Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

NAGR
82943
HEXOSAMINIDASE A
Reference values have not been established for patients that are less than 5 years of age.
> or =5 years: 56-80%
Clinical References: 1. Gravel RA, Kaback MM, Proia RL, et al: The GM2 gangliosidosis. In The
Metabolic and Molecular Bases of Inherited Disease. 8th edition. Edited by CR Scriver, AL Beaudet,
WS Sly, et al. New York, McGraw-Hill Book Company, available at www.ommbid.com. Accessed
11-17-10 2. Maegawa GH, Stockley T, Tropak M, et al: The natural history of juvenile or subacute
GM2 gangliosidosis: 21 new cases and literature review of 134 previously reported. Pediatrics 2006
Nov;118(5):e1550-1562 3. O'Brien JS, Okada S, Chen A, Fillerup DL: Tay-Sachs disease: detection of
heterozygotes and homozygotes by hexosaminidase assay. N Engl J Med 1970;283:15-20
Hexosaminidase A and Total, Leukocytes/Molecular Reflex
Clinical Information: Macromolecules have a finite half-life and must, therefore, be catabolized.
Cleaving enzymes are present in lysosomes, where, at an acidic pH, they are active in cleaving complex
carbohydrates, lipids, and peptides. Hexosaminidase A is required for the breakdown of the ganglioside
GM2. Deficiency of beta-N-acetyl-hexosaminidase A isoenzyme (Hex A) is diagnostic for Tay-Sachs
disease (TSD). The deficiency causes storage of its substrate, ganglioside GM2, in the brains of children
affected by the disease. A second hexosaminidase isoenzyme (Hex B) is present in at least normal
amounts in these patients, but is unable to effectively hydrolyze GM2. The test for TSD is the
measurement of the Hex A isoenzyme component in serum, leukocytes, cultured fibroblasts, or in
amniocyte specimens for prenatal diagnosis. Total hexosaminidase (Hex A + B) activity may be normal
or slightly decreased in TSD, but a dramatic deficiency of the A component is diagnostic. Tay-Sachs
carriers have less than the normal amount of Hex A in serum and leukocytes. Deficiency of both A and
B isoenzymes occurs in patients with Sandhoff disease (Tay-Sachs hexosaminidase AB variant). While
most Hex A assays are performed to identify Tay-Sachs carriers, the test also can suggest that a person
is a carrier of Sandhoff disease. Refer to Carrier Testing for Tay-Sachs Disease and Other GM2
Gangliosidosis Variants: Supplementing Traditional Biochemical Testing with Molecular Methods,
Mayo Medical Laboratories Communique 2004 Jul;29(7) for more information regarding diagnostic
strategy.
Useful For: Diagnosing Tay-Sachs disease and carriers of Tay-Sachs, Sandhoff disease, and carriers
of Sandhoff disease Preferred test for determining carrier status
Interpretation: Hex A usually makes up >62% of the total hexosaminidase activity in leukocytes
(normal=63%-75% Hex A). The key factor for determining whether an individual is a carrier of, or
affected with, Tay-Sachs disease is the percent Hex A in leukocytes: -63% to 75% Hex A=normal
(noncarrier) -58% to 62% Hex A=ambiguous (molecular testing performed to discern carriers from
noncarriers and to provide the most options for prenatal diagnosis) -