Type of test Blood

Normal findings

 Hexosaminidase A: 7.5-9.8 units/L (SI units)

 Total hexosaminidase: 9.9-15.9 units/L (SI units) (Check with the laboratory because of the variety of testing methods.)

Test explanation and related physiology

 Tay–Sachs disease (TSD) is a lysosomal storage disease (LSD), which, in infancy and early childhood, is characterized by loss of motor skills. Major categories of LSD include mucopolysaccharidoses, oligosaccharidoses, neuronal ceroid lipofuscinoses, and sphingolipidoses.

TSD, like other LSDs, is usually a result of a mutation in an autosomal recessive gene. Thus the affected person must have inherited a mutated gene from each parent in order to have TSD. Ashkenazi (Eastern European) Jews may particularly be carriers for these mutations. Eighty different mutations inhibit the function of this important gene. This gene encodes the synthesis of an enzyme called hexosaminidase. Without this enzyme, lysosomes of GM2 accumulate, particularly in the CNS.

Two clinically important isoenzymes of hexosaminidase have been detected in the serum: hexosaminidase A (hex A, made up of 1 alpha subunit and 1 beta subunit) and hexosaminidase B (hex B, made up of 2 beta subunits). Any genetic mutation that affects the alpha unit will cause a deficiency of hexosaminidase A, resulting in TSD. A mutation that affects the beta unit will cause a deficiency in hex A and B. Sandhoff disease, an uncommon variant of TSD, occurs with deficiency of both of these enzymes.

Biochemical testing to identify carriers and those affected by TSD is an important aspect of diagnosis. Hex A has been found to be abnormally low in carriers, whereas hex B is high. Therefore testing for total hexosaminidase is not useful. A carrier has a 25% chance of having a child with TSD if the other biological parent is also a carrier. In communities in which the Ashkenazi Jewish population is high, hex A screening has been very effective for identifying carriers. Furthermore, hex A is used to diagnose TSD in infants, young children, and adults.

Because accumulating analytes can be detected in urine, screening for LSDs typically begins with an analysis to detect disease-specific metabolite patterns. If positive, panel gene sequencing for LSDs is useful to corroborate the identification of an affected person or a carrier.

Interfering factors

• Hemolysis of the blood sample can cause inaccurate test results.

• Pregnancy can cause markedly increased values. For this rea son, blood tests are not done during pregnancy.

* Oral contraceptives can falsely increase levels.

Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no • Blood tube commonly used: red

* Emphasize the importance of this test to Jewish couples of Eastern European ancestry who plan to have children. Explain that both must carry the defective gene to transmit TSD to their offspring.

• Professional genetic counseling should be provided to every person considering undergoing this test.

• Check with the laboratory regarding withholding contraceptives.

• Note that pregnant women can be evaluated by amniocentesis or chorionic villus biopsy.

• Note that infants may have blood obtained by heel sticks. Neonates often have blood drawn through the umbilical cord.

Abnormal findings

Decreased hexosaminidase A

- Tay–Sachs disease

Decreased hexosaminidase A and B

- Sandhoff disease

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مواضيع ذات صلة

Lab Diagnosis of Yellow fever

hemoglobin electrophoresis (Hgb electrophoresis)

growth hormone stimulation test (GH provocation test, Insulin tolerance test [ITT], Arginine test, Glucagon Stimulation test)

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Diagnosis of Prion Disease

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fluorescein angiography (FA, Ocular photography)

Bone Alkaline Phosphatase

Molecular Allergy Diagnostics