Venipuncture
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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, other medical staff and chiropractors 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.
Phlebotomy (incision into a vein) is also the treatment of certain diseases such as hemochromatosis and primary and secondary polycythemia.
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[edit] Equipment
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.
[edit] Venipuncture with evacuated or vacuum tubes
Vacuum tubes were first marketed by U.S. company Becton Dickinson under the trade name Vacutainer tubes. Today, many companies sell vacuum tubes as the patent for this device is in the public domain. Some models are a type of test tube that contains a vacuum that automatically aspirates blood into itself. The tubes are made of glass or plastic. The tubes are attached to a needle and hub. Multiple vacuum tubes can be attached to and removed in turn from a single needle, allowing multiple samples to be obtained from a single procedure. This is possible due to a Japanese invention called the multiple sample sleeve, which is basically a plastic cap fitting over the posterior end of the needle cannula, thus keeping blood from draining onto both health care worker and patient.
Becton Dickinson used this invention to create a sort of system that included the vacuum pressurized sleeve, and a cannula with sleeve attached to a holder that holds the needle/sleeve combination, and guides the negatively pressured collection vial that is inserted once the needle is in the patient.
Unfortunately, the sleeve that stops blood from moving until its seal is perforated by the collection tube's insertion, also prevents flashback, the tell-tale sign the vein has been entered properly.
They are commonly used in US, UK, and Australian hospitals, private doctors' offices and community labs, and are available in various sizes to suit the age of the patient and the type of sample needed.
Tubes have colour-coded stoppers to indicate what type of anticoagulant or other substance is in the tube. Some tubes have a gel substance which, when centrifuged, separates the cells from the serum. This is to keep the cells from leaching nutrients from the serum that is about to be tested. Tubes may also contain no additives or have a variety of anticoagulants or preservatives (e.g. sodium heparin, sodium citrate, sodium fluoride and potassium oxalate or potassium EDTA).
[edit] 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. This is particularly helpful with patients that have small veins that collapse under the vacuum of BD vacutainers. 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.
[edit] Blood cultures
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 the use of iodine that is allowed to dry, seeing that isopropyl alcohol only cleans and does not sterilize. This is disputed in some labs, where iodine is thought to degrade the rubber stopper through which blood enters the bottle, thus allowing contaminates to enter the container.
[edit] 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.
