Whole Blood

Adult Minimum: 6 mL whole blood in pink top tube
Pediatric Minimum: 2-3 mL whole blood in lavender top tube

Deliver to laboratory immediately after collection.

14 days upon receipt at reference laboratory

Several genes in the critical region of the SMA locus have been identified. One of the genes in this region, SNM, has been shown to be homozygously deleted in approximately 95% of the type I and type II cases and 80% of the type III cases. Although the exact functional role of the SNM gene in the pathogenesis of SMA is unknown, detected of absence of both copies of the SNM can be used to confirm the SMA diagnosis in the majority of cases. Carrier status is established by the accurate determination of the SMN copy number. Non carriers have normal 2 copy dosage of SMN, whereas carriers have single copy dosage of the SMN gene. The SMN competitive PCR gene-dosage assay identified two copies of the SMN gene, consistent with the noncarrier state. The finding of normal dosage significantly reduces the carrier risk to approximately 5%, which should then be entered as a conditional probability during the Bayesian risk calculation. It has been found that approximately 5% of the parents with affects children have normal 2 copy dosage. It is possible for a carrier to have normal 2 copy dosage, if both copies of the SMN gene are on 1 chromosome and the other chromosome is homozygously deleted. The SMN gene has also been shown to have a de- novo mutation rate of about 2%. The dosage analysis does not test for SMN point mutations which occur in about 5% of affected individuals. Lastly, although DNA testing is highly accurate, diagnostic errors can occur. It is recommended that the genetic results be conveyed through a genetic counselor.

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