BLOOD OBTAINING METHOD, COLLECTING A BLOOD SAMPLE, Venipuncture

COLLECTION OF BLOOD:- Inside the body (in vivo), blood is in a liquid form. But outside the body in vitro), it clots within a few minutes. If left undisturbed in a tube, this clot begins to retract or shrink, and a pale yellow fluid, called serum, separates from the clot, appearing in the upper part of the tube. During this process of coagulation or clotting, certain factors or constituents in the blood are used up and most of the cells (white, red and platelets) are trapped in the clot.

Such a blood sample is totally unsuitable for haematological investigations. Therefore, it is necessary to prevent coagulation of blood by using anticoagulants. If the anticoagulated, well mixed blood is centrifuged, it separates into 3 main layers . The bottom layer consists of packed red cells which normally makes up for about 40-47% of the total blood volume. A thin whitish layer appears on top of this layer and normally makes up about 1 % of the volume. This layer is called 'buffy coat' and contains leucocytes and platelets.

The uppermost liquid layer makes up about 52-57 % of the total volume. This pale yellow fluid is the plasma. In a normal, healthy individual, these three constituents are in a state of equilibrium in relation to the demand and supply of the body functions. This state of equilibrium is known as homeostasis. Blood samples for haematological study are usually obtained either by finger puncture (capillary blood) or venipuncture (venous blood).

of cells and for estimation of haemoglobin. The capillary blood can be obtained from the tip of the finger from adults, and from the heel or the large toe from infants. However, the use of capillary blood should be avoided as far as possible because of the high risk of sampling error and of infection. Repeat testing is usually restricted because of the smallness of the quantity of blood collected. Capillary blood should be used only when the venous blood is not advisable, for example, in new-born infants, burn cases, amputees or in patients whose veins prove to be difficult to locate.

Technique:-  Select an appropriate site for puncture. The ball of the middle finger is usually satisfactory. Clean the area vigorously with 75 % alcohol and allow it to dry. This disinfects the skin and promotes circulation. For skin puncture, various types of disposable lancets are available. Use of non-disposable lancets is not recommended because of the risk of cross-infections. Make a firm, quick stab with the lancet simultaneously applying a little pressure. This ensures a free flow of blood. Wipe away the first one or two drops of blood using a dry cotton swab. Carefully draw blood into an appropriate pipette (e.g. haemoglobin, RBC or WBC pipette) by applying gentle suction through the mouth-end of the rubber tubing attached to the pipette. Draw blood exactly up to the mark, avoid air bubbles. Wipe the outer surface of the pipette and deliver the blood into an appropriate diluent by blowing slowly. Mix to prevent coagulation.

Collection of Venous Blood:-The most commonly used sites for venipuncture are the veins inside the bend of the elbow (the antecubital fossa). The three main veins in this area are the cephalic, median cubital and median basilica veins. Other sites, such as the veins in the wrist or ankle may be used if necessary. Technique Apply a tourniquet to the upper arm sufficiently tightly to restrict the venous flow and make the veins stand out. The patient should be asked to keep the arm straight and clench the fist.

Usually the veins are obvious by this time. It is advisable to feel the veins so that the most suitable one can be selected. A little tapping or gently massaging the arm from the wrist to the elbow helps in dilation of the veins. Swab the selected vein and site with 75 % alcohol and allow it to dry.

vacuum tube system for blood collection

Prepare the syringe and appropriate containers. Usually a 21 gauge needle is appropriate for very fine veins, 22 or 23 gauge needles may be used only if necessary. Using the left thumb, press just below the puncture site to anchor the vein. Insert the needle smoothly with the bevel facing upwards, at an angle of 20° to 30° to surface of the arm, and in a direct line with the vein. When the needle has entered the vein, blood is withdrawn into the syringe and tourniquet released. When a sufficient quantity of blood is collected, loosen the touniquet, place a wad of cotton wool at the puncture site and withdraw the needle gently. The puncture sight should be kept pressed to stop the flow of blood. Ask the patient to release the clenched fist. Detach the needle and discard in an appropriate disposal container. Dispense the blood in the sample tubes as required. Mix the blood to be anticoagulated. Apply a strip dressing at the puncture site.

Note

1. Infection may be transmitted from patient to staff during the bloodcollection procedure. Viral agents are the greatest hazard and sometimes potentially lethal. All skin lesions on the hands of the collecting staff must be covered with waterproof dressing and gloves should be worn. All swabs, used sy ringes and needles should be secured in suitable containers and disposed off by incineration. Working surfaces should be disinfected frequently as they may get contaminated by droplets.

Comparison of blood collection procedure

2. It is advisable to use disposable syringes and needles to avoid the risk of cross infection due to improper sterilisation. In recent years, instead of traditional needle and syringe, some laboratories use vacuum tube system (Fig. 1.2). This uses a double ended needle in a plastic holder. One end of the needle is inserted into the pa tient's vein and the other is pushed into the rubber stopper of a vacuum tube. The blood will then flow into the tube. Such vacuum tube systems are commercially available. 

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