Test Catalog

Test ID: FAO    
Fatty Acid Oxidation Probe Assay, Fibroblast Culture

Useful For Suggests clinical disorders or settings where the test may be helpful

In vitro confirmation of biochemical diagnoses of the following fatty acid oxidation disorders:

-Short-chain acyl-CoA dehydrogenase (SCAD) deficiency

-Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

-Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency

-Trifunctional protein deficiency

-Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency

-Carnitine palmitoyl transferase deficiency type II (CPT-II)

-Carnitine-acylcarnitine translocase (CACT) deficiency


Confirmation of the following organic acid disorders:

-2-Methylbutyryl-CoA dehydrogenase (SBCAD) deficiency

-Isobutyryl-CoA dehydrogenase (IBD) deficiency


This test is not useful for prenatal testing.


This assay is not informative if the deficient enzyme is physiologically not expressed in skin fibroblasts.

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

When this test is ordered, a fibroblast culture and cryopreservation for biochemical studies will always be performed at an additional charge. However, for multiple assays on a patient utilizing fibroblast culture, only 1 culture is required regardless of the number of assays ordered. If viable cells are not obtained within 30 days, client will be notified.


See Newborn Screening Follow-up for Isolated C5 Acylcarnitines Elevations (also applies to any plasma or serum C5 acylcarnitine elevations) in Special Instructions.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Mitochondrial fatty acid beta-oxidation plays an important role in energy production, particularly in skeletal and heart muscle, and in hepatic ketone body formation. Disorders of fatty acid oxidation (FAO) are characterized by hypoglycemia, hepatic dysfunction, encephalopathy, skeletal myopathy, and cardiomyopathy. Most FAO disorders have a similar presentation and their biochemical diagnosis can, at times, be difficult. Commonly used metabolite screens such as urine organic acids, plasma acylcarnitines, and fatty acids are influenced by dietary factors and the clinical status of the patient. This often leads to incomplete diagnostic information or even false-negative results. Enzyme assays are limited to one enzyme per assay, and molecular assays for common genetic variants are limited by the frequent occurrence of compound heterozygous patients with uncommon, private alterations, which must be distinguished from unaffected carriers. Furthermore, neither specific enzyme assays nor molecular genetic testing are available for all of the known defects. The purpose of the in vitro probe assay is to offer screening for several defects of FAO and organic acid metabolism under controlled laboratory conditions using fibroblast cultures.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

Abnormal results will include a description of the abnormal profile in comparison to normal and abnormal controls. In addition, the concentration of those acylcarnitine species that abnormally accumulated in the cell medium are provided and compared to the continuously updated reference range based on analysis of normal controls. Interpretations of abnormal acylcarnitine profiles also include information about the results' significance, a correlation to available clinical information, possible differential diagnoses, recommendations for additional biochemical testing and confirmatory studies if indicated, name and phone number of contacts who may provide these studies, and a phone number to reach one of the laboratory directors in case the referring provider has additional questions.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Sometimes, an abnormal acylcarnitine profile cannot differentiate between 2 disorders. In such instances, independent biochemical (eg, specific enzyme assay) or molecular genetic analyses are required. Recommendations for such testing will be included in the report.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Ensenauer R, Vockley J, Willard JM, et al: A common mutation is associated with a mild, potentially asymptomatic phenotype in patients with isovaleric acidemia diagnosed by newborn screening. Am J Hum Genet. 2004;75(6):1136-1142. doi: 10.1086/426318

2. Rinaldo P, Matern D, Bennet MJ: Fatty acid oxidation disorders. Ann Rev Physiol. 2002;64:477-502

3. Shen JJ, Matern D, Millington DS, et al: Acylcarnitines in fibroblasts of patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency and other fatty acid oxidation disorders. J Inherit Metab Dis. 2000;23:27-44. doi: 10.1023/a:1005694712583

4. Matern D, Huey JC, Gregersen N, et al: In vitro diagnosis of short-chain acyl-CoA dehydrogenase (SCAD) deficiency. J Inherit Metab Dis. 2001;24(Suppl.1):66

5. Merritt JL, Norris M, Kanungo S: Fatty acid oxidation disorders. Ann. Transl. Med. 2018 Dec;6(24):473. doi: 10.21037/atm.2018.10.57

Special Instructions Library of PDFs including pertinent information and forms related to the test