Blood collection and storage techniques

Tourniquets are often used to collect blood samples as they block venous return and cause dilatation enabling identification of entry points. However, due to this phenomenon typically, for each minute of use due to loss of water and electrolytes from plasma, plasma protein increases by up to 1%. The stasis of blood flow can produce different metabolic products such as lactate, and, if the patient is asked to clench their fist this may cause an artifactual hyperkalemia (elevated potassium levels). Clearly these disadvantages do not justify the non-use of tourniquets but may be worth considering if results appear unlikely based on symptoms or unreliable.