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PubMed This is a summary of 21 peer-reviewed journal articles Updated
Medical Genetics · Very Long-Chain Acyl-CoA Dehydrogenase Deficiency

The Science of VLCADD: From Genes to Diagnosis

At a Glance

An elevated C14:1 newborn screen does not by itself prove VLCADD. Doctors combine repeat acylcarnitine testing, ACADVL gene results, and sometimes cell-based functional testing to distinguish VLCADD from carrier status or a temporary false-positive result.

While the first page of this guide focused on the emotions of receiving a screening result, this page dives into the “why” and “how” of a VLCADD diagnosis. Understanding the biology of this condition can help you make sense of the tests your medical team is ordering.

The Biology of VLCADD

Every person has two copies of the ACADVL gene—one from each parent [1]. This gene provides the instructions for making an enzyme (a specialized protein) called Very Long-Chain Acyl-CoA Dehydrogenase, or VLCAD for short [2].

Think of the VLCAD enzyme as the first “gatekeeper” in a multi-step factory line that turns long-chain fats into energy [3]. When a baby has VLCADD, both copies of their ACADVL gene have mutations (changes), meaning the “gatekeeper” isn’t working correctly [4]. This makes it difficult for the body to break down long-chain fats, which are a major source of fuel, especially during times of fasting or illness [5][6].

VLCADD is an autosomal recessive condition. This means both parents must be carriers (having one changed gene and one working gene) for a child to be born with the condition. Carriers usually have no symptoms because their one working gene produces enough enzyme for normal life [7][8].

Why the Screen Was Abnormal: The C14:1 Marker

When the body cannot break down long-chain fats properly, “leftover” pieces of these fats begin to build up in the blood. One specific piece, called C14:1 (tetradecenoylcarnitine), is the primary marker used to screen for VLCADD [9][10].

However, an elevated C14:1 level does not always mean a baby has the condition. Several factors can cause a “false positive” or a misleadingly high result:

  • Catabolism (Weight Loss): If a baby loses significant weight shortly after birth or goes too long between feedings, their body starts burning its own fat stores very quickly. This “fat-burning mode” can temporarily drive up C14:1 levels even in a healthy baby [11][12].
  • Carrier Status: Babies who are only carriers of one gene change sometimes have mildly elevated C14:1 levels that trigger the screen, even though they aren’t actually sick [13][8].
  • Maternal Nutrition and Metabolism: In some cases, a mother’s nutritional status (such as a riboflavin deficiency) or her own metabolic conditions can affect the baby’s acylcarnitine profile early on. This is one of several specialist-considered causes, and parents should never self-treat with supplements without medical guidance [14].

Confirming the Diagnosis

To move past the initial screen, your metabolic team will perform a series of “confirmatory” tests. These tests look at the problem from different angles: the blood chemistry, the DNA, and the actual function of the cells. Diagnosis and management integrate the clinical phenotype, acylcarnitines, molecular findings, and—when needed—functional testing.

  1. Repeat Plasma Acylcarnitine Profile: This is a more detailed version of the initial screen. Doctors look not just at C14:1, but at the ratios between different fat pieces (like C14:1 compared to C12:1). These ratios are often better at distinguishing true VLCADD from carriers or false positives [13][15].
  2. Genetic Testing (ACADVL Sequencing): This test looks directly at your baby’s DNA to see if there are changes in the ACADVL gene. Finding two disease-causing changes helps confirm the diagnosis at a molecular level [10][16].
  3. Functional Testing: If the blood work and genetics are unclear or discordant, doctors may consider specialized functional assays to see how well the baby’s cells actually process fat.
    • Enzyme Activity: This measures how much “gatekeeper” enzyme is working in the baby’s white blood cells [17].
    • LC-FAO Flux: Doctors take a small skin sample (fibroblasts) and measure how much fat the cells can burn. While a very low “flux” often points to a more severe form, it is a specialized test that is not required for every infant, and it cannot perfectly predict an individual child’s course [18][19].

Confirmatory Diagnosis: Tests the Team May Consider

A complete evaluation typically includes several components to ensure accurate diagnosis and a better understanding of severity. Not every patient will need every test on this list:

  • [ ] Repeat Acylcarnitine Profile: To see if the C14:1 elevation is persistent and check important ratios.
  • [ ] ACADVL Gene Sequencing: To identify the specific genetic changes.
  • [ ] Urine Organic Acids: To look for other metabolic markers that might explain the result.
  • [ ] Parental Testing: Often recommended to confirm the “recessive” inheritance pattern and help with future family planning.
  • [ ] Functional Assay: (Enzyme activity or FAO Flux) Considered if genetic results are “uncertain” or to assist in risk assessment [20][21].

