Tests not covered in main page
SWEAT TESTS
Non-urgent advice: SWEAT TESTS
The determination of sweat chloride concentration is useful in the diagnosis of cystic fibrosis. Sweat testing can be performed after 2 weeks of age on infants greater than 3 kg that are normally hydrated and without significant systemic illness. If clinically important, sweat testing can be attempted after one week of age but will need repeating if insufficient sweat is collected. A repeat test is recommended when the result is abnormal or borderline and the genotype is not confirmatory.
National guidelines for sweat collection are available.
- Indications
-
Phenotype suggestive of CF (respiratory infection, exocrine pancreatic insufficiency)
Positive newborn screening test
- Sample Type
-
Sweat collected into a Wescor Macroduct tube. National guidelines state not less than 1g/m2/min.
A minimum sweat volume of 60 mL (approximately 185 mm) is required to enable duplicate analysis.
For Addenbrooke’s patients sweat collection can be arranged with the CF Clinical Nurse Specialists.
- Method
-
Pilocarpine, transported by iontophoresis is used to induce sweating. Sweat is collected via the capillary ‘macroduct’ system. After collection the tube is sealed at both ends to prevent evaporation.
The concentration of chloride ions is determined using an ion selective electrode.
- Turn Around Time
-
2 days
- Reference range
-
Age related range provided on report
VERY LONG CHAIN FATTY ACIDS, PRISTANATE & PHYTANATE
Non-urgent advice: VERY LONG CHAIN FATTY ACIDS, PRISTANATE & PHYTANATE
Peroxisomes are responsible for b-oxidation of very long chain fatty acids (fatty acids with a carbon length more than 22), bile acid metabolism and plasmalogen synthesis. Peroxisomal disorders can be classified into 2 categories; defects in peroxisomal biogenesis disorders (eg Zellweger syndrome, infantile Refsum disease) and defects in specific peroxisomal enzymes. Very long chain fatty acids are very sensitive for the diagnosis of X-linked adrenoleukodystrophy in males. However approximately 15% of symptomatic female carriers have normal very long chain fatty acids.
Phytanate and pristanate are assayed as part of the plasma very long chain fatty acid profile. They are useful in the diagnosis of Refsum disease, a-methyl-acylCoA racemase deficiency and rhizomelic chondrodysplasia punctata (depending on the age of the patient). Pristanate and phytanate may be normal in young infants with peroxisomal biogenesis defects as both compounds are derived from exogenous, dietary sources.
- Indications
-
[Note: (+) indicates ‘occurring with other features’]
Idiopathic adrenal insufficiency in males
Neurological abnormalities
Leukodystrophy
Ataxia
Seizures (+)
Hypotonia
Ocular abnormalities
Skeletal abnormalities
Dysmorphic features
Liver dysfunction (+)
Hepatomegaly
- Sample Type
-
0.5 mL EDTA plasma, send to laboratory as soon as possible
- Method
-
GC-MS of methylesters
- Turn Around Time
-
3 weeks
- External laboratory information
-
Separate plasma from cells within 2 hours of collection
Store frozen until dispatch
Send by 1st class post or hospital transport