Carnitine is required for the transport of fatty acids into mitochondria, where they are used as a source of energy. Patients with systemic carnitine deficiency are unable to utilise fatty acids, and become entirely dependent on carbohydrate as an energy source. They are therefore susceptible to severe hypoglycaemia.
The deficiency may also lead to muscle weakness and myopathy, cardiomyopathy, lipidosis, and the lack of phospholipids may contribute to a variety of abnormalities. Valproate and other related anticonvulsants, renal disease, a poor protein intake and the ketogenic diet can all lead to low free and total carnitine levels. An acylcarnitine profile should be requested if investigating suspected metabolic disease.
The acylcarnitine profile, measured by tandem mass spectrometry, is one of the most important metabolic investigations. It is quick, accurate and relatively cheap. It can be performed on a single blood spot. It is the test of choice for the fatty acid oxidation defects and should be performed on all patients with otherwise unexplained hypoglycaemia, episodic sickness (vomiting, collapse), cardiomyopathy, cardiac arrhythmias, myopathy and rhabdomyolysis. It is also a useful test for some of the more important organic acidemias. It is not usually critical to perform the test when the patient is acutely unwell.