Merck Manual

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How Blood Is Obtained

How Blood Is Obtained

Blood is obtained from a vein with a needle to fill one or more sample tubes or sometimes from the fingertip by a needle prick to get just a drop of blood.

A health care practitioner determines which vein to use, usually one on the inside surface of the person's elbow. A tourniquet is applied around the upper arm, causing the veins below it to fill with blood so that they may be more easily seen or felt.

After the skin immediately surrounding the vein is cleaned thoroughly, a needle is inserted into the vein. A stinging sensation is usually felt when the needle is first inserted, but otherwise the procedure is painless.

Blood moves through the needle and into a syringe or collecting tube. Once enough blood is collected, the tourniquet is removed, the needle is then removed from the vein, and pressure is applied to the area to prevent bleeding from the puncture site.

If only a small amount of blood is needed, the area, usually a finger (the heel in infants), is cleaned and a needle is used to prick the skin.