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Test Code GALT Galactose-1-Phosphate Uridyltransferase (GALT), Blood

Performing Laboratory

Mayo Medical Laboratories in Rochester

Reporting Name

Gal-1-P Uridyltransferase, RBC

Specimen Type

Whole Blood EDTA


Advisory Information


This test is for enzyme testing only. The preferred test to evaluate for possible diagnosis of galactosemia, routine carrier screening, and follow-up of abnormal newborn screening results is GCT / Galactosemia Reflex, Blood.

 

This assay is not appropriate for monitoring dietary compliance; see GAL1P / Galactose-1-Phosphate (Gal-1-P), Erythrocytes.

           

This assay will not detect UDP-galactose 4' epimerase (GALE) deficiency For epimerase deficiency, see GALE / UDP-Galactose 4' Epimerase (GALE), Blood.

 

This assay will not detect galactokinase deficiency. For galactokinase deficiency, see GALK / Galactokinase, Blood.



Necessary Information


Patient's age is required.



Specimen Required


Container/Tube:

Preferred: Lavender top (EDTA)

Acceptable: Green top (sodium heparin) or yellow top (ACD)

Specimen Volume: 5 mL


Reject Due To

Hemolysis

Mild OK; Gross reject

Lipemia

NA

Icterus

NA

Other

NA

Specimen Stability Information

Specimen Type Temperature Time
Whole Blood EDTA Refrigerated (preferred) 28 days
  Ambient  14 days

Specimen Minimum Volume

2 mL

Day(s) and Time(s) Performed

Monday, Wednesday, Friday; 7 a.m. set up (specimen must be received the day prior)

Specimen Retention Time

2 months

Analytic Time

4 days (not reported on Saturday or Sunday)

Reference Values

≥24.5 nmol/h/mg of hemoglobin

Useful For

Diagnosis of galactose-1-phosphate uridyltransferase deficiency, the most common cause of galactosemia

 

Confirmation of abnormal state newborn screening results

Testing Algorithm

See Galactosemia Testing Algorithm in Special Instructions.

Method Name

Enzyme Reaction Followed by Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

82775

LOINC Code Information

Test ID Test Order Name Order LOINC Value
GALT Gal-1-P Uridyltransferase, RBC In Process

 

Result ID Test Result Name Result LOINC Value
8333 Gal-1-P Uridyltransferase, RBC 24082-0
2296 Interpretation (GALT) 59462-2
58115 Reviewed By 18771-6

Clinical Information

Galactosemia is an autosomal recessive disorder that results from a deficiency of any 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 occur.

 

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.

 

Galactose-1-phosphate (Gal-1-P) accumulates in the erythrocytes of patients with galactosemia. The quantitative measurement of Gal-1-P is useful for monitoring compliance with dietary therapy. Gal-1-P is thought to be the causative factor for development of liver disease in these patients and, because of this, patients should maintain low levels and be monitored on a regular basis.

 

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 Los Angeles variant, which consists of N314D and a second mutation, L218L, is associated with higher levels of GALT enzyme activity than the Duarte-variant allele.

 

Newborn screening for galactosemia is performed in all 50 US states, though the method by which potentially affected individuals are detected varies from state to state and may include the measurement of total galactose (galactose and Gal-1-P) and/or determining the activity of the GALT enzyme. The diagnosis of galactosemia is established by follow-up quantitative measurement of GALT enzyme activity. If enzyme levels are indicative of carrier or affected status, molecular testing for common GALT mutations may be performed. If 1 or both disease-causing mutations are not detected by targeted mutation analysis and biochemical testing has confirmed the diagnosis of galactosemia, sequencing of the GALT gene is available to identify private mutations.

 

See Galactosemia Testing Algorithm in Special Instructions for additional information.

Interpretation

An interpretive report will be provided.

 

See Galactosemia Testing Algorithm in Special Instructions for additional information

Cautions

The results of testing performed in erythrocytes are invalid following a transfusion, including analysis of enzymes, biochemical phenotyping, or galactose-1-phosphate.

Clinical Reference

1. Berry GT: Classic Galactosemia and Clinical Variant Galactosemia. In GeneReviews. Edited by RA Pagon, MP Adam, HH Ardinger, et al. Retrieved 03/11/2015. Available at www.ncbi.nlm.nih.gov/books/NBK1518/

2. Walter JH, Fridovich-Keil JL: Galactosemia. In The Metabolic and Molecular Bases of Inherited Disease. Edited by D Valle, AL Beaudet, B Vogelstein, et al. New York, McGraw-Hill, 2014. Accessed January 26, 2016. Available at http://ommbid.mhmedical.com/content.aspx?bookid=971&Sectionid=62672411

Method Description

Ninety mcL of an aqueous mixture containing CLR H2O, uridine diphosphate (UDP)-glucose, (13)C(2)-labeled galactose-1-phosphate, and UDP-n-acetylglucosamine (internal standard) is added to 30 mcL hemolysate aliquot. The mixture is then vortexed briefly and incubated while rotating at 225 RPM and 37° C for 15 minutes. The enzyme galactose-1-phosphate uridyltransferase (GALT) (EC 2.7.7.12) will convert (13)C(2)-labeled galactose-1-phosphate to (13)C(2)-labeled UDP-galactose during a 15 minute incubation at 37° C.

 

Enzyme reaction:

(13)C(2)-galactose-1-phosphate + UDP-glucose -> (13)C(2)-UDP-galactose + glucose-1-phosphate

 

After incubation the reaction is quenched and extracted with 250 mcL of acetonitrile, vortexing for 30 seconds on a multi-tube vortexer. The mixture is then centrifuged at 3,000 rpm for 15 minutes in an ambient centrifuge. The top acetonitrile extract layer is then transferred to a 96-well (Nunc, polypropylene) plate. Then 10 mcL is injected onto a liquid chromatography-tandem mass spectrometry (LC-MS/MS) API 3200 system for chromatographic separation and measurement of analytes. The MS/MS is operated in the multiple reaction monitoring (MRM) negative mode to follow the precursor to product species transitions for (13)C(2)-labeled UDP-galactose (567.0 to 322.9 m/z) and internal standard of UDP-n-acetylglucosamine (606.2 to 384.7 m/z). The ratio of the extracted peak area of (13)C(2)labeled UDP-galactose to its internal standard UDP-n-acetylglucosamine as determined by liquid chromatography-tandem mass spectrometry is used to calculate the concentration of product analyte in the sample. The concentration of the product is then normalized using the calculated hemoglobin concentration to determine the patient's enzyme level in nmol/h/mg of hemoglobin.(Unpublished Mayo method)

Genetics Test Information

Enzyme testing only. See GCT / Galactosemia Reflex, Blood for comprehensive diagnostic, carrier testing, and follow-up of abnormal newborn screening results.

Disease States

  • Galactosemia

Forms

1. New York Clients-Informed consent is required. Please document on the request form or electronic order that a copy is on file. An Informed Consent for Genetic Testing (T576) is available in Special Instructions.

2. Biochemical Genetics Patient Information (T602) in Special Instructions.