What is being tested?
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 the bleeding and create a blood clot to plug the hole.  During this process, threads of a protein called fibrin are produced.  These threads are crosslinked (glued together) 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 fibrin in the clot are called fibrin degradation products (FDP).  One of the FDPs produced is D-dimer, which consists of variously sized pieces of crosslinked 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. Measurement of D-dimer can tell your doctor that something has increased above normal the body's clotting mechanisms.

How is the sample collected for testing?
Typically, a blood sample is drawn from a vein in your arm. As an alternative, particularly in pediatric care, the blood sample is drawn from the fingertip.