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Phenylketonuria (PKU)


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Definitions of terms


Definitions of Terms

Enzyme Assay, ELISA

A rapid test where an antibody or antigen is linked to an enzyme as a means of detecting a match between the antibody and antigen.

This versatile test is widely used in the medical laboratory. It allows your health care provider to:

  • test your blood with an antigen (e.g., virus or bacteria) to see if your immune system recognizes it as something it has seen before, or
  • test your blood with an antibody to see if a particular substance like a hormone (an antigen) is present in your system.

Alternative Names:
Enzyme-linked immunoassay; EIA
How the test is performed:
Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to cause the vein to swell with blood.
A needle is inserted into the vein, and the blood is collected in air-tight vials or a syringe. During the procedure, the band 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.

The sample is then taken to the laboratory where it is tested for the target antibody or antigen. If the target substance is present in the sample, the test solution has an intense color reaction generated by the attached enzyme indicating a positive test result.

How to prepare for the test:
If your child is to have this test performed, it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen, and why, the less anxiety he or she will feel.

Infants and children:
The preparation you can provide for this test depends on your child's age, previous experience, and level of trust. For general information regarding how you can prepare your child, see the following:

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 is a preliminary screening tool used to detect either the presence of antigen or antibody in your blood. It is frequently used to screen for present or past infections.
Normal Values:
Normal values depend on the type of substance your health care provider is screening for. In some people, a positive result may be normal.
What abnormal results mean:
Abnormal values depend on the type of substance your health care provider is screening for. In some people, a positive result may be normal.
What the risks are:
Risks associated with venipuncture are slight:

  • excessive bleeding
  • fainting or feeling light-headed
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures 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.


Review Date: 11/3/2003
Reviewed By: Sherry Woodhouse, M.D., Department of Pathology and Laboratory Medicine, Cleveland Clinic Florida, Weston, FL. Review provided by VeriMed Healthcare Network.
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