Common questions in this guide

Does a high C14:1 result mean my baby has VLCADD?
No. C14:1 is the main blood marker used to screen for VLCADD, but an elevated result can also occur with rapid weight loss, long gaps between feedings, carrier status, or some maternal factors. Follow-up testing is needed to determine what the result means.
How do doctors confirm VLCADD after a positive screen?
The team usually reviews a repeat plasma acylcarnitine profile, including C14:1 and related ratios, and performs ACADVL gene testing. Urine organic acids and parental testing may also be considered. If results are unclear or do not agree, a functional test can measure enzyme activity or fat processing in cells.
What does ACADVL genetic testing tell us?
ACADVL sequencing looks for changes in the gene that makes the VLCAD enzyme. Finding disease-causing changes in both copies supports a molecular diagnosis of VLCADD, while one change may indicate carrier status. A result called a variant of uncertain significance may need additional evidence or functional testing before its meaning is clear.
Why might my baby need a functional test?
Functional testing may be considered when the screening, blood chemistry, and genetic results are unclear or do not match. It can measure enzyme activity in white blood cells or how well skin cells burn fat. A very low result may suggest a more severe form, but it cannot perfectly predict an individual child's course.
Can weight loss affect a VLCADD screening result?
Yes. Significant weight loss or going too long between feedings can put a baby into a temporary fat-burning state and raise C14:1, even without VLCADD. The care team will interpret the result alongside the baby's feeding, weight history, repeat testing, and genetic findings.
How is VLCADD inherited?
VLCADD is an autosomal recessive condition, meaning a baby generally needs a changed ACADVL gene from each parent. Parents who carry one changed copy usually have no symptoms because their other copy can make enough enzyme. Parental testing can help clarify the inheritance pattern and inform family planning.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What specific ACADVL gene variants were found, and are they classified as pathogenic or variants of uncertain significance?
  2. 2.What was the exact C14:1 level and the C14:1/C12:1 ratio on the repeat test, and how do these compare to the first screen?
  3. 3.Do we need to perform a functional test, like a fibroblast FAO flux assay, to better understand my baby's specific case?
  4. 4.Could my baby's initial weight loss or other temporary factors have influenced the first test result?
  5. 5.If my baby is a 'carrier' with only one gene change, what follow-up is needed to ensure they are safe?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (21)
  1. 1

    Characterization of Variants of Uncertain Significance in ACADVL Gene From a Very-Long-Chain Acyl-CoA Dehydrogenase Deficiency Patient.

    Wang Q, Yang J, Xu Y, et al.

    Molecular genetics & genomic medicine 2025; (13(7)):e70120 doi:10.1002/mgg3.70120.

    PMID: 40678976
  2. 2

    ACADVL Deep Sequencing in a Case Study: Beyond the Common c.848T>C Pathogenic Variant.

    Baldo F, Zupin L, Magnolato A, et al.

    Genes 2025; (16(5)) doi:10.3390/genes16050538.

    PMID: 40428360
  3. 3

    Structural basis for defective membrane targeting of mutant enzyme in human VLCAD deficiency.

    Prew MS, Camara CM, Botzanowski T, et al.

    Nature communications 2022; (13(1)):3669 doi:10.1038/s41467-022-31466-2.

    PMID: 35760926
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    Newborn screening and genetic characteristics of patients with short- and very long-chain acyl-CoA dehydrogenase deficiencies.

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    Evidence that Oxidative Disbalance and Mitochondrial Dysfunction are Involved in the Pathophysiology of Fatty Acid Oxidation Disorders.

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    Very-Long-Chain Acyl-CoA Dehydrogenase Deficiency: Family Impact and Perspectives.

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    Four Years' Experience in the Diagnosis of Very Long-Chain Acyl-CoA Dehydrogenase Deficiency in Infants Detected in Three Spanish Newborn Screening Centers.

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    Recurrent ACADVL molecular findings in individuals with a positive newborn screen for very long chain acyl-coA dehydrogenase (VLCAD) deficiency in the United States.

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    Tracer-based lipidomics enables the discovery of disease-specific candidate biomarkers in mitochondrial β-oxidation disorders.

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    Very Long-Chain Acyl-CoA Dehydrogenase Deficiency: High Incidence of Detected Patients With Expanded Newborn Screening Program.

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    Characteristic Findings of Infants with Transient Elevation of Acylcarnitines in Neonatal Screening and Neonatal Weight Loss.

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    False positive cases of elevated tetradecenoyl carnitine in newborn mass screening showed significant loss of body weight.

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    Serum C14:1/C12:1 ratio is a useful marker for differentiating affected patients with very long-chain acyl-CoA dehydrogenase deficiency from heterozygous carriers.

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    Abnormal VLCADD newborn screening resembling MADD in four neonates with decreased riboflavin levels and VLCAD activity.

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This page explains VLCADD genetics and diagnostic testing for educational purposes and does not replace medical advice. Your baby's metabolic team should interpret screening and confirmatory results.

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