A tourniquet restricts the blood from continuing on its path through the circulation, resulting in the veins becoming distended and increasing their visibility and palpability. Good phlebotomy technique minimizes the length of time that the tourniquet is in place, not only to minimize injury to the tissues where the tourniquet is applied, but to avoid the effects of the blood stasis.
As a rule of thumb, a tourniquet should be in place for no more than one minute, which helps to avoid the pre-analytical variability associated with hemoconcentration. Hemoconcentration is when blood pools at the venipuncture site. A number of measurands are known to be falsely increased when a tourniquet is left in place for a prolonged period of time, including:
- Hemoglobin
- Total protein
- Albumin
- Calcium
- Glucose
- Potassium
- Cholesterol
- Lactic acid
