Failure to obtain blood by venipuncture may occur due to all of the following except:

The correct choice highlights that tying the tourniquet too tightly is not typically a reason for failure to obtain blood by venipuncture. In fact, applying a tourniquet is a standard practice used to engorge the veins and make them more prominent. If a tourniquet is tied too tightly, it could potentially impede venous flow, but it usually does not prevent blood from being drawn; rather, it may complicate the process.

In contrast, incorrect needle positioning can result in the needle not being in the vein, leading to unsuccessful venipuncture. Similarly, inadequate vacuum in the collection tube can prevent the necessary suction needed to draw blood into the tube, and a collapsed vein can block blood flow altogether, making venipuncture difficult or impossible. Each of these factors directly interferes with the ability to successfully collect blood, unlike a tightly tied tourniquet, which generally still allows for blood collection as long as the vein is properly accessed.