Testing for Fragile X

Fragile X-associated disorders, including Fragile X syndrome, are diagnosed by DNA testing.

The test that is required is called DNA studies for fragile X syndrome.

This is generally a blood test, although some laboratories offer testing via a cheek swab or saliva sample.

A general practitioner, paediatrician or any medical doctor can order the test for a patient.

The blood sample can be collected by the doctor or a pathology centre. Test results are usually available approximately 4 to 8 weeks after the sample is taken, but the time taken can vary.

The cost of the genetic test for Fragile X alone is approx. $150-$200. Where there is a family history of Fragile X-associated disorders or where a Fragile X-associated disorder is suspected testing may be covered under Medicare. Where a genetic test is screening for Fragile X together with other genetic conditions, the cost will vary. For example, a 3-in-1 test for Fragile X, cystic fibrosis and spinal muscular atrophy, may cost around $400. For tests for a greater number of genetic conditions the cost will be greater.

The test results will state a “repeat number” which indicates the number of copies of the CGG repeats the person tested has in their FMR1 gene. Women may have two repeat numbers on their test results because women have two X chromosomes.

Where the number of repeats is 44 or below, the result is considered normal or negative. When the repeat number is 45-54, it is known as intermediate and the person is not at risk for Fragile X premutation symptoms.

Where the repeat number is in the range 55-200, the result is called premutation and is seen in Fragile X carriers. A result of above 200 CGG repeats is known as the full mutation and the person tested generally has Fragile X syndrome.

Image credit: Fragile X syndrome parts 1 & 2, J Cohen, A Archibald, S Metcalfe, Medical Observer, June 2012.

It is recommended that test results showing the Fragile X premutation or Fragile X full mutation be discussed with a genetic counsellor.

Advances in testing for Fragile X have become more accurate over the years. Tests for Fragile X carried out before 1991 may not be correct and the results of ‘cytogenetic’ tests since then may be inaccurate. If there is concern about the accuracy of an older test result for Fragile X, it is recommended that a DNA test for Fragile X be done.

To learn more:

The Fragile X syndrome 

Professor Graeme Suthers, Sonic Genetics, 2017.


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