C0/AC Total Ratio (Free Carnitine to Total Acylcarnitines)

This test measures the ratio of free carnitine (often called C0) to the total amount of acylcarnitines in your blood. Carnitine helps move fatty acids into cells so they can be used for energy, while acylcarnitines form when fatty acids are being processed. Looking at the balance between these two gives a snapshot of how your body is handling fat metabolism.

It is typically performed as part of a metabolic evaluation or acylcarnitine profile. Your clinician may use it to understand your carnitine status, to assess how well your body is converting fats to energy, or to monitor carnitine supplementation.

The C0/AC ratio can help identify patterns seen in carnitine deficiency and in disorders of fatty acid oxidation. It is often ordered after an abnormal newborn screen, or when there are symptoms such as poor feeding in infants, low energy, muscle weakness, episodes of low blood sugar, heart enlargement, or unexplained liver issues. In adults, it may be used when there is concern for secondary carnitine depletion due to illness or medications.

This ratio can also help monitor response to L-carnitine therapy and may flag drug effects that lower carnitine stores. Your provider uses it alongside other tests and your history to decide whether more metabolic or genetic testing is needed.

Your healthcare provider will interpret the ratio in the context of your age, symptoms, recent diet, and any supplements or medicines. A higher ratio can be seen when free carnitine is relatively abundant or acylcarnitines are reduced, including during carnitine supplementation or recovery from illness. A lower ratio can occur when carnitine is depleted or when acylcarnitines are increased by metabolic stress or certain inherited conditions.

If your result is unexpected, your clinician may repeat testing when you are clinically stable, review your fasting status, and order related studies such as a full acylcarnitine profile, free and total carnitine, urine organic acids, ketones, or genetic testing. Results are best interpreted with a specialist familiar with metabolic testing, and changes to diet or medication should be made only after medical advice.