Fatty Acid Oxidation & Organic Acid Disorders

Acylcarnitines; Free and Total Carnitine

Description:
Many genetic errors affecting fatty acid and branched-chain amino acid catabolism have been identified using this method. For most of the disorders involving fatty acid beta-oxidation, this is the most informative screening test. 
Quantitative acylcarnitine analysis can also be used to evaluate the treatment during follow-up of patients with these disorders.

Specimen Requirements:
Plasma – 1-2 mL plasma (from whole blood collected in sodium heparin tube [dark green top]). Keep specimen cold. Freeze only during off-hours. 

Dried blood spots – At least 3 properly filled spots

Method: 
Acylcarnitine species in plasma are measure by electrospray tandem mass spectrometry using deuterated acylcarnitine species as internal standards. An aliquot of plasma is extracted with an acidified acetonitrile solution containing internal standards. The supernatant is evaporated and the residue heated with n-butanolic HCl yielding the acylcarnitines for analysis as their n-butyl esters. Values are reported in relation to the appropriate age-specific reference range.

Limitations: 
In a few instances, false-negative results occur in the analysis of acylcarnitine profiles. Patients with carnitine deficiency may not exhibit abnormally high acylcarnitine concentrations. For some disorders, such as medium-chain acyl-CoA dehydrogenase deficiency, the calculation of ratios between different acylcarnitine species provides a discriminant factor to overcome such problems. Where applicable, the calculation of such ratios will be incorporated in the routine acylcarnitine analysis.

If the results are indicative of carnitine deficiency, the interpretation will include a remark that this limits the diagnostic value of the test and repeat analysis may be considered following carnitine supplementation.
Informative profiles may also not be detected in some disorders where the accumulation of diagnostic acylcarnitines is a reflection of the residual activity of the defective enzyme, the dietary load of precursors, the anabolic/catabolic state, and the treatment status of a patient.  In some cases, additional methods of higher specificity and sensitivity such as in vitro enzyme assays or molecular genetic testing are advisable as opposed to stressing the patient’s metabolism (e.g., fasting test) prior to repeating acylcarnitine analysis.