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Fragile X

Item Value
Approval req'd? No
Available Stat? No
Utilization Guidelines Used in the assessment of patients with mental retardation, adult onset tremor/ataxia in men and/or premature ovarian failure in women.

Tests with long turn-around times (ie. Molecular based tests and Microarrays) should only be requested on an inpatient if the result is going to affect the inpatient management.

If the patient will likely be discharged before the result will be available, the test should be requested after discharge. (NOTE: UCSF Medical Center is not reimbursed for inpatient testing).

An exception to the above may be appropriate if there is a possibility the patient will not survive to be discharged and the information is important for diagnosis and/or family decisions/management (ie. recurrence risk).
Test code FRX
Performed by Medical Genomics - Molecular Diagnostics
In House Availability Run 2x per week, Wednesday & Friday, day shift only
Method Southern blot for methylation analysis
PCR and capillary electrophoresis for number of CGG repeats.
Collection Instructions Do not collect sample in heparin. Keep sample refrigerated for overnight or longer storage.

Container type Lavender top preferred, Blue (citrate) and Yellow (ACD) tops acceptable
Amount to Collect 3 mL blood
Sample type EDTA whole blood, Amniocyte culture
Preferred volume 3 mL blood
Min. Volume 2 mL blood
UCSF Rejection Criteria Inadequate sample. Samples collected in heparin.
Processing notes Do not centrifuge the specimen. Store at room temperature. Refrigerated samples are acceptable.
Normal range Results are reported as "No Mutation Detected", "Pre-mutation Present" or "Full Mutation Present".

Number of CGG repeats is also reported.

Result CGG Repeats
Normal < 45
Intermediate 45-54
Premutation 55-200
Full mutation >200
Synonyms Mental retardation; ataxia; ovarian failure; FRAXA; FMR-1
Turn around times 10-14 days
Additional information An interpretation of this test by a laboratory physician will automatically be performed and billed for separately.

Presence of a full mutation with complete methylation is associated with mental retardation. Methylation mosaicism and/or premutation along with a full mutation has been shown to reduce the severity of mental retardation.

A premutation in a female is not associated with mental retardation. The presence of a full mutation in a female carries approximately 30% risk of mild mental retardation.

The chance that a premutation could expand to a full mutation when passed from a female carrier to her offspring is shown in the following table:
Premutation CGG repeats Risk of Expansion to Full Muatation
55-59 4%
60-69 5%
70-79 31%
80-89 58%
90-99 80%
>99 approx 100%

Adult onset disease:
Fragile X-associated tremor / ataxia syndrome (FXTAS). An FMR-1 premutation may confer an adult risk for tremor and ataxia. Not all men with an FMR-1 premutation will develop FXTAS. The risk of developing FXTAS is age dependent and ranges from 17% (age 50- 59), 38% (age 60-69), 45% (age 70- 79) and 75%(age ≥ 80). However, it is estimated that only 20-30% of men with an FMR-1 premutation will develop the syndrome.

FMR-1 related premature ovarian failure (POF): Women with an FMR-1 premutation have an approximately 21% risk of developing POF as opposed to 1% in the general population.

If a mutation is detected it is recommended that the patient seek genetic counseling.

This test was developed and its performance characteristics determined by the Clinical Laboratories at the Medical Center at UC San Francisco. It has not been cleared or approved by the U.S. Food and Drug Administration.
CPT coding 81243, 81244
LOINC code 36913-2
LDT or Mod FDA? Yes
Last Updated 10/30/2017 7:41:34 PM
Entry Number 393
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