Myotonic Dystrophy Type 1 (DMPK) CTG Expansion
Ordering Recommendation

Diagnose myotonic dystrophy type 1 (DM1) in symptomatic individuals. Screen for DM1 for adults with a family history.

Polymerase Chain Reaction/Capillary Electrophoresis
7-10 days
New York DOH Approval Status
Specimens from New York clients will be sent out to a New York DOH approved laboratory, if possible.
ARUP Consult®
Disease Topics
Specimen Required
Patient Preparation
Lavender (K2EDTA), Pink (K2EDTA), or Yellow (ACD Solution A or B). 
Specimen Preparation
Transport 5 mL whole blood. (Min: 3 mL) 
Storage/Transport Temperature
Unacceptable Conditions
Ambient: 1 week; Refrigerated: 1 month; Frozen: 6 months 
Reference Interval
By report
Interpretive Data
Interpretive Data:
Background Information for Myotonic Dystrophy Type 1 (DMPK):
: Myotonic dystrophy type 1 (DM1) is a multisystem disorder characterized by myotonic myopathy with involvement of the eye, heart, endocrine system and central nervous system. Clinical findings span a continuum from mild to severe, with overlap in the three recognized clinical subtypes of DM1: mild, classic and congenital. Mild DM1 is adult-onset and features include mild myotonia and premature cataracts or baldness. Onset of classic DM1 is typically between 10-30 years of age and findings include distal muscle weakness, myotonia, cataracts, GI disturbances, and cardiac conduction abnormalities. Congenital DM1 may present prenatally with polyhydramnios and reduced fetal movement, and postnatal features commonly include infantile hypotonia, respiratory insufficiency, facial diplegia, and intellectual disability.
Prevalence: 1:20,000.
Inheritance: Autosomal dominant.
Penetrance: Age-related, approaches 100 percent by age 50.
Cause: Expanded number of CTG repeats in the DMPK gene.
Normal: 5-34 CTG repeats, stably transmitted, not associated with DM1 manifestations.
Premutation: 35-49 CTG repeats, may be unstably transmitted, not associated with DM1 manifestations.
Full-penetrance disease allele: 50 or more CTG repeats, unstably transmitted, associated with DM1 manifestations.
Clinical Sensitivity: >99 percent for DM1.
Triplet repeat-primed polymerase chain reaction (PCR) followed by size analysis using capillary electrophoresis to assess the CTG repeat in the DMPK 3' untranslated region. Specific allele sizing estimates cannot be determined for CTG repeats of >150. Repeat sizing precision is approximately +/- 2 repeats for alleles with 5-24 repeats and +/- 4 repeats for alleles with 77 to 150 repeats.
Analytical Sensitivity and Specificity
: 99 percent.
Limitations: Diagnostic errors can occur due to rare sequence variations. This assay will not detect myotonic dystrophy type 2.

PhenotypeNumber of CTG Repeats
Normal alleleLess than or equal to 34
Premutation35 - 49
Mild50 - approx. 150
Classicapprox.100 - approx 1000

Compliance Statement C: For human genetic inheritable conditions and mutations. This test was developed and its performance characteristics determined by ARUP Laboratories. The U. S. Food and Drug Administration has not approved or cleared this test; however, FDA clearance or approval is not currently required for clinical use. The results are not intended to be used as the sole means for clinical diagnosis or patient management decisions.

Counseling and informed consent are recommended for genetic testing. Consent forms are available online.

Hotline History
View Hotline History
Component Test Code*Component Chart NameLOINC
3001908Myotonic Dystrophy (DM1) - Specimen31208-2
3001909Myotonic Dystrophy (DM1) - Allele 135751-7
3001910Myotonic Dystrophy (DM1) - Allele 235750-9
3001911Myotonic Dystrophy (DM1) Interpretation50398-7
* Component test codes cannot be used to order tests. The information provided here is not sufficient for interface builds; for a complete test mix, please click the sidebar link to access the Interface Map.
  • DM1
  • DMPK
  • Myotonic dystrophy
  • Steinert disease