Makindo Medical Notes

Related Subjects: |Cystic Fibrosis |Sweat Test |Encopresis in Children |Enuresis/Bedwetting in Children |Acute Glomerulonephritis in Children |Nephrotic Syndrome in Children |Acute Appendicitis in Children |Gastro-oesophageal reflux in Children |Intussusception in Children |Panayiotopoulos Syndrome in Children |Reflex anoxic attacks in Children |Paediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections

A sweat chloride concentration of less than 40mmol/L is considered normal, suggesting a low probability of cystic fibrosis (CF). A sweat chloride concentration of greater than 60mmol/L supports the diagnosis of CF. Intermediate results (40-60mmol/L) are suggestive but not diagnostic of cystic fibrosis and may require further investigation.

Purpose of the Test

• The sweat test is the most reliable diagnostic test for cystic fibrosis (CF) in both children and adults.
• This test measures the concentration of chloride in sweat, which is abnormally high in individuals with cystic fibrosis due to defective chloride channels.

Procedure

• Caveat: The sweat test is technically challenging and can yield false positives or false negatives. It is important to have the test conducted by an experienced practitioner.
• Induction of sweat: Pilocarpine is used to stimulate sweating. It is applied to the skin via iontophoresis, a process involving a mild electrical current to enhance the absorption of the drug.
• Collection of sweat: Sweat is collected on filter paper or a specially designed collector for a duration of 30-60 minutes.
• Measurement: The chloride concentration in the collected sweat is then measured using a sweat analyzer.
• False-negative sweat test: Edema is the most common cause of false negatives. Inadequate technique or improper collection may also lead to inaccurate results.

False-Positive Sweat Test Results

• Atopic eczema
• Adrenal insufficiency
• Ectodermal dysplasia
• Glycogen storage diseases
• Hypothyroidism
• Dehydration
• Malnutrition

Normal and Abnormal Results

• Normal result: Chloride levels less than 30-40mmol/L are considered normal, indicating a low likelihood of cystic fibrosis.
• Abnormal result: Chloride levels greater than 60mmol/L are highly suggestive of cystic fibrosis, especially when the result is consistent with clinical symptoms and a family history.
• Intermediate result: Chloride levels between 40-60mmol/L are considered borderline and may require repeat testing or additional diagnostic procedures.

Indications for the Test

• Presence of symptoms suggestive of cystic fibrosis, such as chronic respiratory infections, failure to thrive, and gastrointestinal issues.
• A positive family history of cystic fibrosis or known carriers of the CF gene.
• Abnormal newborn screening results suggesting cystic fibrosis.

Test Limitations

• The sweat test can be influenced by the patient’s age. Young children, in particular, may produce insufficient sweat, making the test more difficult to perform accurately.
• In rare cases, patients with cystic fibrosis may have borderline or normal sweat chloride levels, requiring further testing, such as genetic testing or a detailed clinical evaluation.

Follow-up

• If the sweat test result is borderline or there is uncertainty in the diagnosis, further genetic testing (e.g., CFTR gene analysis) may be needed for confirmatory diagnosis.
• Early diagnosis of cystic fibrosis allows for timely intervention, including enzyme replacement therapy, nutritional support, and airway clearance techniques to manage the disease and improve quality of life.

Availability in the UK

• The sweat test is widely available across the UK, typically performed in specialist cystic fibrosis centers, pediatric departments, or hospitals with respiratory medicine services.
• It is included in routine newborn screening for cystic fibrosis across the UK, enabling early detection in newborns.