Fragile X syndrome chromosome analysis


This test invloves a specific analysis of white blood cells, looking for a certain deformity in the X chromosome, which carries genetic information. The white blood cells, which are obtained from a blood sample, must be grown in a special folate-deficient medium in order to visualize the “fragile site.”

This test is rarely performed any more, having been largely replaced by a more modern, convenient, and reliable test called Molecular Fragile X testing.

In order for accurate diagnosis and proper counseling to be given to a patient and/or family, the molecular testing and correlation of clinical findings must be performed by a professional experienced in the complicated molecular genetics involved with this syndrome.

How the test is performed 

Adult or child:
After appropriate consent is obtained, blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein.

This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe.

During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or less commonly, a lancet. The blood may be collected in a pipette (small glass tube), or into a small container. Cotton or a bandage and pressure may be applied to the puncture site, if there is any continued bleeding.

The sample is shipped promptly to a cytogenetics laboratory for evaluation.

How to prepare for the test 
No special preparation is necessary. Genetic counseling is often given prior to the test.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:

  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed 

This test is performed to detect fragile X syndrome or identify carriers of the disorder.

Normal Values  
In the laboratory, a number of cells are evaluated, and a percentage is established comparing the number of abnormal cells to the number of normal cells. A normal result would indicate that less than 4% of the cells has a constricted section on the long arm of the X chromosome.

What abnormal results mean  

If more than 4% of the cells evaluated test positive for fragile X chromosomes, fragile X syndrome is indicated. Again, this test is virtually obsolete in practice since molecular analysis has identified the presence of “pre-mutations.”

It is usual to repeat a fragile X study performed earlier by chromosomal analysis with the molecular analysis that is now the standard of care.

What the risks are  
Risks associated with venipuncture are slight:

  • excessive bleeding that typically resolves with adequate pressure and a dressing  
  • fainting or feeling light-headed  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures may be necessary to locate veins

Special considerations 
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Johns Hopkins patient information

Last revised: December 7, 2012
by Mamikon Bozoyan, M.D.

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