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In medicine venipuncture or venepuncture (also known as phlebotomy, venesection, blood draw, drawing blood or taking blood) is the process of obtaining a sample of venous blood. Usually a 5 ml to 25 ml sample of blood is adequate depending on what blood tests have been requested. In many circumstances it will be done by a phlebotomist, although nurses, doctors, some EMTs, paramedics and other medical staff are also trained to take blood.
Blood is most commonly obtained from the median cubital vein, on the anterior forearm (the side within the fold of the elbow). This vein lies close to the surface of the skin, and there is not a large nerve supply.
Minute quantities of blood may be taken by fingersticks sampling and collected from infants by means of a heel stick or from scalp veins with a butterfly needle.
Additional recommended knowledge
There are many ways which blood can be drawn from a vein. The best method varies with the age of the patient, equipment available and tests required.
Most blood collection in the US and UK is done with an evacuated tube system, such as the BD Vacutainer system or similar blood collection equipment consisting of a plastic hub, a hypodermic needle, and a vacuum tube. Under certain circumstances, a syringe may be used, usually with a butterfly needle, which is a plastic catheter attached to a short needle. In the developing world, a needle and syringe are still the most common method of drawing blood.
Blood is usually drawn from a vein in the crook of the elbow, the antecubital region. Tubes have color-coded stoppers to indicate what type of anticoagulant or other substance is in the tube. Tubes may contain no additive, a gel substance which separates cells from serum, or a variety of anticoagulants or preservatives (e.g. sodium heparin, sodium citrate, sodium fluoride and potassium oxalate or potassium EDTA).
Venipuncture with evacuated or vacuum tubes
Vacuum tubes, or BD Vacutainer tubes are a type of test tube that contain a vacuum that automatically aspirates blood into itself. They are made of glass or plastic. They are commonly used in US and UK hospitals, private doctors' offices and community labs, and are effective in most adult patients and older children. For infants or toddlers, there are smaller size BD Vacutainer tubes available.
Equipment needed includes: a plastic needle holder or "barrel", needles designed specifically for the needle holder, an adjustable tourniquet, appropriate tubes, alcohol swabs, 2x2 gauze pads (cotton balls used to be used, but they have been found to dislodge the clot and the patient bleeds again causing larger hematomas), sticky tape, and a pen for labeling.
Below is a sample of the procedure used when blood is collected with BD Vacutainer tubes.
To prevent cross-contamination of samples with additives from other tubes, it is necessary to insert and withdraw BD Vacutainer tubes in a set order, referred to by phlebotomists as the "Order of Draw." When drawing blood for very sensitive testing, it is sometimes advisable to first draw a red top (no additive) tube, which is then discarded, to eliminate from the sample the interstitial fluid that comes from the puncture site rather than from the bloodstream. Subsequent tube order is determined by the need to prevent a tube's contamination by additives used in a previous tube that might affect laboratory analysis.
Venipuncture with needle and syringe
A syringe is used to manually extract blood from a patient. The very young, very old and anyone with problematic veins are all candidates for this old-fashioned method. Because syringes are manually operated, the amount of suction applied may be easily controlled. The procedure is similar to what is described in the section above. After the needle is inserted into the vein, the phlebotomist receives confirmation of success when a small amount of blood appears in the back of the needle. This is often referred to as 'flash'. The plunger is then pulled backwards and blood fills the syringe. If the syringe plunger is pulled back too quickly red blood cells may be broken (hemolyzed) by turbulence or physical forces as they are forced through the needle. The blood is usually transferred quickly to a BD Vacutainer tube before clotting sets in using a similar defined tube order; normally the opposite of the order of draw when using a BD Vacutainer tube.
Most often, blood cultures are drawn using a "butterfly" needle. When drawing blood cultures, the aerobic culture should be drawn first. Aerobic refers to bacteria that grow in the air and anaerobic refers to bacteria that do not grow in the air. Since there is air in the "butterfly" needle tubing anaerobic specimens cannot be drawn without first removing the air from the tubing. The tops of any containers used when drawing a blood culture should be sterilized with isopropyl alcohol.
Blood alcohol tests
It is generally not advisable to use isopropyl alcohol to cleanse the venipuncture site when obtaining a specimen for a blood alcohol (BAC) test to test blood alcohol levels. Using soap and hot water is an acceptable alternative to isopropyl alcohol when using venipuncture to collect blood sample for BAC tests.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Venipuncture". A list of authors is available in Wikipedia.|