D-dimer

When a vein or artery is injured and begins to leak blood, a sequence of clotting steps and factors (called the coagulation cascade) is activated by the body to limit bleeding and create a blood clot to plug the hole.  During this process, threads of a protein called fibrin are produced.  These threads are cross-linked (glued together by a protein called thrombin) to form a fibrin net that catches platelets and helps hold the forming blood clot together at the site of the injury.

Once the area has had time to heal, the body uses a protein called plasmin to break the clot (thrombus) into small pieces so that it can be removed.  The fragments of the disintegrating clot are called fibrin degradation products (FDP).  One of the FDPs produced is D-dimer, which consists of variously sized pieces of cross-linked fibrin.  D-dimer is normally undetectable in the blood and is produced only after a clot has formed and is in the process of being broken down.

Typically, a blood sample is taken from a vein in your arm. As an alternative, particularly in paediatric care, the blood sample is taken from the fingertip.