Sweat Electrolytes, Cystic Fibrosis and Diagnosis
Cystic Fibrosis
Sample for Cystic Fibrosis
- At Least 100 mg of sweat is collected.
- Sweat is collected by applying the chemical to the skin.
- Sweat may be collected on paper after pilocarpine and low electric current stimulation.
Precautions for Cystic Fibrosis
- The infant should be at least 48 hours of age.
- A proper collection of sweat is very difficult.
- Take care of evaporation and contamination.
- Use gauze or filter paper which should be low in electrolyte contents.
- Wash and dry the patient’s skin thoroughly after the stimulation and before collection with deionized water.
- Minimum sweat weight or volume is critical to get an accurate result.
Indications for sweat electrolytes
- This test is done to confirm the diagnosis of cystic fibrosis.
- Sweat is tested for chloride.
Pathophysiology of Cystic Fibrosis
Definition of Cystic Fibrosis:
- This is inherited, an autosomal recessive disease that affects nearly all the exocrine glands in the body.
- Cystic fibrosis transmitted autosomal recessive trait.
- Cystic fibrosis is also called mucoviscidosis of the pancreas.
- Cystic fibrosis is a disorder of mucous secretion with occlusion of exocrine glands function.
- This will produce sweat with very high contents of sodium and chloride.
Cystic Fibrosis presentation:
- It is most common in European, with a frequency of 1 in 2000 live births.
- In Afro-Americans, its incidence is around 2%.
- It is rare in Asians.
- Cystic fibrosis is present at birth and persists throughout life.
- Cystic fibrosis is a quite common genetic disorder in the Caucasian population.
- Almost all exocrine glands produce abnormal mucus that obstructs the glands and ducts and damages tissue.
- Because of this, abnormal exocrine secretion in cystic fibrosis develops mucous plugs that obstruct the pancreatic ducts.
- The pancreatic enzymes like amylase, lipase, trypsin, and chymotrypsin cannot be expelled into the duodenum.
- These secretions will either be absent or very low in the duodenal aspirate.
- There are abnormal secretions in:
- Lung bronchi.
- Small intestine.
- Pancreatic duct.
- Bile ducts.
- Skin (sweat glands).
There is a wide spectrum of the disease:
- Chronic obstructive pulmonary disease.
- There is recurrent productive cough, dyspnoea, and wheezing.
- There are recurrent airway infections.
- A pancreatic deficiency (pancreatic insufficiency) may occur due to an abnormally viscous fluid, causing a decrease in amylase and lipase activity.
- There is decreased trypsin and chymotrypsin level.
- There is a decreased bicarbonate level.
- Pancreatic insufficiency also affects glucose metabolism through delayed insulin response.
- There may be recurrent pancreatitis.
- There may be distal intestinal obstruction syndrome.
- There is decreased fat absorption.
- There is nutritional deficiency.
- There are chronic liver diseases.
- The patient may show the effects of the disease through a reduced level of Vitamin A, carotenoids, Vitamin E, cholesterol, and essential fatty acids.
- In the USA, 1/3 of the patients with cystic fibrosis are adults.
- These patients have an increased risk of malignancies of the GI tract, arthropathies, and osteopenia.
- There are electrolytes in the sweat.
Mechanism of cystic fibrosis
- The mucus glands produce abnormal viscid secretions.
- This abnormal secretion may inspissate and plug the ducts of the glands.
- This process will lead to obstructive complications.
- In the lungs may lead to recurrent bronchopneumonia, which is a fatal complication.
- The most common pathogens are Pseudomonas and Staphylococcus aureus.
- Next, the most common complication is the destruction of the exocrine portion of the pancreas;
- The pancreatic disease will lead to malabsorption, steatorrhea, digestive abnormalities, and malnutrition.
- Less common abnormalities are biliary liver cirrhosis due to bile duct obstruction.
- In newborns, meconium may lead to paralytic ileus (meconium ileus) and intestinal obstruction.
- There is an increased loss of chloride and sodium in the sweat. This factor is the bases for the diagnosis of cystic fibrosis.
Clinical presentation of Cystic Fibrosis
- Cystic fibrosis is suspected in a young adult if there is a history of:
- Chronic lung disease (Bronchiectasis).
- A chronic cough with sputum production.
- Recurrent hemoptysis.
- There are sinus tenderness, purulent nasal secretion, and nasal polyps.
- Infertility.
- Pancreatitis.
- Chronic lung disease (Bronchiectasis).
- There may be abdominal pain, diarrhea, and steatorrhea.
