Tests to Diagnose Cystic Fibrosis

Cystic fibrosis, also named “Mucoviscidosis”, due to its pathogenesis of increase in viscosity of mucus, which forms the main reason for the clinical manifestations of the patient. As the disease is more prevalent in countries like United States and in European countries, the knowledge of clinical features and tests to diagnose cystic fibrosis before birth in newborn and in adults, is essential.

The diagnosis of cystic fibrosis mainly rests on the clinical features when patient presents to the hospital. In most of the countries the newborn screening is missed and patient progresses to symptoms and complications when they are diagnosed as having cystic fibrosis with the help of genetic testing and blood tests. Due to its high incidence, newborn screening is done in most of the hospitals in United states.

All the tests are based upon abnormal function of CFTR gene.

The tests done to diagnose Cystic fibrosis are:

1. Blood tests to diagnose Cystic fibrosis

2. Sweat chloride test – Confirmatory test of Cystic fibrosis

3. DNA mutational analysis

4. Nasal potential difference test

5. Tests to diagnose infections associated with Cystic fibrosis

6. Tests ordered for particular organ/system involvement

Blood tests to diagnose Cystic fibrosis:

The blood tests to diagnose Cystic fibrosis are done in the period after a baby is born to detect for “Immunogenic Trypsinogen” in the blood which is a digestive enzyme. It is raised in the babies with CFTR abnormal gene, it is not a confirmatory test as the enzyme levels are raised even in carriers of the disease in whom only one gene is abnormal – Heterozygous, and they do not progress to exhibition of clinical manifestations.

The positivity of blood test with raised Immuno trypsinogen levels in the newborn strongly suggests the need to undergo ‘Sweat chloride test’ which is a confirmatory test.

Sweat chloride test – confirmatory test:

Sweat chloride test is ordered in high risk infants or when the blood test shows raised Immunogenic trypsinogen levels or when the adult shows suspected clinical manifestations.

Increased sodium and chloride concentrations in the sweat of the Cystic fibrosis patients forms the basis of sweat chloride test.

Procedure of ‘Sweat chloride’ test:

The skin [forearm, thigh] of newborn or suspected adult is stimulated for the production of sweat with the help of two electrodes, one electrode directly applied to skin and the other on the area of skin precoated with drugs like pilocarpine. Mild current is passed through the electrodes which is painless and once the sweat is generated, it is collected by gauze piece and tested for raised chloride levels, the test is usually repeated twice on two separate days to confirm the test result without any bias.

The chloride concentration of greater than 70 meq/l confirms the diagnosis of cystic fibrosis. The test is highly predictive of the disease though in 1 to 2 % of patients normal sweat chloride levels are noticed inspite of the presence of disease.

DNA mutational analysis:

DNA mutational analysis is done to know whether the person is carrier or diseased. The blood sample of the patient is collected or sometimes the scrapings from the cheek are taken and the cells are tested for presence of mutation in CFTR gene. The genetic material is amplified to know if the mutational gene is present. As more than 1600 mutations are discovered, it is not possible to test for all the mutations. The basic mutational analysis referred to as ’23 mutational panel’ tests for most common mutations. At least 23 mutations are to be tested and the test is positive in 7o to 90% of Cystic fibrosis patients. The basic test is recommended by” American college of medical genetics”.

Two copies of CFTR gene alleles are present in each cell. If one copy of gene has mutation, he is a carrier and if both alleles are having mutation, he is diagnosed to have Cystic fibrosis.

DNA mutational analysis helps to know the pancreatic genotype – phenotype relationship of the affected individual and the mutation- class specific treatments are developed which help in categorizing the affected individual and planning the treatment course accordingly for the child/adult.

Nasal potential difference test:

The test is an alternative confirmatory test in patients of Cystic fibrosis. The patients nasal mucosal potential difference is decreased in patients with Cystic fibrosis. In normal patients, the normal CFTR gene creates a chloride channel which lines the nasal mucosal epithelium as well as epithelium of airway of the lungs. The normal transport of sodium and chloride ions across the epithelial membrane maintains the normal electrical potential. In Cystic fibrosis patients due to abnormality of CFTR gene, the function of chloride transport channel is defective, thus the base line electrical potential is decreased

procedure:

Two electrodes are placed, one on the nasal mucosa and other on the subcutaneous portion of skin, generally the forearm and the electrical potential difference between two is noted. In normal persons the measurement is in between -10mv to -30mv, where as in Cystic fibrosis patients, a more negative baseline difference is seen from -40mv and can decrease further based on progression and severity of disease, say -50, -60 and so on. The negativity further decreases when a substance like amiloride, a sodium channel blocker is applied on the nasal mucosa and the test is done. The drug blocks the sodium entry and decreases the electrical potential more aggressively in Cystic fibrosis patients than in normal individuals.

Nasal potential difference measurement is not on a routine basis as the technique is difficult and the equipment is not owned by all hospitals, but present in research centres.

The importance of nasal potential difference arises when sweat chloride test and DNA mutational analysis both are negative, but the newborn is of high risk category or adult is showing clinical manifestations.

Tests to diagnose specific infections:

The infections associated with Cystic fibrosis are mostly lung infections due to the remodelling and mucus accumulation in the lung. Common organisms are staph, hemophilus and can be complicated with the superinfection of pseudomonas, Mostly bacterial infections are noticed, but in severe conditions fungal invasion can be present the most common fungal infection in Cystic fibrosis pateints being ‘ Allergic pulmonary aspergillosis ‘.

The sputum is taken for culture to diagnose specific infections.

Oher tests based on speific systems involved:

Lungs :

Chest X-ray

CT scan

MRI

PULMONARY FUNCTION TESTS: Pulmonary function tests are performed to know the capacity of lungs, how fast the breathe in and out of air takes place and the amount of oxygenation the tissues get in the lungs.

Intestines :

CT SCAN

MRI

Pancreas:

Pancreatic function can be illustrated by measurement of enzyme levels like trypsin and chymotrypsin in stools, Fecal fat estimation in stools, by performing secretin stimulation test and screening for presence of diabetes by measuring blood sugar levels.

Prenatal diagnosis of Cystic fibrosis can improve the quality of life and life expectancy of the patient and it plays major role among all diagnostic tests.