A tourniquet restricts the blood flow resulting in 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; this helps to avoid the pre-analytical variability associated with hemoconcentration. Hemoconcentration occurs 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
