Conditions Citrullinemia, Type I

Citrullinemia is a condition with multiple forms, which each have different treatments and outcomes. Citrullinemia, type I is only one form of the condition. You can read about a different form of the condition, citrullinemia type II, here.

Condition Type

Amino Acid Disorders


Citrullinemia, type I affects one out of every 57,000 babies born in the United States.

Also known as

  • CIT
  • Classic form citrullinemia
  • Argininiosuccinate synthetase deficiency
  • Arginininosuccinic acid synthetase deficiency
  • AS deficiency
  • ASS deficiency
  • Citrullinuria
  • CTLN1
  • Citrullinemia

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of CIT (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 citrullinemia, type I (CIT) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your child 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 CIT 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 levels of acids and toxins. Certain acids and toxins build up in the body when a child has an amino acid condition, so measuring the amounts of these substances in your baby’s body can help doctors determine if your baby has a condition. High levels of ammonia and an amino acid called citrulline in the blood and orotic acid in the urine might indicate that your baby has CIT. Sometimes follow-up testing may also include testing a very small sample of skin.

About Citrullinemia, Type I

Early Signs

There are two main forms of citrullinemia, type I (CIT): the “classic” form seen in babies and a “mild” form that is first seen later in childhood or adulthood. Some signs of the mild form are different from the classic form seen in babies.

Babies usually start to show signs within the first few days of life. Signs of classic CIT include:

  • Sleeping longer or more often
  • Tiredness
  • Poor appetite
  • Vomiting
  • Seizures (Epilepsy)
  • Irritability
  • Delayed growth

Mild CIT may share some of the signs of classic CIT. For more information about the signs seen in mild CIT, check out STAR-G’s website.

Many of these signs may occur when your baby eats foods that their body cannot break down. They can be triggered by long periods of time without eating, illnesses, and infections.

If your baby shows any of these signs, be sure to contact your baby’s doctor immediately.


Dietary Treatments

Your baby may need to follow a restricted diet in order to avoid foods that your baby cannot break down. A dietician or nutritionist can help you plan the right diet for your child.

Your baby’s doctor might also recommend special formulas and foods for babies with citrullinemia, type I (CIT). These formulas will likely need to be continued through adulthood.

Supplements and Medications

Your baby’s doctor may prescribe medications to help lower ammonia levels in your baby’s blood. Everyone has some ammonia is their blood, but high levels can be toxic.

Arginine supplements may also be helpful for babies with CIT. Arginine is a natural substance that can help prevent the build-up of ammonia in individuals with CIT. Your baby’s doctor can write a prescription for these supplements.

Expected Outcomes

Babies who receive early treatment for citrullinemia, type I (CIT) can have healthy growth and development. This is why newborn screening for CIT is so important.

Some children can still have high ammonia levels, even with treatment. These children may need to seek treatment in a hospital to remove ammonia from their blood.

It is important to treat CIT early because babies who do not receive treatment are at risk for coma, brain damage, or death.


When we eat food, enzymes help break it down. Some enzymes help break down proteins into their building blocks, called amino acids. Other enzymes help break down these amino acids. When we break down proteins and their amino acids, our bodies need even more enzymes to get rid of waste products, like ammonia.

In citrullinemia, type I (CIT) babies do not make enough of the enzyme arginosuccinate synthetase (ASS1). When ASS1 is not working correctly, the body cannot remove ammonia through the urine. This causes a dangerous build-up of ammonia in the blood.

CIT is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for CIT, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition. While having a child with CIT is rare, when both parents are carriers, they can have more than one child with the condition. Learn more about autosomal recessive inheritance.

Support for Citrullinemia, Type I

Support Services

Support groups can help connect families who have a child or other family member affected with citrullinemia with a supportive community of people who have experience and expertise in living with the condition. These organizations offer resources for families, affected individuals, health care providers, and advocates.

Accessing Care

Work with your baby’s doctor to determine the next steps for your baby’s care. Your baby’s physician 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. Depending on the signs and symptoms your child experiences, your baby’s doctor may work with other doctors who specialize in treating the liver or brain. Some children with citrullinemia (CIT) have developmental delays. If you think that your baby is not meeting their developmental milestones, ask your baby’s doctor about the next steps in accessing a developmental evaluation and care.

Because CIT is a genetic condition, you may want to talk with a genetics specialist. A genetic counselor or geneticist can help you understand the causes of the condition, discuss genetic testing for CIT, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Clinic Services Search Engine offered by the American College of Medical Genetics and Genomics (ACMG) and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good resources for you or your baby's health care provider to use in identifying local specialists.

Families' Experiences

Coubly is a healthy young boy with citrullinemia, diagnosed as a result of newborn screening. Read about Coulby’s diagnosis and progress since the diagnosis on the blog,A Tale of Two C's, that his family created.

Lynlee was diagnosed with citrullinemia when she was four-days-old, after a number of early health challenges. Read about her family’s experience in the days after Lynlee's birth and in her successful recovery on her mother’s blog.

References & Sources

Visit the Screening, Technology And Research in Genetics (STAR-G) Project for more information on citrullinemia

Visit MedlinePlus Genetics for more condition information

Visit GeneReviews for more information on citrullinemia, type I

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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