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Huntington Disease Mutation (8000101296)
Test Mnemonic:

HD
Specimen Requirements:

Whole Blood
Container:

Lavender (EDTA), pink (K2EDTA), blue (Na Citrate) or yellow (Acid Citrate Dextrose) collection tube
Minimum Volume:

2ml
Storage/Transport:

Ambient
Stability:

30 days
Causes for Rejection:

Incorrect tube, inadequate volume
Reference Range:
Turnaround Time:

4-10 days
Methodology:

Polymerase Chain Reaction / Fragment Analysis
Performed:
Lab:

Molecular Diagnostics Laboratory
Synonyms:

Huntington's, HD, HTT
CPT 4 Code:

81401
Note:

Phenotype Characteristics and Indications of the Test: Huntington disease (HD) is a progressive neurodegenerative disorder with characteristic motor, cognitive, and psychiatric symptoms. HD is caused by an expanded trinucleotide CAG repeat in the gene encoding huntingtin (HTT). This test is indicated to confirm the clinical diagnosis of HD in affected individuals, to provide presymptomatic predictive diagnosis of HD in individuals with positive family history, and to identify individuals at risk of having affected offspring. It is strongly recommended that pre- and post-test counseling be incorporated in any presymptomatic testing, and that predictive testing not be offered to individuals until they are at least 18 years old.

Incidence: 1 in 15,000.

Inheritance: Autosomal dominant.

Cause: Expanded number of CAG repeats in HTT gene.

HD allele: Greater than or equal to 40 CAG repeats
HD allele with reduced penetrance: 36-39 CAG repeats

Mutable normal allele: 27-35 CAG repeats
Normal allele: Less than or equal to 26 CAG repeats

Clinical Sensitivity: 99 percent.

Methodology: Chimeric polymerase chain reaction (PCR) followed by size analysis using capillary electrophoresis. This test was developed and its performance characteristics were determined by UTMB Pathology Molecular Diagnostics Laboratory. It has not been cleared or approved by the U.S. Food and Drug Administration (FDA). The FDA has determined that such clearance or approval is not necessary. This test is used for clinical purposes. It should not be regarded as investigational or for research. This laboratory is certified under the Clinical Laboratory Improvement Amendments of 1988 (CLIA-88) as qualified to perform high complexity clinical laboratory testing.

Analytic Sensitivity and Specificity: 99 percent.

Limitations: Diagnostic errors can occur due to rare sequence variations. The number of CAG repeats does not strongly indicate the onset, severity, or the cognitive ability of the affected person. Alleles with 36-39 CAG repeats may have reduced penetrance and it may not be possible to predict if an asymptomatic individual with an allele size in this range will develop HD symptoms. This test may not detect very large CAG expansions of HTT gene (over 105 repeats).

Counseling and informed consent are recommended for genetic testing.

References:

Online Mendelian Inheritance in Man (OMIM) number: 143100

Online GeneReviews profile for Huntington disease:

https://www.ncbi.nlm.nih.gov/books/NBK1305/
When ordering tests for which Medicare or Medicaid reimbursement will be sought, physicians should only order tests that are medically necessary for the diagnosis or treatment of the patient. Components of the organ or disease panels may be ordered individually. The diagnostic information must substantiate all tests ordered and must be in the form of an ICD-10 code or its verbal equivalent.
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