The sweat test remains the gold standard for confirming the diagnosis of cystic fibrosis (Littlewood, 1986). The test should always be performed by experienced laboratory personnel in centres who carry out a minimum of 50 sweat tests per annum. It should be repeated at least twice if genotyping does not fully support the diagnosis (Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002).
When to perform a sweat test
The sweat test may be attempted in term infants after seven days of age if clinically indicated although a follow up test will need to be carried out at a later date if an insufficient quantity of sweat is collected
Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002. In term infants, sweat electrolytes can be raised in the first seven days after birth, especially in the first 48 hours of life (Hardy et al, 1973). It is more routine to undertake sweat testing of infants after two weeks of age as long as their weight is greater than three kg, they remain normally hydrated and in the absence of significant systemic illness Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002). The administration of flucloxacillin is not a contraindication to sweat testing. There is little published data on the effects of other antibiotics (Williams et al , 1988). Sweat electrolytes do not appear to be affected by the use of diuretics or intravenous fluids in clinically stable patients (Littlewood, 1986); Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002).
When to avoid sweat testing
Sweat testing should be delayed in subjects who are either dehydrated, underweight (infants), systemically unwell, have a cutaneous rash affecting the potential stimulation site (eczema may increase sweat electrolytes), peripheral oedema or sytemic steroids administration (Littlewood, 1986; MacLean et al, 1973, Brand et al, 1996; Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002).
Age and the sweat test
Sweat chloride concentrations appear to increase with age in normal children between one to 12 years of age (Kirk et al, 1992). In contrast sweat chloride concentrations do not appear to increase in children with cystic fibrosis or healthy subjects over the age of 12 years. A number of studies have reported an age related increase in sweat sodium concentration and sodium to chloride ratio.
The sweat test
Sweat should be collected for not more than 30 minutes and not less than 20 minutes using quantative pilocarpine nitrate iontophoresis. The flexor surface of the forearm is the preferred site for sweat collection, although other locations such as the back, chest and thigh have been used successfully (Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002). A weight of at least 100 mg of sweat is required.
The sweat test remains central to the diagnosis of CF and it is essential that it is performed by experienced personnel in accordance with national guidelines (National Committee for Clinical Laboratory Standards, 2000; Association for Clinical Biochemistry, 2002).
When analyzing a sweat test the chloride concentration shows greater discrimination than sodium (Green et al, 1985; Hall et al, 1990; Gleeson et al, 1991, Kirk et al, 1992, Shwachman et al, 1981). A chloride value of >60 mmol/l is considered positive. There is controversy as to what should be considered a borderline sweat chloride. Original guidelines suggested a sweat chloride of 40–60 mmol/l, 40 mmol/l representing the mean +2 standard deviations in carriers. Recent evidence suggests that a proportion of patients with “classical CF” have chloride concentrations of 30-60 mmol/l. Sweat chloride concentrations of 30-60 mmol/l are seen in about 4% of sweat tests and 23% of these will subsequently be found to have two CF causing mutations. CF affected patients occur with equal frequency in the 30-40 mmol/l as they do in the 40-60 mmol/l ranges (Lebecque et al, 2002). A chloride sweat concentration of 30-60 mmol/l in infancy is suspicious of CF (Massie et al, 2000). Sodium concentrations should not be interpreted without a chloride result. A sodium value of less than 60 mmol/l is unlikely to be associated with cystic fibrosis (not in atypical CF). Values above 90 mmol/l support the diagnosis. A chloride concentration which is less than the sodium concentration or a discrepancy between the two of > 20 mmol should be regarded with suspicion.
The European Diagnostic Working Group – Diagnosis and sweat test
The European Diagnostic Working Group proposes the following terminology( De Boeck et al, 2006). Patients are diagnosed with classic or typical CF if they have one or more phenotypic characteristics and a sweat chloride concentration of >60 mmol/l. The vast majority of CF patients fall into this category. Patients with non-classic or atypical CF have a CF phenotype in at least one organ system and a normal (<30 mmol/l) or borderline (30–60 mmol/l) sweat chloride level. In these patients confirmation of the diagnosis of CF requires detection of one disease causing mutation on each CFTR gene or direct quantification of CFTR dysfunction by nasal potential difference measurement.
Certain clinical conditions are associated with a false positive test result. In adults and older children the mineralocorticoid suppression adaptation sweat test has been used to try and discriminate between an equivocal and a negative sweat test. Patients undergoing this test receive oral fludrocortisone, (dose; children 3 mg/sqm/day for 2 days, adults 5mg a day for 2 days) prior to pilocarpine iontophoresis. In healthy individuals the sweat Na values have been reported to fall significantly after fludrocortisone priming (Lobeck & McSherry, 1963; Hodson et al, 1983). However the recent UK guidelines for the performance of the sweat test suggest that there is no routine place for this test due to paucity of data.
“Non Classical CF”
Some mutations such as R117H, 3849 + 10kb C-T, R334W, P67L are associated with borderline sweat tests (Augarten et al, 1995, Desmarquest et al, 2000; Gilfillan et al, 1998). In these cases the sweat sodium concentration is often higher than chloride and patient is pancreatic sufficient. The diagnosis can remain uncertain in those patients with clinical features of CF, intermediate and one or less identified CF mutation. Very rarely, the sweat test can be normal in a patient with a CF genotype (Highsmith et al, 1994; Augarten et al, 1993; Strong et al, 1991, Stewart et al, 1995; Guidelines for the performance of the sweat test; Multi-Disciplinary working group, 2002).
Examples of borderline sweat tests in patients with cystic fibrosis.
Case 1: R117H/5T + (3849+10KB C>T)
Sweat test Na 72 / Cl 58, Na 60 / Cl 49, Na 58 / Cl 47
Case 2: R117H/5T + (3849+10KB C>T)
Sweat Na 52 / Cl 44 ,Na 56 / Cl 42
Case 3: DF508 + 3849+10Kb (C>T)
Sweat test Na 56 / Cl 50, Na 54 / Cl 49
False-positive sweat test result
Adrenal insufficiency, anorexia nervosa, atopic dermatitis, autonomic dysfunction, coeliac disease, ectodermal dysplasia, familial cholestasis (Byler’s disease), fucosidosis, G6PD deficiency, glycogen storage disease type 1, hypogammaglobulinemia, hypoparathyroidism, hypothyroidism, Klinefelter’s syndrome, malnutrition, mucopolysaccharidosis type 1, nephrogenic diabetes insipidus, nephrosis, pseudohypoaldosteronism, psychosocial problems (Rosenstein & Cutting, 1998, Ruddy et al, 1987).
• The test should always be performed by experienced laboratory personnel in centres who carry out a minimum of 50 sweat tests per annum
• A chloride value of >60 mmol/l is considered positive
•A sweat chloride result in the range 30-60 mmol/l is suggestive, but not diagnostic of CF.
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