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Galactosemia is an inborn
error of carbohydrate metabolism caused by the deficiency of
galactose-1-phosphate uridyl transferase (GALT), which performs the second
enzymatic step in the conversion of galactose to glucose-1-phosphate. As a
result of the enzyme deficiency, excess galactose is excreted in the urine and
the substrate for GALT, galactose-1-phosphate, accumulates throughout the body.
Manifestations of the
disease appear within days of the initiation of milk feedings, and include
vomiting, jaundice and failure to thrive. If left untreated for a prolonged
period of time, patients will develop hepatomegaly, cataracts and mental
retardation. Speech delay and premature ovarian failure are also observed in
many galactosemia patients, even in those who are diagnosed early and put on
treatment.
Galactosemia is typically
detected very early because of newborn screening programs, which measure GALT
enzyme activity in dried blood spots. The measurement of galactose-1-phosphate
is performed to confirm a new diagnosis of galactosemia and to periodically
monitor the effectiveness of treatment in patients known to have the disease.
Sequencing of the GALT
gene is also available. Contact the lab (1-800-473-9411) for further
information.
Specimen requirements:
Galactose-1-phosphate measurements must be performed in washed red blood cells.
Transport:
The laboratory will isolate red blood cells for use in the assay if a whole
blood sample is sent. Collect blood in a sodium heparin green top tube (5-10 mL)
and send via courier or 24-hour delivery at room temperature (not frozen). Red
blood cells must be isolated within 24 hours of sample collection.
Standard of analysis:
Galactose-1-phosphate (Gal-1-P) is measured by first converting endogenous
galactose-1-phosphate to galactose using alkaline phosphatase, and then
converting galactose to galactonolactone using β-galactose dehydrogenase.
During the second enzyme reaction, NAD is reduced to NADH, the accumulation of
which can be measured using its absorbance at 340 nm. Results are reported in
mg Gal-1-P / dL RBC.
Time required:
Analysis should be complete within 2 weeks of sample receipt.
Reporting of results:
Written reports with interpretation of results will be mailed and/or faxed to
the referring physician. When an abnormality is detected, the results will be
communicated verbally to the referring physician by telephone.
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