C18OH (3-hydroxyoctadecanoyl)

This test measures C18OH, also called 3-hydroxyoctadecanoylcarnitine, a long-chain hydroxyacylcarnitine in your blood. It is formed when your body processes certain long-chain fats inside the mitochondria, the parts of cells that make energy. The measurement is usually part of an acylcarnitine profile used to evaluate how well fat is being broken down for energy.

Because hydroxyacylcarnitines reflect steps in mitochondrial beta-oxidation, this marker can help identify patterns linked to specific enzyme problems, including long-chain 3-hydroxyacyl-CoA dehydrogenase or related protein complexes. It is often used in newborns, children, and adults when doctors are assessing possible inherited metabolic disorders or unexplained metabolic symptoms.

Abnormal levels can signal fatty acid oxidation disorders that may lead to low blood sugar, muscle weakness, heart rhythm issues, liver stress, or episodes of lethargy during illness or fasting. Finding these problems early supports timely treatment, tailored nutrition, and prevention of complications.

Your clinician may order this test to follow up an abnormal newborn screen, investigate symptoms triggered by fasting, infection, or exercise, or monitor a known metabolic condition. Results are interpreted alongside other acylcarnitines, organic acids, clinical history, and sometimes genetic testing to narrow the diagnosis and guide care.

Your result is interpreted in the context of your age, health status, and other laboratory markers. A value that is higher than expected may suggest reduced capacity to process long-chain fats or a period of metabolic stress, but on its own it does not confirm a specific disorder. Lower or undetectable amounts can be normal and are assessed with other parts of the acylcarnitine profile.

If your result is unexpected, your clinician may repeat testing under stable conditions, review diet and medications, and consider additional tests such as a full acylcarnitine panel, plasma free carnitine, urine organic acids, or genetic studies. Avoid making major diet or supplement changes without guidance, especially for infants and children.