Diagnosis of MSUD

DIAGNOSTICS FOR MSUD

Timing of screening:

All infants should be tested at discharge from the hospital, on the 5th day of life, or as soon as possible after death.

The diagnosis can usually be suspected from the characteristic odor of the urine and is confirmed by the abnormal levels of amino acids and keto acids in blood and urine. Other symptoms include metabolic acidosis, and depressed serum alanine levels. The enzyme defect may be detected in leukocytes and fibroblasts.

The method screening before birth is for deficiency of branched-chain alpha-ketoacid dehydrogenase activity in cultured chorionic villi or amniocytes. Emory’s Genetic Laboratory is the only one that gives details on testing for MSUD. They have a prenatal diagnosis in which they need a urine sample containing simultaneous measurements of isovaleryl, 3-methyl crotonyl, tiglyl, hexanoyl and suberyl glycine.

Screening test (for acute episodes)

0.1% solution of 2,4-dinitrophenylhydrazine reagent in 2N HCl

mix equal volumes of DNPH solution, urine, and distilled H2O

positive test ranges from a mild turbidity (1+) to a heavy yellow-orange precipitate (4+)

the yellow precipitate is the dinitrophenylhydrazone derivative of the keto acids present

Laboratory tests and required action

Test for elevated blood leucine on a filter paper blood specimen: a bacterial inhibition assay in which the presence of leucine allows the growth of mutant B. subtilis on an agar plate.

Normal leucine levels are < 4 mg/dL (0.4 mg/ml). Even transient elevation of plasma leucine in the normal newborn is unusual unless the infant is premature and/or receiving IV amino acid preparations.

Abnormal test results:

Leucine > 4 mg/dL:

Interpretation: classic MSUD

Confirmatory test: quantitative determination of leucine, isoleucine and valine in a blood specimen by HPLC.

Leucine 3-4 mg/dL:

Interpretation: MSUD possible

Test for blood leucine may depend to some degree upon the infant's accidentally ingested protein. Since the accumulation of leucine in the blood is slower than accumlation of the ketoacid of leucine, blood leucine in affected infants may not become elevated above the cut-off level until 8 to 12 hours after birth.

Normal Values and Criteria for Follow-up of Abnormal Results from Newborn Screening Tests

Condition screened for	Substance tested for	Normal value	Repeat filter paper (or serum) specimen requested when
Maple syrup urine disease	Leucine	< 4mg/dL	4mg/dL	(>4mg/dL)
Phenylketonuria	Phenylalanine	<2 mg/dL	2-6 mg/dL	(6 mg/dL)
Homocystiniuria	Methionine	< 1mg/dL	1-2 mg/dL	(>2mg/dL)
Galactosemia	Galactose Galactose-1-phosphate uridyl transferase	<6 mg/dL enzyme activity present	6-8 mg/dL enzyme activity low or absent	(>8mg/dL)

Return to Intro to MSUD