• Patients and families

• Health professionals

• Contacts

• Links

Information for health care professionals

• Home
• Newborn screening
• Diagnosis
• Airway cultures
• Staph aureus MSSA
• MRSA
• Pseudomonas
• Cepacia
• NTM
• Infection control
• Oral antibiotics
• Intravenous antibiotics
• Hearing loss
• Inhaled antibiotics
• DNase and HS
• Pancreatic enzymes
• Vitamin supplements
• Low vitamin D
• Salt supplements
• ABPA
• Bone disease
• CFRD
• DIOS
• Haemoptysis
• Liver disease
• Meconium ileus
• Transition

Diagnosis

The sweat test

The sweat test remains the gold standard.

How to do a sweat test

A sweat test is a quantitative measurement of electrolytes in sweat. Sweat is collected after pilocarpine iontophoresis and accurate results are dependent on careful technique. Sweat is collected for 20–30min. The sweating rate should exceed 1g/m2/min giving an adequate volume of sweat for analysis: 15µL if collected in a Macroduct system or 100mg weight if collected on filter paper.

How to interpret a sweat test

All the clinics in the OCCFN use the Macroduct system and using this system the first measurement obtained will a conductivity. All conductivities below 40mEq/L will be normal (the approximate sweat chloride can be derived from the equation: sweat chloride = [conductivity x 1.2] - 36). Ideally all sweat samples should be sent to the lab for a formal chloride measurement; certainly all those with a conductivity above 40mEq/L must go to the lab. A sweat chloride concentration of > 60mmol/L supports a diagnosis of CF. The majority of children with CF will have a sweat chloride in excess of 90mmol/L. Borderline sweat tests (chloride 40mmol/L-60mmol/L in older children, and >30mmol/L in early infancy) should be repeated and interpreted in the context of the clinical picture. Sweat tests are valid from after the first 48h of life, although it can be difficult to get enough sweat from babies under 4kg, and this can delay definitive diagnosis if genetic analysis is inconclusive. Sweat chloride levels are lower in normal neonates than in older children and adults. The mean sweat chloride in infants at around 6 weeks of age is 10mmol/L compared with adult levels of around 20-25mmol/L. Healthy heterozygotes have slightly higher sweat chlorides - a mean of around 15mmol/L in early infancy. A sweat chloride of 40mmol/L is more than 3 standard deviations above this level and should be considered to be highly suggestive of cystic fibrosis in this age group. Sweat chlorides above 30mmol/L should be considered borderline, particularly if there are other factors in favour of a diagnosis of CF. Sweat sodium should not be interpreted without a chloride result.

Genetic analysis

Standard genetic analysis testing for the commonest 32 mutations can be used as a diagnostic test and will confirm the diagnosis (2 disease-causing mutations found) in 70% of patients (of northern European descent). In the remainder, only one or occasionally no CF mutations will be found using standard methods. With newer sequencing methods it is now possible screen all of the coding regions and most of the splicing regions of the CFTR gene; this approach will identify the majority of rarer mutations and may lead to a confirmation of the diagnosis. Sequence analysis takes 2-3 months, and it is best suited as a diagnostic test only for the very occasional patients with mild disease and normal or border-line sweat test values in whom CF is still suspected. All patients diagnosed with CF should have blood sent for genetic analysis, both to allow genetic counselling and to provide information that may become important when considering novel therapies.