Venipuncture:
patient should be seated or supine for at least 20 minutes before sampling. An arm with an inserted intravenous line should be avoided. The median cubital vein in the anticubital fossa is the preferred site for collection of blood in adults. After selection of the vein, the area should be cleaned with a prepackaged alcohol swab or with a gauze pad saturated with 70% isopropanol. The skin should be allowed to dry in the air. No alcohol should remain on the skin, because traces may cause hemolysis. After cleaning, a tourniquet is applied above the intended site to apply pressure upon the vein.
Tourniquet should not be left in position for more than one minute (marked changes of hemoconcentration is observed after 3 min.), and the patient should not allowed to pump his or her fist while the tourniquet is in place (it cause an increase in plasma potassium, phosphate, and lactate concentration).
Stress associated with blood collection can have effects in patients at any age. Plasma concentration of cortisol and growth hormone may increase.
Blood is then could be collected using either an evacuated collection tube or by syringe. During collection of blood you should avoid vigorous suction in the syringe and avoid forceful transfer to the receiving vessel as this may cause hemolysis of blood. Hemolysis will interfere with chemical procedures which are based on colorimetry. Erythrocytes have higher concentration of potassium, thus any hemolysis will lead to erroneous higher potassium in plasma or serum.
Hemolysis may be avoided by making sure that the container of blood (blank tube) is not wet because presence of water (which is hypotonic solution) will cause hemolysis. If we want to collect SERUM, then blood should be transferred to a clean, dray tube slowly and allowed to clot for 15-30 minutes at room temperature for completion of coagulation, the clot is dislodged from the wall by mean of a glass rod then the tube is centrifuged for 5 minutes to provide a clear supernatant serum for further use in analysis.
Serum is usually the standard sample for performing most of the biochemical blood tests like sugar, urea, creatinine, uric acid, protein, albumin, electrolytes, cholesterol, most enzymes, calcium, phosphate, etc. Serum is kept in the dark if the sample will be used for bilirubin estimation If we want to collect PLASMA or whole blood, then blood should be transported from the syringe to a tube with an anticoagulant and make sure to mix the blood with the anticoagulant by rotating the container between hands and to avoid shaking and forth production. Whole blood is rarely required for clinical tests; indeed only for blood gas, ammonia, and some trace elements.