When drawing blood, is it necessary to keep the tourniquet on once you've cannulated the vein?

Question

I've always been taught to apply the tourniquet so the vein is easier to see. Once I've entered the vein with a needle/cannula/butterfly, why do most people draw the blood with the tourniquet still on? Wouldn't it be easier to take it off once the needle is in the vein and draw blood that way?

Answer 1

You leave the tourniquet on for the same reason you put it on in the first place. To restrict the flow back to the heart. You want the blood to fill the vein you've cannulated to make it easier to fill your syringe. Otherwise, you may just suck the vein closed and not get any blood.

When starting an I.v. for fluid therapy, however, you immediately remove the tourniquet so the fluid can enter the blood stream.

Answer 2

Maybe to keep the blood in that generalized area, instead of rushing back to the extremities and hence losing the flow.

Answer 3

For drawing blood it usually makes little difference. As a pediatrician I was drawing blood on squirmy infants and even squirmier toddlers and small children. It took both hands to stabilize the extremity and draw the blood without the needle slipping out of the vein. I needed a third hand to loosen the tourniquet. Thus it remained on.

Answer 4

No.

Answer 5

Once you get a good flow established, it isn't necessary to keep the tourniquet on.  As long as the vein holds, it will continue to give blood.