Venipuncture
From Wikipedia, the free encyclopedia
In medicine venipuncture or venepuncture (also known as phlebotomy, 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 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 opposite 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 (removal of blood) is also the treatment of certain diseases such as hemochromatosis and primary and secondary polycythemia.
Contents |
[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 is done with a Vacutainer 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, which is a plastic catheter attached to a short needle. 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).
[edit] Venipuncture with vacuum tubes (vacutainers)
Vacutainers are a type of test tube that contain a vacuum that automatically aspirates blood into itself. 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.
Equipment needed includes: a plastic needle holder or "barrel", hypodermic needles designed specifically for the needle holder, an adjustable tourniquet, appropriate tubes, alcohol swabs, cotton wool, sticky tape, and a pen for labelling.
Below is a sample of the procedure used when blood is collected with Vacutainers.
- First of all, the patient should be identified -- a test result is useless if it's done on the wrong person. The test subject should be asked their name and date of birth. This should not be a yes or no question, e.g. Are you Stanley Jones?, as many people will just agree, especially if they are elderly and have hearing difficulties or senility.
- The vacutainer tubes needed for the tests required should be determined, and nearby ready to be used. The order that the tubes are used is often important, and should be known before puncturing the skin. Other equipment that will or may be needed should also be handy.
- A tourniquet should be placed on the arm where blood is to be collected. This should be just tight enough that blood collects in the veins.
- The vein to be used should be palpated; its depth, size and direction should be determined, and the best angle to puncture the skin planned.
- The skin around the area should be wiped with an alcohol swab (or iodine prep if drawing for blood culture) and allowed to air dry. An appropriate needle should be placed into a holder.
- The needle should be inserted into the vein with the bevel facing upwards. It should be pushed quickly and smoothly through the skin to minimize pain and to prevent blood from spouting out through the bevel.
- Once the needle is in the vein, a vacutainer should be pushed into the needle holder. Care should be taken to ensure the needle does not move too much. The tube will fill itself up with blood.
- The tube can be removed once full, and the next one pushed into the holder.
- On the last tube, the tourniquet should be loosened or removed once blood starts to enter the tube. Make sure the tourniquet does not stay on longer than one minute.
- The last tube should be removed from the holder before the needle is pulled from the vein.
- The needle should be pulled out of the skin on the same angle it went in.
- Once the needle is removed, clean cotton wool should be pressed onto the wound, and the patient instructed to apply pressure to it. Doing this may minimize bruising.
- The tubes should be well mixed by inverting them gently a number of times, and labelled straight away. The wound under the cotton wool should be checked, then the cotton wool taped to the patient's arm.
- The patient should be instructed not to lift heavy objects or perform strenuous activities for about 24 hours, otherwise bruising is likely to result.
To prevent cross-contamination of samples with additives from other tubes, it is necessary to insert and withdraw 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.
[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. 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 'flashback'. 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 (hemolysed) by turbulence or physical forces as they are forced through the needle. The blood is usually transferred quickly to a vacutainer before clotting sets in using a similar defined tube order..
