approach typically follows a stepwise pattern: identifying at-risk individuals through newborn screening, confirming the diagnosis through biochemical testing, and then using genetic and additional assessments to fully characterize the disease and guide management. Newborn Screening Most developed regions implement universal newborn screening for CF, which involves measuring immunoreactive trypsinogen (IRT) levels in blood spots collected from newborns (typically on day 2-3 of life). Why IRT? In CF, pancreatic tissue damage and ductal obstruction causes trypsinogen to leak into the bloodstream at elevated concentrations. This elevation is detectable in newborns, making IRT an excellent screening marker. The advantage of early detection through newborn screening is substantial: infants identified before symptoms appear can begin treatment immediately, which significantly improves long-term lung function and nutritional status compared to those diagnosed later through symptom recognition. Sweat Chloride Test The sweat test (pilocarpine iontophoresis) is the gold standard confirmatory test for CF diagnosis. Here's how it works: The Procedure: A small electric current delivers pilocarpine (a drug that stimulates sweat glands) through the skin. This induces localized sweating, and the sweat is collected on filter paper or in a capillary tube. Why This Works: CF is caused by defective CFTR (cystic fibrosis transmembrane conductance regulator) protein in cell membranes. In the sweat glands, the CFTR normally reabsorbs chloride ions from sweat as it's produced. When CFTR is defective, chloride reabsorption fails, causing abnormally high chloride concentration in the sweat. Interpreting Results: Chloride concentration >60 mEq/L = CF diagnosis confirmed This elevated chloride is essentially "liquid evidence" of defective CFTR function The sweat test has excellent sensitivity and specificity—it doesn't just detect CF, it directly measures the biochemical consequence of the genetic defect. Genetic Testing Once CF is suspected or confirmed, genetic testing identifies the specific CFTR mutations causing the disease. This serves two important purposes: confirming the diagnosis at the molecular level and informing family screening. Standard Panels: Most laboratories use panels that detect the 30–96 most common CFTR mutations, which identify >90% of CF cases. These panels look for the mutations found with highest frequency in the population being tested. When Full Sequencing Is Needed: If standard panels don't identify mutations in a patient with confirmed CF (positive sweat test), full gene sequencing is performed. This identifies rare or previously unknown mutations. Full sequencing can detect nearly 100% of mutations when they're actually present. Practical Value: Genetic testing also identifies carriers (people with one mutated allele who are unaffected), enabling family screening and reproductive counseling. Additional Assessments Beyond the initial diagnosis, several additional tests help characterize disease severity and guide ongoing management: Sputum Culture Sputum samples are cultured to identify which bacteria have colonized the airways. Different CF patients are colonized by different pathogens (commonly Pseudomonas aeruginosa, Burkholderia cepacia, or Staphylococcus aureus). Identifying these organisms enables targeted antibiotic therapy rather than empiric treatment. Imaging Studies High-resolution computed tomography (HRCT) of the lungs evaluates: Presence and extent of bronchiectasis (abnormal airway dilation) Severity of bronchial wall thickening Pattern of air trapping Overall structural lung disease burden These findings help establish baseline disease severity and are used to monitor progression over time. Pulmonary Function Testing Spirometry measures forced expiratory volume in one second (FEV₁), expressed as a percentage of predicted normal value for the patient's age, height, and sex. Serial spirometry tracking over months and years reveals whether lung function is stable, declining, or improving with treatment. This is one of the most important outcome measures for CF care.