Fragile X syndrome chromosome analysis
Key points
- This test invloves a specific analysis of white blood cells, looking for a certain deformity in the X chromosome, which carries genetic information.
- 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 area is cleansed with antiseptic and punctured with a sharp needle or less commonly, a lancet.
- Genetic counseling is often given prior to the test.
Definition
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
Adults:
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)
Written and revised by
David Cohen, M.D.
Our doctors write and review articles for accuracy and keep them up to date.
health.am provides general information, not medical advice. Always consult a qualified health professional about your own health.