Institute of Genomic Medicine
Defects of the Carnitine System
BACKGROUND:
Carnitine (4-N-trimethylammonio-3-hydroxybutanoate) is a molecule required for the transport of long chain fatty acids across the mitochondrial membrane into the matrix where the fatty acyl group is metabolized. It also establishes the steady-state intramitochondrial ratio of acyl-CoA/CoASH. In some metabolic disorders, this ratio can be disturbed if total carnitine concentration is decreased. Therefore, the measurement of carnitine in serum and urine has become an accepted procedure to assess the carnitine metabolic status in the body.
INDICATIONS FOR TESTING:
The analysis of carnitine in plasma and urine is indicated in patients who fail to thrive, have hypotonia, chronic muscle weakness, cardiomyopathy, myopathy, encephalopathy, hypoglycemia, metabolic acidosis, or nonketotic dicarboxylic acidurias.
PROCEDURE:
A radioisotopic method is used to determine total and free carnitine levels. L-carnitine + acetyl CoA <*> acetyl carnitine + CoASH
Radioactive acetate-labelled acetyl CoA is used, and the radiolabelled product acetylcarnitine is separated from the substrate, acetyl CoA.
SAMPLE REQUIREMENTS:
Serum:
2-3cc blood drawn in a red top vacutainer tube. Spin down and store frozen until ready for assay.Urine:
10cc urine, random collection or 12 or 24 hour specimen when desired.
INTERPRETATION:
The ratio of free and total carnitine is determined, and the patient's results are compared to serum and urine reference ranges established by the laboratory. The serum and urine concentration of total and free carnitine depends on the amount of carnitine in the diet, the state of hydration of the patient, the degree of physiological stress, and the metabolic state and general health of the patient. Results on the same patient may be very different, for example, if one sample was drawn when the patient was clinically stable and another when the patient was symptomatic. The results may also be used to monitor carnitine therapy.
