T (Thymus Derived) Lymphocyte Count

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How is the Test 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).