Conditions Medium-Chain Ketoacyl-CoA Thiolase Deficiency

Medium-chain ketoacyl-CoA thiolase deficiency (MCAT) is a condition in which the body is unable to break down certain fats. It is considered a fatty acid oxidation condition because people affected by MCAT are unable to change some of the fats they eat into energy the body needs to function. This can cause too many unused fatty acids build up in the body. If left untreated, MCAT can cause vomiting, liver problems, and death. The effectiveness of treatment is unknown.

Condition Type

Fatty Acid Oxidation Disorders


Medium-chain ketoacyl-CoA thiolase deficiency (MCAT) is a very rare condition. At least one case of it has been reported. The actual number of individuals affected with MCAT is unknown.

Also known as

  • MCAT
  • MCKAT deficiency

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of MCAT (see Early Signs below). If your baby has certain signs, your baby’s doctor may suggest starting immediate treatment.

If your baby’s newborn screening result for medium-chain ketoacyl-CoA thiolase deficiency (MCAT) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your baby to have additional testing. It is important to remember that an out-of-range screening result does not necessarily mean that your child has the condition. An out-of-range result may occur because the initial blood sample was too small or the test was performed too early. However, as a few babies do have the condition, it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated MCAT can occur soon after birth, follow-up testing must be completed as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve checking your baby’s urine and blood samples for harmful amounts of acids and toxins. Certain acids and toxins build up in the body when a child has a fatty acid oxidation condition, so studying the amounts of these substances in your baby’s body can help doctors determine if your baby has a condition.

About Medium-Chain Ketoacyl-CoA Thiolase Deficiency

Early Signs

There has only been one reported case of medium-chain ketoacyl-CoA thiolase deficiency (MCAT). In this case, the baby began showing signs two days after birth. The signs of MCAT included:

  • Vomiting
  • Weight loss
  • Poor appetite
  • Diarrhea
  • Trouble breathing

In the one case of medium-chain ketoacyl-CoA thiolase deficiency (MCAT), the baby was treated with IV fluids to lower the acid levels in his blood. While everyone has some acid in their blood, high levels can be toxic.

Expected Outcomes

There has only ever been one reported case of medium-chain ketoacyl-CoA thiolase deficiency. The baby died at 13 days of age.


When we eat food, enzymes help break it down. Some enzymes break down fats into their building blocks, called fatty acids. Other enzymes break down these fatty acids. The enzyme medium-chain ketoacyl-CoA thiolase helps break down certain fatty acids.

Fatty acids are built like chains and come in a variety of lengths. They are categorized as either short, medium, long, or very long. Medium-chain ketoacyl-CoA thiolase helps break down medium length fatty acid chains. If you have a question about what makes a “medium” fatty acid, read about it here.

In medium-chain ketoacyl-CoA thiolase deficiency (MCAT), this enzyme is not working correctly. It may be that the body does not make enough of the enzyme or that the body makes non-working enzymes. When this enzyme does not work, the medium length fatty acids cannot be broken down for energy.

Fatty acids are an important source of energy for the heart, especially when sugars are low, such as in between meals. When fatty acids are not completely broken down, harmful substances build up in the body. MCAT causes dangerously high acid levels. While everyone has some acid in their blood, high levels can be toxic.

We do not yet know the genetic basis of MCAT.

Support for Medium-Chain Ketoacyl-CoA Thiolase Deficiency

Support Services

Support groups can help connect families who have a child or other family member affected with medium-chain ketoacyl-CoA thiolase deficiency with a supportive community of people who have experience and expertise in living with similar genetic conditions and fatty acid oxidation conditions. These organizations offer resources for affected individuals and their families, as well as health care providers:

Accessing Care

Work with your baby’s doctor to determine the next steps for your child’s care. Your baby’s doctor may help you coordinate care with a doctor who specializes in metabolism, a dietician who can help plan your child’s specialized diet, and other medical resources in your community.

Families' Experiences

At this time we have not located a family story for this particular condition. If your family is affected by medium-chain ketoacyl-CoA thiolase deficiency (MCAT) and you would like to share your story, please contact us so other families may benefit from your experience.

Each family's experience with a condition is unique, but there are some parts of the evaluation and treatment of MCAT that are similar to other fatty acid oxidation conditions. You may find it useful to read the stories of families affected by other fatty acid oxidation conditions. Read these stories on the family story page of the FOD Family Support Group website.

References & Sources

Visit OMIM to search its online catalog of human genes and disorders

Visit the Newborn Screening Coding and Terminology Guide by the U.S. Library of Medicine for condition information

Kamijo T, Indo Y, Souri M, et al. Medium chain 3-ketoacyl-coenzyme A thiolase deficiency: a new disorder of mitochondrial fatty acid beta-oxidation. Pediatr Res. 1997; 42(5): 569-76.

ACT Sheets

Healthcare professionals can learn more about confirmatory testing by reading the American College of Medical Genetics and Genomics' Algorithm for Diagnosis and ACT Sheet, a guide for follow-up after newborn screening. You can visit this page of the ACMG website here.

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