Laboratory Studies

The diagnosis of cystic fibrosis (CF) is based on typical pulmonary manifestations, GI tract manifestations, a family history, and positive sweat test results.

• Sweat test⁶
  • Several methods are used to conduct a sweat test. Performed properly, the quantitative pilocarpine iontophoresis test (QPIT) to collect sweat and perform a chemical analysis of its chloride content is currently considered to be the only adequately sensitive and specific type of sweat test (see Cystic Fibrosis Foundation).
  • For reliable results, collect at least 50 mg or, preferably, 100 mg of sweat. Current macroduct collection methods allow adequate analysis with smaller volumes of sweat. If a macroduct coil is used for collection, then sweat must be stimulated with a disposable Pilogel electrode using the Webster Sweat Inducer for 5 minutes. After 30 minutes, the minimum acceptable sample is 15 µL. The sweat chloride reference value is less than 40 mmol/L, and a value of more than 60 mmol/L of chloride in the sweat is consistent with a diagnosis of cystic fibrosis. In babies aged 3 months or younger, results of 30-60 mEq/L are considered borderline and require retesting.⁶
  • The sweat test must be performed at least twice in each patient, preferably several weeks apart. Values of 40-60 mmol/L are considered borderline, and the test must be repeated because these values have been found to be consistent with the diagnosis in some patients with typical features. Repeat a sweat test to confirm positive results. Repeat a sweat test with negative results if clinical features suggestive of cystic fibrosis are present. Some patients with genetically documented cystic fibrosis and typical symptoms have consistently negative results on sweat tests.
  • Other causes of elevated levels of sweat chloride include the following:
    • Untreated adrenal insufficiency
    • Glycogen storage disease
    • Type I fucosidosis
    • Hypothyroidism
    • Vasopressin-resistant diabetes insipidus
    • Ectodermal dysplasia
    • Malnutrition
    • Mucopolysaccharidosis
    • Panhypopituitarism
    • Familial cholestasis
    • Familial hypoparathyroidism
    • Atopic dermatitis
    • Iatrogenic causes (ie, infusion of prostaglandin E₁, improper technique)
• Neonatal screening: Multiple states in the United States have already implemented neonatal screening for cystic fibrosis. All screening algorithms in current use in the United States rely on testing for immunoreactive trypsinogen (IRT) as the primary screen for cystic fibrosis.⁷ The presence of high levels of IRT, a pancreatic protein typically elevated in infants with cystic fibrosis, warrants second level testing in the form of repeat IRT testing, DNA testing, or both. Within these 2 categories, various modifications are used. Evidence suggests that newborn screening is decreasing the incidence of cystic fibrosis.⁸

Imaging Studies

• Chest radiography: Initial changes are hyperinflation and peribronchial thickening. Progressive airtrapping with bronchiectasis may be apparent in the upper lobes. With advancing pulmonary disease (see Media file 1), pulmonary nodules resulting from abscesses, infiltrates with or without lobar atelectasis, marked hyperinflation with flattened domes of the diaphragm, thoracic kyphosis, and bowing of the sternum develop. Pulmonary artery dilatation and right ventricular hypertrophy associated with cor pulmonale is usually masked by marked hyperinflation. Several radiologic scoring systems are recognized.
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  Chest radiograph of a patient with advanced cystic fibrosis. Note marked hyperinflation, peribronchial thickening, and bilateral infiltrates with evidence of bronchiectasis especially of the upper lobes.

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  Chest radiograph of a patient with advanced cystic fibrosis. Note marked hyperinflation, peribronchial thickening, and bilateral infiltrates with evidence of bronchiectasis especially of the upper lobes.

• Sinus radiography: Panopacification of the sinuses is present in almost all patients with cystic fibrosis, and its presence is strongly suggestive of the diagnosis. Conversely, absence of panopacification strongly suggests that cystic fibrosis is not present.

