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Interpretive Handbook

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Test 80341 :
Galactose-1-Phosphate Uridyltransferase Biochemical Phenotyping, Erythrocytes

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Galactosemia is an autosomal recessive disorder that results from a deficiency of 1 of the 3 enzymes catalyzing the conversion of galactose to glucose: galactose-1-phosphate uridyltransferase (GALT), galactokinase (GALK), and uridine diphosphate galactose-4-epimerase (GALE). GALT deficiency is the most common cause of galactosemia and is often referred to as classic galactosemia. The complete or near-complete deficiency of GALT enzyme is life-threatening if left untreated. Complications in the neonatal period include failure to thrive, liver failure, sepsis, and death; even with survival, long-term intellectual disability can result. Galactosemia is treated by a galactose-restricted diet, which allows for rapid recovery from the acute symptoms and a generally good prognosis. Despite adequate treatment from an early age, individuals with galactosemia remain at increased risk for developmental delays, speech problems, and abnormalities of motor function. Females with galactosemia are at increased risk for premature ovarian failure. Based upon reports by newborn screening programs, the frequency of classic galactosemia in the United States is approximately 1 in 30,000, although literature reports range from 1 in 10,000 to 1 in 60,000 live births.

 

Duarte-variant galactosemia (compound heterozygosity for the Duarte mutation, N314D, and a classic mutation) is generally associated with higher levels of enzyme activity (5%-20%) than classic galactosemia (<5%); however, this may be indistinguishable by newborn screening assays. Typically, individuals with Duarte-variant galactosemia have a milder phenotype, but are also often treated with a low galactose diet during infancy. The LA variant, which consists of N314D and a second mutation, L218L, is associated with higher levels of GALT enzyme activity than the Duarte-variant allele.

 

In general, molecular genetic analysis with a panel of common mutations is typically performed to determine the specific genotype. If the enzymatic and molecular results are incongruent, biochemical phenotyping and/or molecular sequence analysis may be beneficial to help clarify results to determine a treatment strategy and recurrence risks.

 

For more information regarding diagnostic strategy, refer to Galactosemia: Current Testing Strategy and Aids for Test Selection, Mayo Medical Laboratories Communique 2005 May;30(5).

 

See Galactosemia Testing Algorithm in Special Instructions for additional information.

Useful For Suggests clinical disorders or settings where the test may be helpful

Determining the biochemical phenotype for galactosemia when enzymatic and molecular results are incongruent

 

A quantitative galactose-1-phosphate uridyltransferase level (GALT / Galactose-1-Phosphate Uridyltransferase [GALT], Blood) is required for accurate interpretation.

Interpretation Provides information to assist in interpretation of the test results

An interpretive report will be provided.

 

See Galactosemia Testing Algorithm in Special Instructions for additional information.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

A more comprehensive interpretation can be provided when parental specimens are also submitted for testing.

 

Since transfusion results in replacement of significant number of red cells, the assay should be deferred for 90 days post-transfusion.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Descriptive report

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Elsas LJ: Galactosemia. NCBI GeneReviews. Updated 2010, Oct 26. Available from www.ncbi.nlm.nih.gov/books/NBK1518

2. Holton JB, Walter JH, Tyfield LA: Galactosemia. In The Metabolic and Molecular Basis of Inherited Disease. Vol. 1. Eighth edition. Edited by CR Scriver, AL Beaundet, WS Sly, et al. New York, McGraw-Hill Book Company, 2001, pp 1553-1587


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