A tourniquet is a simple yet powerful device used in phlebotomy, the process of drawing blood from a vein. Its purpose is to temporarily restrict venous blood flow in a limb, typically the arm, while arterial flow continues unimpeded. This restriction causes the veins below the application site to swell with blood, making them more visible and easier for the phlebotomist to feel before the needle is inserted. The proper application of this device is a foundational step in ensuring a successful venipuncture procedure.
The Critical Time Limit for Tourniquet Application
The universally accepted maximum duration for a tourniquet to remain applied during a blood draw is one minute, or 60 seconds. This time limit is dictated by clinical guidelines to maximize vein visibility without introducing pre-analytical errors into the collected blood sample. The goal is to ensure the integrity of the blood sample.
If a phlebotomist anticipates difficulty finding a vein, they may briefly apply the tourniquet to examine the site but must release it if the minute is reached without a successful puncture. Once the venipuncture is successful and blood flow is established, the tourniquet should be released immediately. If the draw takes longer than one minute, the phlebotomist must release the tourniquet, wait briefly, and then reapply it before continuing the collection.
Physiological Consequences of Extended Application
Exceeding the one-minute limit causes a rapid physiological change in the local blood composition known as hemoconcentration. This occurs because the tourniquet pressure prevents venous blood from flowing back toward the heart. However, smaller molecules, primarily water, are still forced out of the blood vessel and into the surrounding tissue space. The plasma, the liquid component of the blood, essentially pools and stagnates below the tourniquet.
This restriction increases the concentration of larger, non-filterable blood components, such as proteins and cells, within the remaining plasma volume inside the vein. Prolonged application also restricts oxygen flow to the tissues, leading to localized venous congestion and hypoxia. Extended pressure can cause physical trauma, including discomfort, bruising, and the formation of a hematoma (a localized collection of blood outside the vessel). Nerve compression leading to temporary numbness, tingling, or even permanent damage may occur if the pressure is excessive or the application time is significantly extended.
How Prolonged Tourniquet Use Affects Lab Results
The hemoconcentration caused by extended tourniquet use translates into inaccurate laboratory test results, potentially leading to misdiagnosis or inappropriate treatment. When plasma water leaves the vessel, the remaining blood sample falsely shows elevated levels of substances that cannot easily pass through the capillary wall. These are typically large molecules, including total protein, albumin, and various enzymes.
Prolonged application can artificially increase the concentration of several analytes:
- Total protein and albumin
- Lipids, such as cholesterol and total glycerol
- Liver enzymes like alkaline phosphatase and gamma-glutamyl transferase
- Calcium levels, as circulating calcium is bound to concentrated protein
In some cases, prolonged venous stasis can also cause cells to rupture, a process called hemolysis. Hemolysis releases high concentrations of intracellular substances like potassium into the plasma, resulting in a falsely high potassium reading. These analytical errors compromise the diagnostic value of the blood sample.