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Ency. home > Test > H > Hematocrit

Hematocrit   

Overview | Risks | Results

Alternative names:

HCT

Definition:

The hematocrit is the percent of whole blood that is comprised of red blood cells. The hematocrit is a compound measure of RBC number and size.

The HCT is almost always ordered as part of the complete blood count (CBC). The CBC measures the number of red blood cells (RBCs), white blood cells (WBCs), the total amount of hemoglobin in the blood, and the fraction of the blood composed of cells (hematocrit).

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, some of the blood is centrifuged (spun in a machine). This forces the cells to the bottom of the container. The cellular portion is compared to the total amount of blood and expressed as a percent. The cellular portion is almost entirely red blood cells; the percent that is WBCs is very small.

How to Prepare for the Test

Adults:
No special preparation is necessary for this 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:

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

The hematocrit indicates the proportion of cells and fluids in the blood.

Ency. home > Test > H > Hematocrit

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