Other Tests

• Genotyping
  • More than 1600 cystic fibrosis mutations have been identified.⁵ In the commercially available cystic fibrosis gene sequencing method, the entire coding region, splice junction sites, and promoter region of the CFTR gene are amplified from genomic DNA by polymerase chain reaction (PCR) and then subjected to nucleotide sequence analysis on an automated capillary DNA sequencer. This test can detect more than 98% of disease-causing mutations. The detection rate is lower in black, Hispanic, and Asian populations; therefore, failure to find 2 abnormal genes does not exclude the disease.
  • A finding of 2 CFTR mutations in association with clinical symptoms is diagnostic, but negative results on genotype analysis do not exclude the diagnosis.
• Semen analysis: Obstructive azoospermia, in the absence of any other obvious cause (eg, vasectomy), provides additional corroborative evidence for the diagnosis of cystic fibrosis. Confirm results from semen analysis by obtaining a testicular biopsy.
• Nasal potential difference measurement
  • Potential difference (PD) (voltage) measured from nasal mucosa and the reading obtained by a reference electrode inserted into the forearm correlates with the movement of sodium across cell membranes, which is a physiologic function rendered abnormal by a CFTR mutation. The nasal PD (NPD) is a sensitive test of electrolyte transport that can be used to support or refute a diagnosis of cystic fibrosis. A normal mean value standard error (SE) is 0.9-24.7 mV; an abnormal value is 1.8-53 mV. When measurements are repeated after mucosal perfusion with amiloride to block an epithelial sodium channel, the drop in PD is greater in patients with cystic fibrosis (73%) than in control subjects (53%). Normally, subsequent perfusion with chloride-free solution and isoproterenol produces a sharp increase in the PD but has little effect when CFTR function is abnormal.
  • As a result of the lack of commercially available equipment and the practical difficulties with NPD measurement, this test is performed in only a few research centers to diagnose cystic fibrosis in patients in whom making a diagnosis is difficult or a sweat test is not technically possible because of skin problems.

Procedures

• Pulmonary function testing (PFT)
  • Standard spirometry may not be reliable until patients are aged 5-6 years; however, some younger patients can be taught to do reproducible maneuvers. Partial flow-volume curves may show abnormalities, in addition to an elevated airway resistance, and hyperinflation.
  • The recently described forced oscillation technique (FOT), which uses the impulse oscillometry system (IOS), can be used successfully in younger children. Airway resistance measured by IOS has been found to be similar to the airway resistance measured by body plethysmography, and this technique has been successfully used to measure lung function in young patients with cystic fibrosis unable to perform spirometry.⁹
  • Typically, peripheral airway involvement resulting from cystic fibrosis manifests as an obstructive defect with airtrapping and hyperinflation; oxyhemoglobin desaturation may occur because of a ventilation-perfusion mismatch. In the early stages, forced expiratory volume in 1 second (FEV₁) may be normal and forced expiratory flow (FEF) after 25-75% of vital capacity has been expelled (FEF 25-75) is reduced, suggesting small airway involvement. As the disease progresses, FEV₁ is also reduced.
  • The associated airtrapping results in an elevated ratio of residual volume to total lung capacity (RV/TLC). With hyperinflation, TLC is also increased. In patients with advanced disease, extensive lung changes with fibrosis are reflected as restrictive changes characterized by declining TLC and vital capacity.
• Bronchoalveolar lavage: Airway inflammation is the hallmark of cystic fibrosis lung disease. Studies suggest airway inflammation even in the absence of infection. Bronchoalveolar lavage fluid usually shows a high percentage of neutrophils, and recovery of Pseudomonas aeruginosa from bronchoalveolar lavage fluid supports the diagnosis of cystic fibrosis in a clinically atypical case.
• Sputum microbiology: The most common bacterial pathogens in the sputum of patients with cystic fibrosis are Haemophilus influenzae, Staphylococcus aureus, P aeruginosa, Burkholderia cepacia, Escherichia coli, and Klebsiella pneumoniae. Findings of P aeruginosa, especially the mucoid form, support the diagnosis of cystic fibrosis in children.