Venepuncture

Blood samples are best taken from an antecubital vein using a 21G needle and a Vacutainer™ system or syringe. If a large volume of blood is required a 21G butterfly may be inserted to facilitate changing the vacutainer sample bottle or the syringe.

A tourniquet should be gently applied to the upper arm and the antecubital fossa inspected and palpated for veins. In an obese individual antecubital veins may be more easily palpated than seen. The skin over the vein should be 'sterilised' (alcohol swab or Mediswab™) and allowed to dry. The needle should then be gently introduced along the line of the chosen vein at an angle of 45° to the skin surface. It may be helpful to attempt to penetrate the skin with the initial introduction of the needle and then slowly penetrate the vessel wall by continuing the forward movement of the needle. The tourniquet on the upper arm should be loosened once the needle has been inserted into the vein to reduce haemoconcentration. If a syringe is used the piston should be withdrawn slowly to prevent collapse of the vein. Once an adequate sample has been obtained, the tourniquet should be completely removed, a dry cotton wool ball applied gently above the site of venepuncture and gentle pressure increased as the needle is removed. Firm pressure should be directly applied to the venepuncture site for 3-5 minutes to ensure haemostasis and prevent extravasation and bruising. A small elastoplast or if allergic, suitable light dressing, should be applied to the venepuncture site.

In patients in whom it is difficult to obtain a sample, the arm should be kept warm, a sphygmomanometer cuff inflated on the upper arm to the diastolic pressure and the vein may be dilated by smacking the overlying skin. With patience it is rarely impossible to obtain a venous sample. In very obese individuals or those in whom iatrogenic thrombosis or sclerosis has occurred in the antecubital veins, the dorsal veins of the hand may be used for sampling, though a smaller gauge needle (23G) or butterfly is often necessary.

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