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Blood test
Blood test

T (thymus derived) lymphocyte count
Definition:
A test that measures the amount of T-lymphocytes (a type of white blood cell) in the blood.

Alternative Names:
T cell counts; CD4 count; CD3 count; T-lymphocyte count

How the test is performed:

Adult or child:
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 a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

In the laboratory, the white blood cells are separated from other blood cells. A stain or other substance that "labels" the cells is added to help identify which type of white blood cells are present.

B and T lymphocytes look the same under an ordinary microscope. However, they may be differentiated with an electron microscope. B or T lymphocytes may also be counted using immunofluorescence or by flow cytometry. T cells can also be differentiated by the E rosette test (Blood is mixed with sheep red blood cells. The T cells will cling to the sheep cells and produce a distinctive rosette pattern).
How to prepare for the test:
Adults:
No special preparation is necessary.

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:
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:
A count of the T cells may be helpful in the diagnosis of immunodeficiency diseases and lymphocytic diseases. It is also used to monitor the response to therapy for these type of diseases.

Lymphocytes are white blood cells that are part of the immune system. T lymphocytes (T-cells) act directly (cellular immunity) to fight diseases or organisms, stimulate B lymphocytes (helper T-cells), or suppress some B lymphocyte functions (suppressor T-cells).
Normal Values:

Lymphocytes make up 16-45% of white blood cells. Of those lymphocytes, about half are T cells, a quarter are B cells, and another quarter are natural killer cells.

  • Helper T-cells, also known as absolute CD4+ or T4 count: more than 400 per microliter, or 32-68% of lymphocytes
  • Suppressor T-cells, also known as absolute CD8+ or T8 count: 150-1000 per microliter
What abnormal results mean:
Greater-than-normal T-cell levels may indicate: Less-than-normal T-cell levels may indicate:
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
Note: This test is often performed on people with altered immune systems. Therefore, the risk for infection may be somewhat greater than when venipuncture is performed on a person with a normal immune system.
Special considerations:
This test can be affected by:
  • Stress
  • Surgery
  • Chemotherapy medications
  • Corticosteroids
  • Immunosuppressive medications
  • X-rays
A flow cytometric measurement is more accurate than manual counts.

People with decreased T lymphocytes may not react to diagnostic skin tests.

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: 1/5/2002
Reviewed By: Camille Kotton, M.D., Infectious Diseases Division, Massachusetts General Hospital and Brigham and Women's Hospital, Boston, MA. Review provided by VeriMed Healthcare Network.
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