An anticoagulant is a solution used to prevent blood from clotting in a vial during blood tests.

Typically, it is a gel-like substance that is either sprayed inside the vial or sits at the bottom. After filling the vial, it is important to invert it several times to disperse the additive with the blood.

Avoid shaking the vial, as this could damage the sample. It's worth noting that the anticoagulant won't visibly mix with the blood since it's a gel-like substance. So there is no need to be concerned if you don't notice any changes.