MedlinePlus Medical Encyclopedia: T (thymus derived) lymphocyte count

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T (thymus derived) lymphocyte count

Contents of this page:
Illustrations Alternative names Definition How the test is performed How to prepare for the test How the test will feel	Why the test is performed Normal Values What abnormal results mean What the risks are Special considerations

Illustrations

Blood test

Alternative names Return to top

T cell counts; CD4 count; CD3 count; T-lymphocyte count

Definition Return to top

This is a blood test that measures the number of T-lymphocytes (a type of white blood cell).

How the test is performed Return to top

Blood is drawn from a vein on 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 apply pressure and restrict blood flow through the vein. This causes veins below the band to 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 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.

For an 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 the 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 Return to top

No special preparation is necessary.

For infants and children:
The preparation you can provide for this test depends on your child's age and experience. For specific information regarding how you can prepare your child, see the following topics:

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 Return to top

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 Return to top

A T-cell count of may be helpful in the diagnosis of immunodeficiency diseases and lymphocytic diseases. It is also used to monitor the response to therapy for these types 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 Return to top

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

There are several ways to count T cells and the normal range may vary with the method used.

What abnormal results mean Return to top

Greater-than-normal T-cell levels may indicate:

Less-than-normal T-cell levels may indicate:

Congenital T-cell deficiency (rare)
Congenital immunodeficiency disorders
Acquired immunodeficiency disorders
Leukemias
Cancer
Hodgkin's
Acute viral infection
Waldenstrom's macroglobulinemia
AIDS
Side effect of radiation therapy
Effect of aging

What the risks are Return to top

Risks associated with having blood drawn 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 blood is drawn from a person with a normal immune system.

Special considerations Return to top

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.

Update Date: 2/9/2004

Updated by: Frank A. Greco, M.D., Ph.D., Director, Biophysical Laboratory, The Lahey Clinic, Burlington, MA. Review provided by VeriMed Healthcare Network.

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