Positive Newborn Screen for MCAD -- Now What?

Stephanie L. Austin, MS, MA; Laura A. Stokowski, RN, MS


July 10, 2014

Editorial Collaboration

Medscape &

MCAD Deficiency

You briefly explain MCAD deficiency to the parents, emphasizing that you do not yet know whether the infant has MCAD deficiency. However, until it has been ruled out, it is important that the parents monitor the baby for any health and behavior changes, especially poor feeding, vomiting, diarrhea, lethargy, flaccidity, or fever. If any of these occur, they should take the infant to the emergency department of the local hospital. Likewise, if the infant develops any signs of illness or infection (stuffy nose, fever, eye drainage, redness or swelling of circumcision, sleeplessness or sleepiness, irritability), they should call you. The baby should be breastfed at least every 4 hours, including at night, until the test results are known.

MCAD deficiency is the most common disorder of fatty acid oxidation and one of the most frequently detected disorders in newborn screening, especially in individuals of northwestern European origin.[9] Depending on the population, MCAD deficiency has an incidence of 1:4000 to 1:17,000. MCAD, a mitochondrial flavoprotein with a major role in the production of energy during times of fasting or physical exertion, catalyzes the first reaction in the beta-oxidation of medium-chain length (C6-C10) fatty acids. Normally, fatty acid oxidation results in the production of ketones, a substrate for energy in the brain, heart, and skeletal muscles. In the presence of MCAD deficiency, however, medium-chain fatty acids cannot be used for energy, so fasting or acute illness results in hypoketotic hypoglycemia and increased levels of medium-chain fatty acids, which in turn lead to the accumulation of glycine- and carnitine-esters and dicarboxylic acids, metabolites that are detectable in body fluids.[9] Even a seemingly mild illness can produce severe hypoglycemia in people with MCAD deficiency.

MCAD deficiency can be clinically silent, even into adulthood, and is typically asymptomatic in the newborn, becoming apparent between the ages of 3 and 24 months when a metabolic stress occurs (loss of appetite, mild illness), with the onset of hypoglycemia, metabolic acidosis, hyperammonemia, hepatomegaly, lethargy, seizures, and coma. A simple respiratory or gastrointestinal illness with vomiting and diarrhea that induces the mobilization of stored fatty acids for energy can rapidly precipitate an acute metabolic crisis in affected individuals,[14] and the clinical picture can be confused with Reye syndrome.[9] Numerous cases of young infants dying before diagnosis have been reported, and typical autopsy findings include fatty liver and cerebral edema.


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