The Test Sample
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 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.
The main use of D-Dimer is in excluding venous blood clots as a cause for symptoms in the legs or lungs – it has something called 'negative predictive value'. If your D-dimer result is negative it is extremely unlikely you have a significant venous blood clot. D-dimer has no 'positive predictive value' – it is raised in many conditions and does not help to narrow down a differential diagnosis if positive.
How is the sample collected for testing?
Typically, a blood sample is taken from a vein in your arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?