C5 Isovaleryl/2-methylbutyryl (acylcarnitine)

This test measures the C5 acylcarnitine in your blood, specifically the isovaleryl and 2-methylbutyryl forms. These molecules form when your body processes the amino acids leucine and isoleucine and then attaches them to carnitine for transport. Measuring C5 helps assess how well certain enzymes in these metabolic pathways are working.

C5 acylcarnitine is part of expanded metabolic screening panels and targeted evaluations for inborn errors of metabolism. It can be checked in newborns as part of follow-up testing, and in children or adults when a metabolic disorder is suspected. Results are interpreted together with other acylcarnitines, urine organic acids, and sometimes genetic testing.

Abnormal C5 levels can point to conditions that affect how the body breaks down specific amino acids. Elevated results may suggest isovaleric acidemia or short/branched-chain acyl-CoA dehydrogenase deficiency, conditions that can cause illness during stress or fasting if not recognized. Identifying these problems early helps prevent complications by guiding diet, supplements, and emergency sick-day care.

Clinicians order this test to confirm an abnormal newborn screen, investigate unexplained vomiting, lethargy, acidosis, or low blood sugar, or to monitor known metabolic disorders. It can also help distinguish between primary metabolic disease and temporary changes from diet, illness, or certain medicines.

Your result will be reviewed in the context of age, symptoms, and other lab findings. A higher C5 value can be a clue to a metabolic condition, but it does not make a diagnosis by itself. Follow-up may include a full acylcarnitine profile, urine organic acids and acylglycines, and sometimes genetic testing of the IVD or ACADSB genes. If a disorder is confirmed, your care team may discuss dietary adjustments, carnitine or glycine supplementation, and plans for times of illness.

If your C5 value is typical for your age, no specific action is usually needed. If it is borderline or unexpectedly high, your clinician may repeat the test when you are well and fasting, or review medications and diet. For newborns, maternal health and feeding status can influence results, so both infant and maternal factors may be considered when planning next steps.