What is the main purpose of using a tourniquet during a blood draw?

The main purpose of using a tourniquet during a blood draw is to engorge the veins for easier access. When the tourniquet is applied to the upper arm, it temporarily restricts venous blood flow while allowing arterial blood flow to continue. This causes the veins below the tourniquet to become fuller and more prominent, making them easier to locate and puncture with a needle. The increased visibility and palpability of the veins greatly aid the phlebotomist in successfully obtaining a sample with minimal discomfort to the patient.

Using a tourniquet effectively enhances the process of venipuncture, which is crucial for obtaining a good quality blood sample for laboratory testing.