C4OH (3-hydroxybutyryl/3-hydroxyisobutyryl) acylcarnitine

This test measures a specific acylcarnitine called C4OH, which includes 3-hydroxybutyryl- and 3-hydroxyisobutyryl-carnitine in the blood. These molecules form when your body breaks down fats and certain amino acids, and carnitine helps transport these byproducts for energy processing.

It is usually performed by tandem mass spectrometry as part of an acylcarnitine profile or newborn screening panel. The result helps show how your body is handling fat and ketone metabolism at the time the sample was collected.

Clinicians use this test to screen for or evaluate possible inborn errors of metabolism that affect fat and ketone handling, and some branched-chain amino acid pathways. It is often included in newborn screening and in diagnostic workups for symptoms such as poor feeding, vomiting, lethargy, abnormal breathing, or episodes of low blood sugar during illness.

C4OH can also rise temporarily during normal physiologic ketosis, such as with fasting, heavy exercise, or intercurrent illness. Interpreting the value in context, alongside other acylcarnitines and clinical findings, helps distinguish a transient change from a metabolic condition that may require treatment and ongoing monitoring.

Your result is interpreted together with your symptoms, medical history, and other laboratory tests like the full acylcarnitine profile and urine organic acids. A single elevated result does not establish a diagnosis. If the value is unexpected, your clinician may repeat the test when you are well, check related markers, and consider genetic testing to clarify the cause.

If a metabolic disorder is suspected, your care team may recommend practical steps such as avoiding prolonged fasting, using sick-day plans during infections, and consulting a metabolic specialist or dietitian. If your result is within the expected range, it generally supports normal fat and ketone metabolism, but your clinician will still consider your overall health and any symptoms you have.