Amino Acid Disorders Screening Health Article

Definition

Amino acid disorders screening is performed to detect inherited and acquired disoders in amino acid metabolism. Tests are most commonly done in the neonatal period.

Purpose

Twenty of the 100 known amino acids are the main building blocks for human proteins. Of these 20 amino acids, ten (essential amino acids) are not made by the body and must be acquired through diet. Proteins regulate every aspect of cellular function. The goal of amino acid disorder screening is early detection and intervention. Errors of amino acid metabolism range from asymptomatic or benign to life-threatening. In some cases, management by dietary restriction of the amino acid(s) involved can prevent damage or symptoms.

Precautions

All neonates should be screened for the most prevalent aminoacidurias within a few days of birth. Usually this includes tests for phenylketonuria, tyrosinemia, lysinemia, maple syrup urine disease, and homocystinuria using a dried spot of blood collected on a paper card. When an infant displays symptoms such as failure to thrive, repeated vomiting, or nervous system impairment and an amino acid disorder is suspected, testing may be done on either plasma or urine. The patient should be fasting for the blood test. Blood samples are usually collected by heel puncture. The nurse or phlebotomist should observe universal precautions for the prevention of transmission of bloodborne pathogens when collecting the sample. Blood samples show considerable variance depending upon the time of sampling, and urine testing avoids this problem. Urine testing should be performed on a first morning voided sample or a 24-hour urine sample. Some drugs may affect the results of amino acid tests by certain methods of analysis. Any medications being given to the patient should be reported to the laboratory performing the test. Antibiotics will adversely affect the growth of the bacterium used in the Guthrie bioassay test for phenylalanine. Mothers of breastfeeding infants should report any medications they are taking, since these can pass from mother to child in breast milk.

Description

An enzyme deficiency that blocks the metabolism of an amino acid is called a primary aminoaciduria. An error in absorption or transport of an amino acid is called a secondary aminoaciduria. Both primary and secondary aminoacidurias may be inherited (congenital) or acquired. Aminoacidurias may be grouped into two types, overflow and renal, based on the mechanism by which the amino acids accumulate. In the overflow type, the plasma level of one or more amino acids will be increased (aminoacidemia). The most common overflow type is phenylketonuria (PKU). The incidence of PKU varies with race and ethnic origin and there are 5 different forms of the disease. The most prevalent is type I which occurs in approximately 1 in 10,000 births in the United States, and accounts for about 50% of PKU cases. Because of its high prevalence all states screen newborns for PKU. This condition is caused by an inherited deficiency of the enzyme phenylalanine hydroxylase. The enzyme is needed to convert phenylalanine to tyrosine. Phenylalanine accumulates in the plasma and is oxidized by phenylalanine transaminase to phenylpyruvic acid which is excreted in the urine. Phyenylpyruvic acid is a ketoacid, hence the name phenylketonuria. Mental retardation occurs as a consequence of PKU, but can be prevented by withholding phenylalanine from the diet. There are many other primary aminoacidurias. Several are caused by enzyme deficiencies of the urea cycle. These give rise to increased blood ammonia levels and an increase of one or more amino acids.

In renal type aminoacidurias, one or more amino acids are excreted in excess in the urine but plasma levels are not increased. Some renal type aminoacidurias are inherited, but many are acquired or secondary to other diseases. A common inherited renal type aminoaciduria is cystinuria which results from a defect in the reabsorption of dibasic aminoacids by the kidney.