C5/C3 IVA

This test measures the ratio of C5 (isovalerylcarnitine) to C3 (propionylcarnitine) in blood. These are acylcarnitines, small molecules that reflect how your body breaks down certain amino acids and fats. The measurement is typically performed by tandem mass spectrometry, often on a dried blood spot or plasma sample.

The ratio helps identify a biochemical pattern associated with isovaleric acidemia, a rare inherited condition that affects the enzyme isovaleryl-CoA dehydrogenase. On its own, the ratio is a screening or adjunct marker and is interpreted together with other lab findings and clinical information.

Doctors use this ratio to help flag possible isovaleric acidemia, especially in newborn screening or when someone has symptoms that could suggest a metabolic disorder. Finding a concerning pattern early can guide confirmatory testing and prompt treatment to prevent metabolic crises.

The result can also support monitoring in people already diagnosed with the condition. Because many factors can shift acylcarnitine patterns, the ratio is not diagnostic by itself; it helps your care team decide whether further evaluation is needed.

If the ratio is higher than expected, it raises suspicion for isovaleric acidemia, but it does not confirm the diagnosis. Your clinician may repeat the test and order additional studies, such as a full acylcarnitine profile, urine organic acids to look for isovalerylglycine, and sometimes genetic testing. Results are interpreted in the context of symptoms, diet, medications, and age.

If the ratio is within the expected range, that is generally reassuring, although follow-up may still be recommended if there are persistent symptoms or other test abnormalities. When results are borderline or influenced by feeding status or illness, a repeat sample after recovery or after regular feeding has been established can clarify the picture.