Biochemical Genetics Unit- Metabolic Tests (Cont)

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.

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-acyl CoA 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.