Overview: CTG/CAG Repeats TestIntroduction: The CTG/CAG Repeats Test counts CTG/CAG repeats in genes to diagnose disorders like myotonic dystrophy, causing muscle weakness or heart issues, aiding in genetic diagnosis. Affecting 1 in 8,000 people, myotonic dystrophy poses diagnostic challenges due to progressive symptoms. Following 2023 American College of Medical Genetics (ACMG) guidelines, it uses PCR/fragment analysis for high accuracy, supporting molecular pathology screening. This test is vital for diagnosis, genetic counseling, and improving outcomes in neurology.
Other Names: Trinucleotide Repeat Test, Myotonic Dystrophy Gene Assay.
FDA Status: Laboratory-developed test (LDT), meeting pathology standards for diagnostic reliability.
Historical Milestone: Repeat testing began in the 1990s with research by Harper, who linked CTG repeats to myotonic dystrophy. PCR/fragment analysis advancements in the 2000s by Applied Biosystems improved detection, surpassing earlier Southern blot methods.
Purpose: Counts CTG/CAG repeats to diagnose myotonic dystrophy, guides supportive therapy, and evaluates patients with muscle weakness, aiming to manage symptoms.
Test Parameters: Number of CTG/CAG repeats
Pretest Condition: No special preparation required. Collect whole blood, buccal swab, or saliva. Report history of muscle issues.
Specimen: Whole Blood (EDTA, 3-5 mL), Buccal Swab (sterile swab, 1-2 swabs), Saliva (sterile container, 1-2 mL); 4 mL whole blood in EDTA tube. Transport in a biohazard container.
Sample Stability at Room Temperature: 24 hours
Sample Stability at Refrigeration: 1 week
Sample Stability at Frozen: 1 month
Medical History: Document muscle weakness or heart issues. Include current medications or family history of genetic disorders.
Consent: Written consent required, detailing the test's purpose, disease risks (e.g., cardiac arrest), and sample collection risks.
Procedural Considerations: Uses PCR/fragment analysis to count repeats, requiring labs with thermal cyclers. Results available in 5-7 days. Performed in labs with strict handling.
Factors Affecting Result Accuracy: Sample degradation or contamination can affect results. Medications may not affect results but require correlation.
Clinical Significance: High repeat count confirms myotonic dystrophy, guiding therapy. Early diagnosis might improve management, while untreated cases lead to worsening. Normal counts may require EMG.
Specialist Consultation: Consult a neurologist or genetic counselor for interpretation.
Additional Supporting Tests: Muscle biopsy, ECG, or genetic panel to confirm diagnosis.
Test Limitations: Specific to tested genes; correlation with symptoms needed. False negatives possible with mosaicism.
References: ACMG Guidelines, 2023; Neurology, Harper PS, 2022.
