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HelpTest Code AFXN Friedreich Ataxia, Repeat Expansion Analysis, Varies
Additional Testing Requirements
All prenatal specimens must be accompanied by a maternal blood specimen; order MATCC / Maternal Cell Contamination, Molecular Analysis, Varies on the maternal specimen as this must be a different order number than the prenatal specimen.
Shipping Instructions
Specimen preferred to arrive within 96 hours of collection.
Specimen Required
Patient Preparation: A previous bone marrow transplant from an allogenic donor will interfere with testing. For instructions for testing patients who have received a bone marrow transplant, call 800-533-1710.
Submit only 1 of the following specimens:
Specimen Type: Whole blood
Container/Tube:
Preferred: Lavender top (EDTA) or yellow top (ACD)
Acceptable: Any anticoagulant
Specimen Volume: 3 mL
Collection Instructions:
1. Invert several times to mix blood.
2. Send whole blood specimen in original tube. Do not aliquot.
Specimen Stability Information: Ambient (preferred) 4 days/Refrigerated
Prenatal Specimens
Due to its complexity, consultation with the laboratory is required for all prenatal testing; call 800-533-1710 to speak to a genetic counselor.
Specimen Type: Amniotic fluid
Container/Tube: Amniotic fluid container
Specimen Volume: 20 mL
Specimen Stability Information: Refrigerated (preferred)/Ambient
Additional information:
1. A separate culture charge will be assessed under CULAF / Culture for Genetic Testing, Amniotic Fluid.
2. All prenatal specimens must be accompanied by a maternal blood specimen; order MATCC / Maternal Cell Contamination, Molecular Analysis, Varies on the maternal specimen.
Specimen Type: Chorionic villi
Container/Tube: 15-mL tube containing 15 mL of transport media
Specimen Volume: 20 mg
Specimen Stability Information: Refrigerated
Additional Information:
1. A separate culture charge will be assessed under CULFB / Fibroblast Culture for Biochemical or Molecular Testing. An additional 3 to 4 weeks is required to culture fibroblasts before genetic testing can occur.
2. All prenatal specimens must be accompanied by a maternal blood specimen; order MATCC / Maternal Cell Contamination, Molecular Analysis, Varies on the maternal specimen.
Acceptable:
Specimen Type: Confluent cultured cells
Container/Tube: T-25 flask
Specimen Volume: 2 Flasks
Collection Instructions: Submit confluent cultured cells from another laboratory.
Specimen Stability Information: Ambient (preferred)/Refrigerated
Additional Information: All prenatal specimens must be accompanied by a maternal blood specimen; order MATCC / Maternal Cell Contamination, Molecular Analysis, Varies on the maternal specimen.
Specimen Type: Blood spot
Supplies: Card-Blood Spot Collection (Filter Paper) (T493)
Container/Tube:
Preferred: Collection card (Whatman Protein Saver 903 Paper)
Acceptable: Perkin/Elmer 266 filter paper, or Blood Spot Collection Card
Specimen Volume: 5 Blood spots
Collection Instructions:
1. An alternative blood collection option for a patient older than 1 year is a fingerstick. For detailed instructions, see How to Collect Dried Blood Spot Samples.
2. Let blood dry on the filter paper at ambient temperature in a horizontal position for a minimum of 3 hours.
3. Do not expose specimen to heat or direct sunlight.
4. Do not stack wet specimens.
5. Keep specimen dry.
Specimen Stability Information: Ambient (preferred)/Refrigerated
Additional Information:
1. For collection instructions, see Blood Spot Collection Instructions
2. For collection instructions in Spanish, see Blood Spot Collection Card-Spanish Instructions (T777)
3. For collection instructions in Chinese, see Blood Spot Collection Card-Chinese Instructions (T800)
4. Due to lower concentration of DNA yielded from blood spots, it is possible that additional specimen may be required to complete testing.
Useful For
Molecular confirmation of clinically suspected Friedreich ataxia
Genetics Test Information
This test assesses for GAA trinucleotide repeat expansions within the FXN gene to confirm a molecular diagnosis of Friedreich ataxia.
Reflex Tests
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| CULFB | Fibroblast Culture for Genetic Test | Yes | No |
| CULAF | Amniotic Fluid Culture/Genetic Test | Yes | No |
| MATCC | Maternal Cell Contamination, B | Yes | No |
Testing Algorithm
For prenatal specimens only:
If amniotic fluid (nonconfluent cultured cells) is received, the amniotic fluid culture will be added at an additional charge.
If chorionic villus specimen (nonconfluent cultured cells) is received, the fibroblast culture will be added at an additional charge.
For any prenatal specimen that is received, maternal cell contamination studies will be added. A maternal whole blood specimen is required to perform this test.
