heparin
Meaning of heparin: Fast on, fast off and reversible with an antidote, which is why hospitals reach for it when clotting risk may change within hours.
Definition of heparin
Heparin is an anticoagulant, a medicine that slows the formation of blood clots. It is a naturally occurring sugar chain, stored in the body's mast cells, and it is given by injection or intravenous drip because the gut does not absorb it. Rather than thinning the blood, it sharply accelerates antithrombin, a natural brake built into the clotting cascade.
How does it stop a clot forming?
Clotting is a chain of enzymes, each switching on the next, and the body keeps its own inhibitors in the blood to stop that chain running away. Antithrombin is the main one. On its own it disables thrombin and factor Xa slowly. Heparin binds to antithrombin and changes its shape so that it does the same job many times faster. The practical effect is that a blood clot cannot extend and new ones struggle to form. An existing clot is not dissolved by heparin; the body's own system breaks that down over time while the drug holds the situation still. This is the standard distinction between an anticoagulant and a clot-dissolving drug.
What is the difference between the heparin types?
| Unfractionated heparin | Low molecular weight heparin | |
|---|---|---|
| How given | Intravenous infusion or injection under the skin | Injection under the skin, usually once or twice daily |
| Predictability | Variable between people, so it is monitored by blood test | Predictable enough that routine monitoring is often unnecessary |
| How it clears | Mixed routes, short-lived once stopped | Mainly through the kidneys, so impaired kidneys prolong it |
| Reversal | Protamine reverses it almost completely | Protamine reverses it only in part |
What are the main hazards?
Bleeding is the obvious one and follows directly from what the drug does. The second is counterintuitive: heparin-induced thrombocytopenia is an immune reaction against heparin bound to a platelet protein, and although it drops the platelet count it pushes strongly toward clotting rather than bleeding, so continuing heparin makes matters worse. Prolonged use over months is also associated with bone thinning. These are the reasons platelet counts and clotting measurements are followed during treatment rather than assumed to be steady.
Used in a sentence
Heparin was administered intravenously to prevent further clot formation.
Good to know
Heparin's effect can be reversed within minutes by protamine, unlike most oral anticoagulants, which is one reason it is chosen where clotting risk may change quickly. A separate reaction, heparin-induced thrombocytopenia, lowers the platelet count while increasing rather than reducing the tendency to clot.
Word origin
Where “heparin” comes from
From Greek hepar, meaning liver, reflecting heparin's original isolation from liver tissue.