Hexosaminidase A and B

Synonym/acronym: N/A.

Common use

To assist in diagnosing Tay-Sachs disease by identifying a hexosaminidase enzyme deficiency.

Specimen

Serum (3 mL) collected in a red-top tube. After the specimen is collected, it must be brought immediately to the laboratory. Once in the laboratory, the specimen must be allowed to clot for 1 to 1.5 hr in the refrigerator. The serum should then be removed and frozen immediately.

Normal findings

(Method: Fluorometry)

Total Hexosaminidase	Conventional Units	SI Units (Conventional Units × 0.0167)
Noncarrier	589–955 nmol/hr/mL	9.83–15.95 units/L
Heterozygote	465–675 nmol/hr/mL	7.77–11.27 units/L
Tay-Sachs homozygote	Greater than 1,027 nmol/hr/mL	Greater than 17.15 units/L

Hexosaminidase A	Conventional Units	SI Units (Conventional Units × 0.0167)
Noncarrier	456–592 nmol/hr/mL	7.62–9.88 units/L
Heterozygote	197–323 nmol/hr/mL	3.29–5.39 units/L
Tay-Sachs homozygote	0 nmol/hr/mL	0 units/L

Hexosaminidase B	Conventional Units	SI Units (Conventional Units × 0.0167)
Noncarrier	12–32 nmol/hr/mL	0.2–0.54 units/L
Heterozygote	21–81 nmol/hr/mL	0.35–1.35 units/L
Tay-Sachs homozygote	Greater than 305 nmol/hr/mL	Greater than 5.09 units/L

Description

Hexosaminidase is a lysosomal enzyme; the deficiency of which results in accumulation of complex sphingolipids and gangliosides in the brain.. There are three predominant isoenzymes: hexosaminidase A, B, and S. There are more than 70 lysosomal enzyme disorders. Testing for hexosaminidase A is done to determine the presence of Tay-Sachs disease, a genetic autosomal recessive condition characterized by early and progressive retardation of physical and mental development. This enzyme deficiency is most common among Ashkenazi Jews, for whom the incidence is 1 in 3,000 and carrier rate is 1 in 30. Genetic testing by DNA polymerase chain reaction analysis can identify as many as 94% of the gene mutations associated with Tay-Sachs disease in persons with Ashkenazi Jewish heritage, 80% of the mutations in persons with French-Canadian ancestry, and 25% of mutations in non-Jewish Caucasians. Genetic testing combined with enzyme screening analysis and correlation of clinical information provides the most reliable means of determining carrier status. The American College of Medical Genetics recommends routine preconceptual or prenatal screening for nine different hereditary diseases common to persons with Ashkenazi Jewish heritage. Counseling and written, informed consent are recommended and sometimes required before genetic testing. Patients who are homozygous for this trait have no hexosaminidase A and have greatly elevated levels of hexosaminidase B; signs and symptoms include red spot in the retina, blindness, and muscular weakness. Tay-Sachs disease results in early death, usually by age 3 or 4.

Hereditary Disease	Estimated Incidence in Ashkenazim	Estimated Carrier Rate in Ashkenazim
Bloom’s syndrome	1:40,000	1 in 100
Canavan disease	1:10,000	1 in 50
Familial dysautonomia	1:3,600	1 in 32
Fanconi’s anemia group C	1:32,000	1 in 89
Gaucher’s disease	1:900	1 in 15
Mucolipidosis IV	1:63,000	1 in 127
Niemann-Pick disease type A	1:32,000	1 in 90
Tay-Sachs disease	1:3,000	1 in 30
Cystic fibrosis	1:3,000	1 in 25

This procedure is contraindicated for

high alert Parents who are not emotionally capable of understanding the test results and managing the ramifications of the test results.

Indications

Assist in the diagnosis of Tay-Sachs disease
Identify carriers with hexosaminidase deficiency

Potential diagnosis

Increased in

Alterations in lysosomal enzymes metabolism are associated with various conditions.
Total
- Gastric cancer
- Hepatic disease
- Myeloma
- Myocardial infarction
- Pregnancy
- Symptomatic porphyria
- Vascular complications of diabetes
Hexosaminidase A
- Diabetes
- Pregnancy
Hexosaminidase B
- Tay-Sachs disease

Decreased in

Sandhoff’s disease (inherited disorder of enzyme metabolism lacking both essential enzymes for metabolizing gangliosides)

Tay-Sachs disease (inherited disorder of enzyme metabolism lacking only the hexosaminidase A enzyme for metabolizing gangliosides)

Sandhoff’s disease

Critical findings

N/A

Interfering factors

Drugs that may increase hexosaminidase levels include ethanol, isoniazid, oral contraceptives, and rifampin.
The serum specimen hexosaminidase assay should not be performed on women who are pregnant or who are taking oral contraceptives. Increases in enzyme activity during pregnancy occur in carriers and noncarriers of the Tay-Sachs gene. Enzyme activity is also notably increased in women taking oral contraceptives regardless of carrier status.

Nursing Implications and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in diagnosing or identifying carrier status for Tay-Sachs disease.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
Obtain a history of the patient’s reproductive system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

Potential complications: N/A
Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
Immediately transport the specimen to the laboratory for processing and analysis.

Post-Test

Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
Recognize anxiety related to test results, and be supportive of fear of shortened life expectancy. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Encourage the family to seek genetic counseling if results are abnormal. It is also important to discuss feelings the mother and father may experience (e.g., guilt, depression, anger) if abnormalities are detected. Educate the patient regarding access to counseling services. Provide contact information, if desired, for the National Tay-Sachs and Allied Diseases Association (www.ntsad.org).
Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

Related tests include ALT, amniotic fluid analysis, bilirubin, biopsy chorionic villus, biopsy liver, chromosome analysis, GGT, liver and spleen scan, and protein total and fractions.
Refer to the Reproductive System table at the end of the book for related tests by body system.