C4 Acylcarnitine (Butyryl/Isobutyryl)

Complement

Butyrylcarnitine/IsobutyrylcarnitineC4C4 acylcarnitine

Review status

Currently under review

Pending specialist review and validation.

What it shows

C4 acylcarnitine (butyryl/isobutyryl) is a small molecule formed when your body links short-chain fatty acids to carnitine. This test measures the amount of C4 acylcarnitine in the blood, usually by tandem mass spectrometry, as part of an acylcarnitine profile.

It helps evaluate how well certain enzymes break down fats and some amino acids for energy. The test is often included in newborn screening and is also used when infants, children, or adults have symptoms that suggest an inherited metabolic condition.

Why it matters

Changes in C4 acylcarnitine can point to disorders such as short-chain acyl-CoA dehydrogenase deficiency or isobutyryl-CoA dehydrogenase deficiency, which can affect energy production. Your clinician may order this test after an abnormal newborn screen or when there are episodes of poor feeding, vomiting, lethargy, low blood sugar, failure to thrive, or unexplained illness.

Finding the cause early can guide nutrition and treatment plans that help prevent metabolic crises and support healthy development. Mild, isolated changes are common and may not mean disease, so results are interpreted carefully and usually confirmed with additional testing.

Understanding your results

Your result is interpreted together with other acylcarnitines, your symptoms, and additional tests. A single out-of-range value does not diagnose a disorder. Patterns across multiple markers, along with urine organic acids or acylglycines, help tell true genetic conditions from temporary changes related to diet or illness.

If your result is higher than expected, your care team may repeat the test, review a full acylcarnitine profile, order urine studies, and consider genetic testing. For newborns, follow-up is typically coordinated with a metabolic specialist. If results are normal but symptoms continue, further evaluation may still be recommended.

Let your clinician know about all medications, special formulas, and supplements. Vitamin status, intercurrent illness, and some drugs can influence acylcarnitine patterns, and your team will take these into account.

Reference ranges

0.11 – 0.35 umol/L

All sexes

0 days – 7 days

0.12 – 0.41 umol/L

All sexes

7 days – 1 month

0.13 – 0.51 umol/L

All sexes

1 month – 12 months

0.12 – 0.45 umol/L

All sexes

12 months – 13 years

0.11 – 0.42 umol/L

All sexes

13 years – 19 years

0.12 – 0.43 umol/L

All sexes

19 years – 150 years

Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.

Factors that could impact C4 Acylcarnitine (Butyryl/Isobutyryl)

Recent illness or fasting
Fever, poor intake, or prolonged fasting shifts the body toward fat use and can temporarily change C4 and related acylcarnitines.
Sample timing and type
Results can differ between dried blood spots and plasma, and whether the specimen is collected soon after birth or after feeding.
Medications and supplements
Some anticonvulsants, antibiotics, and nutrient supplements, especially those affecting mitochondrial function or riboflavin status, may alter acylcarnitine patterns.
Genetic background
Variants in enzymes involved in short-chain fatty acid oxidation can raise C4. Some variants are benign or of uncertain significance and need expert review.
Nutrition and special formulas
High protein intake, specialized medical formulas, or carnitine supplementation can influence acylcarnitine profiles without indicating disease.

2026

References

McGill University Health Centre. (2018, December 03). C4 Butyryl_Isobutyryl (Task CD 709458). Laboratory reference ranges.
McGill University Health Centre. (2019, May 21). C4 Butyryl_Isobutyryl (Task CD 709458). Laboratory reference ranges.
McGill University Health Centre. (2019, June 07). C4 Butyryl_Isobutyryl (Task CD 709458). Laboratory reference ranges.
American College of Medical Genetics and Genomics. (2022). ACT Sheet and algorithm: Elevated C4 acylcarnitine (isobutyryl- or short-chain acyl-CoA dehydrogenase deficiencies).