Special Instructions
- Informed Consent for Genetic Testing
- Hereditary Peripheral Neuropathy Diagnostic Algorithm
- Molecular Genetics: Neurology Patient Information
- Blood Spot Collection Card-Spanish Instructions
- Blood Spot Collection Card-Chinese Instructions
- Informed Consent for Genetic Testing (Spanish)
- Blood Spot Collection Instructions
Method Name
Polymerase Chain Reaction (PCR)
Reporting Name
FXN, Repeat Expansion AnalysisSpecimen Type
VariesSpecimen Minimum Volume
Amniotic fluid: 10 mL
Blood: 0.5 mL
Chorionic villi: 5 mg
Blood spots: 5 punches, 3-mm diameter
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Varies | Varies | ||
Reject Due To
Specimens will be evaluated at Mayo Clinic Laboratories for test suitabilityClinical Information
Friedreich ataxia (FA) is one of the most commonly inherited ataxias and is characterized by progressive gait and limb ataxia, dysarthria, dysphagia, and sensory loss. The phenotypic spectrum includes nonneurologic manifestations, particularly cardiomyopathy and diabetes mellitus. Onset typically occurs between the ages of 10 to 16 years; however, late-onset and early-onset variants have been reported. FA is autosomal recessively inherited. The majority of affected individuals (96%) have homozygous GAA trinucleotide repeat expansions in intron 1 of FXN. The remaining affected individuals have a heterozygous GAA trinucleotide repeat expansion and another disease-causing FXN variant detectable by sequencing or deletion and duplication analysis. Correlation exists between the size of the GAA repeat and disease onset and severity, with larger alleles associated with earlier onset and more severe disease presentation. GAA expansions may demonstrate instability during meiosis and mitosis. The GAA repeat size may expand or contract during transmission to offspring and GAA repeat size may vary in different tissues. The GAA trinucleotide repeat is polymorphic in the general population, with the number of nondisease-associated repeats ranging from 5 to 33. Repeats of 66 or greater are fully penetrant disease-associated alleles; however, the majority of affected individuals have repeat sizes in the 600 to 1200 repeat range. Repeat sizes of 34 to 65 fall within a borderline range. Borderline alleles are of unclear significance and may be associated with clinical symptoms of FA and/or a risk for expansion to a full penetrance allele when transmitted to offspring.
Reference Values
FXN
Normal alleles: <34 GAA repeats
Borderline alleles: 34-65 GAA repeats
Expanded alleles: >65 GAA repeats
An interpretive report will be provided.
Interpretation
An interpretive report will be provided.
Cautions
For familial testing, it is important to first document the molecular etiology of disease in an affected family member to confirm that a repeat expansion is the underlying mechanism of disease in the family. Specifically, this assay will not detect nonrepeat expansion variants (eg, sequence variants, deletions, and duplications).
It is strongly recommended that patients undergoing genetic testing receive genetic counseling.
Test results should be interpreted in the context of clinical findings, family history, and other laboratory data, such as frataxin concentrations (see FFRWB / Friedreich Ataxia, Frataxin, Quantitative, Blood and FFRBS / Friedreich Ataxia, Frataxin, Quantitative, Blood Spot). Errors in test interpretation may occur if the provided information is inaccurate or incomplete.
Rare variants (ie, polymorphisms) may exist, such as intron 1 deletions, which could lead to false-negative results. If GAA-repeat expansion results do not match clinical findings, additional testing should be considered.
Due to somatic mosaicism, GAA repeat-sizes in peripheral blood specimens may not reflect GAA repeat-sizes in other tissues (eg, central nervous system).
Bone marrow transplants from allogenic donors will interfere with testing. Call Mayo Clinic Laboratories at 800-533-1710 for instructions for testing patients who have received a bone marrow transplant.
Method Description
A polymerase chain reaction-based assay is used to amplify across the region of FXN containing GAA repeats.(Unpublished Mayo method)
Day(s) Performed
Monday, Wednesday
Report Available
21 to 28 daysPerforming Laboratory
Mayo Clinic Laboratories in Rochester
Test Classification
This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. It has not been cleared or approved by the US Food and Drug Administration.CPT Code Information
81284
81265-Maternal Cell Contamination (if appropriate)
88233-Fibroblast Culture (if appropriate)
88235-Amniotic Fluid Culture (if appropriate)
88240-Cryopreservation (if appropriate)
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| AFXN | FXN, Repeat Expansion Analysis | 21762-0 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| 609752 | Result Summary | 50397-9 |
| 609753 | Result | 21762-0 |
| 609754 | Interpretation | 69047-9 |
| 609755 | Reason for Referral | 42349-1 |
| 609756 | Specimen | 31208-2 |
| 609757 | Source | 31208-2 |
| 609758 | Method | 85069-3 |
| 609759 | Disclaimer | 62364-5 |
| 609760 | Released By | 18771-6 |