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Spinal Muscular Atrophy (SMA), Diagnostic
Test Code16869
CPT Codes
81329
Physician Attestation of Informed Consent
This germline genetic test requires physician attestation that patient consent has been received if ordering medical facility is located in AK, DE, FL, GA, IA, MA, MN, NV, NJ, NY, OR, SD or VT or test is performed in MA.
Preferred Specimen
4 mL whole blood collected in an EDTA (lavender-top) tube
Minimum Volume
2 mL
Other Acceptable Specimens
Whole blood collected in: ACD (yellow-top) tube, 3.2% sodium citrate (light blue-top) tube, EDTA (pink-top) tube, or sodium heparin (green-top) tube
Instructions
Do not freeze whole blood specimens. Transport at room temperature.
Transport Temperature
Room temperature
Specimen Stability
Room temperature: 8 days
Refrigerated: 14 days
Frozen: Unacceptable
Refrigerated: 14 days
Frozen: Unacceptable
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Clotted specimen • Received frozen
Methodology
Allele Specific Real-Time Polymerase Chain Reaction, ddCt Method
FDA Status
This test was developed and its analytical performance characteristics have been determined by Quest Diagnostics. It has not been cleared or approved by FDA. This assay has been validated pursuant to the CLIA regulations and is used for clinical purposes.
Setup Schedule
Night Sets up 7 days a week.
Clinical Significance
Spinal muscular atrophy (SMA) is one of the most common autosomal recessive diseases. SMA is characterized by the degeneration of the anterior horn cells of the spinal cord which leads to symmetric proximal muscle weakness. The estimated incidence of SMA is 1/6,000 to 1/10,000 live births with a carrier frequency of 1/40 to 1/60. Clinical presentation, mainly based upon age of onset of symptoms, is classified into 4 SMA subtypes: Subtype I, onset before 6 months of age; Subtype II, onset between 6 and 12 months of age; Subtype III, onset in childhood after 12 months of age; And subtype IV, adult onset. Each subtype displays significant variance of clinical prognosis, including lifespan. The survival motor neuron (SMN1) gene has been shown to be responsible for 99% of SMA cases. An adjacent homologous gene, SMN2, encodes a protein identical to that of SMN1. SMA is caused by a critical reduction in the total amount of functional SMN protein. Typically 80% to 90% of SMN protein is derived from functional SMN1 genes, while 10% to 20% is derived from SMN2 genes. Therefore, SMN protein expression, or dosage, is based largely upon SMN1 gene copy number and, to a much lesser extent, SMN2 gene copy number. Loss of functional production of SMN protein most commonly occurs by deletion of SMN1 and/or SMN2 genes, either by homologous recombination or gene conversion (95% of SMA alleles). The remaining 5% of SMA alleles harbor point mutations in the SMN1 gene(s) that effectively eliminates the production of functional SMN protein from those gene copies. Although a diagnosis of SMA depends upon SMN1 gene copy number, a less severe SMA phenotype may be associated with an increased number (greater than or equal to 3 copies) of functional SMN2 gene copies. Conversely, a severe SMA phenotype may be associated with fewer (less than or equal to 2) functional SMN2 gene copies.