- Almost all men have a congenital bilateral absence of the vas deferens with azoospermia.
- There may be the occurrence of gallstones.
- There may be biliary cirrhosis.
Diagnosis of Cystic fibrosis:
- It depends upon the following:
- The clinical symptoms.
- Positive sweat test of increased chloride and sodium amount.
- The serum concentration of immunoreactive trypsin is elevated in a newborn with cystic fibrosis.
- The measurement of these enzymes is the basis of the cystic fibrosis newborn screening program.
- Genetic analysis can be used for counseling families on gene carrier status.
- The serum concentration of immunoreactive trypsin is elevated in a newborn with cystic fibrosis.
Procedure for sweat collection for the diagnosis of cystic fibrosis:
- Sample collected in three stages:
- Stimulation for sweating by pilocarpine (This is a muscarinic alkaloid drug that can induce sweating). There is a local injection into the skin to produce sweat, which is collected.
- This is the iontophoresis procedure.
- Collection of the sweat.
- Sweat can be collected on gauze or filter paper.
- Qualitative or quantitative analysis of sweat.
- Electrode method: Electrode is applied to the skin, where sweat is produced by the colorless chemical. This may take 5 minutes to collect the sweat.
- Electrodes are stimulated, and sweat is put on the gauze or paper.
- This collected sample is sent to the lab.
- The sweat, considered a good collection, should weigh 50 mg. If this is <50 mg, that is not a good sweat collection.
- There is a new method where the blood sample is not taken, but a sweat-analyzing skin patch can be used.
- Sweat chloride >60 meq/L (>60 mmol/L) and sodium >70 meq/L (>70 mmom/L) are considered abnormal.
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The critical issues of sweat electrolytes test:
- The proper collection of sweat is very difficult.
- Avoid evaporation and contamination of the sample.
- Collect a sufficient amount and minimize skin reactions.
- The minimum weight or volume of the sweat is critical for the sweat test.
- In the case of newborns, in the first three days of life, sodium and chloride is higher, and the level comes down after 4th day.
Normal Cystic fibrosis
Source 1
Sweat Chloride (iontophoresis)
- Normal = 5 to35 meq/L
- Marginal = 30 to 70 meq/L
- Cystic fibrosis = 60 200 meq/L
Sweat Sodium ((iontophoresis)
- Child and adult = 10 to 40 meq/L
- Child, X: = 27 meq/L
- Adult, X: = 33 meq/L
- Cystic fibrosis = 70 to 190 meq/L
- To convert into SI unit x 1.0 = mmol/L
Source 2
Sodium values in children
- Normal = <70 meq/L
- Abnormal = >90 meq/L
- Equivocal = 70 to 90 meq/L
Chloride values in children
- Normal = <50 meq/L
- Abnormal = >60 meq/L
- Equivocal = 50 to 60 meq/L
Another source
- Normal Chloride = 5 to 35 mmol/L (<50 meq/L)
- Cystic fibrosis evidence:
- Sweat Chloride = 40 to 59 mmol/L (does not confirm the diagnosis of cystic fibrosis).
- Sweat chloride less or equal to 39 mmol/L in an infant over 6 months old is an unlikely cystic fibrosis diagnosis.
- Sweat Chloride 60 or >60 mmol/L is diagnostic of cystic fibrosis.
- Cystic fibrosis evidence:
- Normal Sodium = <70 mmol/L (<70 meq/L)
- Cystic fibrosis = >90 mmol/L (>90 meq/L)
Sweat electrolytes increased in:
- Cystic fibrosis of the pancreas.
- Sodium
- 50 to 140 meq / L and mean maybe 103 meq / L.
- Chloride
- Normal = 4 to 60 meq / L.
- In diseases is 50 to 120 meq / L.
- Potassium
- Normal mean = 9 meq / L.
- In the disease = mean 15 meq / L.
- Sodium
Other causes than cystic fibrosis are:
- Addison disease (Untreated adrenal insufficiency).
- Pseudohypoaldosteronism.
- Some unusual diseases like Glucose – 6 phosphate deficiency, glycogen storage disease, and diabetes insipidus.
- Anorexia nervosa
- Atopic dermatitis.
- Familial cholestasis.
- Klinefelter syndrome.
- Nephrosis.
- Untreated hypothyroidism.
- Protein-calorie malnutrition.
- Ectodermal Dysplasia.
- Environmental deprivation.
- Nephrosis and nephrogenic diabetes inspidus.
- Psychosocial failure to thrive.
Questions and answers:
Question 1: Can you see sweat test positive in some diseases?
Question 2: What is difficulty in sweat test?