Blood for analysis may be obtained from veins, arteries, or capillaries. Venous blood is usually the specimen of choice, and venipuncture is the method for obtaining this specimen.
In young children skin puncture is frequently used to obtain what is predominantly capillary blood. Arterial puncture is used mainly for blood gas analysis.
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.