The Test Sample
What is being tested?
This test detects and measures antibodies that are produced by some people when they are treated with heparin. Heparin is a common anticoagulant that is given intravenously or through injections to prevent the formation of inappropriate blood clots (thrombosis) or as an initial treatment for those who have a blood clot, to prevent the clot from enlarging. It is often given during some surgeries, such as cardiopulmonary bypass, when the risk for developing blood clots is high. Small amounts of heparin are frequently used to flush out catheters and intravenous lines to keep clots from forming in them.
When a person is given heparin, the drug can combine with a substance found in platelets called platelet factor 4 (PF4) and form a complex. In some people, the body's immune system recognizes the heparin-PF4 complex as "foreign" and produces an antibody directed against it. This antibody can activate platelets and lead to a drop in the number of platelets, a condition known as heparin-induced thrombocytopenia (HIT). It may also lead to the development of new thrombosis or worsening thrombosis.
Platelets are cell fragments that are an important part of the blood clotting system. When a blood vessel is injured and leaks blood, platelets are activated and clump together at the site of the injury, and work with coagulation factors to promote clot formation and stop the bleeding.
Not everyone on heparin produces HIT antibodies, and not everyone with HIT antibodies develops a low platelet count, but about 1% to 5% of those with the antibodies do. In HIT, the antibodies bind to the heparin-PF4 complexes, which then attach to the surface of platelets. This activates the platelets, which in turn, triggers the release of more PF4. This starts a cycle that can cause a rapid and significant drop (e.g., 50% or more) in the number of platelets in the blood. Usually, a decrease in platelets results in a higher risk of bleeding, but in HIT, the activation of platelets by HIT antibodies can paradoxically lead to new and progressive blood clot formation in the veins and arteries. This occurs in about 30% to 50% of those who have the HIT antibody and thrombocytopenia.
This condition, associated with the presence of HIT antibody, low platelet count, and excessive clotting, is formally called immune-mediated heparin-induced thrombocytopenia or HIT type II. It typically develops about 5-10 days after a person starts heparin therapy but may also develop rapidly, within 1-2 days, if a person has been treated with heparin in the last 3 months and starts treatment again.
There is also a non-immune mediated HIT (type I) that occurs when heparin binds directly to platelets, causing activation; it is more common than type II but is transient and a milder form.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the 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?
No test preparation is needed.