Heparins and related parenteral anticoagulants

Quick revision

Most drugs in this group work indirectly, by making antithrombin — a natural clotting brake already in the blood — work far harder than it otherwise would, and among those agents almost every difference comes down to how long the sugar chain is. The intravenous direct thrombin inhibitors are the contrast: they block thrombin themselves and need no cofactor.

• Heparin binds antithrombin III and changes its shape, greatly increasing its ability to inactivate thrombin and factor Xa. (1)
• To inhibit thrombin, the heparin molecule must be long enough to hold antithrombin and thrombin together on the same chain; inhibiting factor Xa needs only the short binding sequence. (2) (1)
• That length requirement is the whole reason unfractionated heparin blocks both thrombin and factor Xa while low-molecular-weight heparin acts mainly through factor Xa. (2)
• Fondaparinux is the logical endpoint of shortening the chain: a synthetic pentasaccharide of just five sugars that potentiates antithrombin's neutralisation of factor Xa by roughly three hundredfold and does not inactivate thrombin at all. (7) (3)
• None of these drugs is absorbed from the gut, so all are given by injection or infusion. (1) (2)
• Unfractionated heparin has a short, dose-dependent half-life measured in well under a couple of hours, which is what makes it the agent of choice when anticoagulation may need to be switched off quickly. (1)
• Unfractionated heparin is monitored with the activated partial thromboplastin time, or with activated clotting time during bypass, extracorporeal support and coronary intervention. (1)
• Low-molecular-weight heparin has a longer half-life and a more predictable response, so it is usually given at a fixed weight-based dose without routine monitoring. (2)
• Heparin-induced thrombocytopenia type II is the signature adverse effect of the class: an immune, antibody-mediated reaction that paradoxically causes clotting rather than bleeding. (3)
• The antibody is directed at a complex of heparin with platelet factor 4, and it activates platelets by binding their Fc receptor, which is what drives the hypercoagulable state. (3)
• Because antibodies take time to form, type II usually appears after about five to fourteen days of heparin, but can appear on the first day if there was exposure within the previous few months. (3)
• Type I is different: a non-immune, mild, early fall in platelets that resolves on its own even if heparin continues. (3) (1)
• Risk of heparin-induced thrombocytopenia tracks chain length — higher with unfractionated heparin than with low-molecular-weight heparin, and fondaparinux does not cause it and does not react with the antibodies. (3)
• Protamine neutralises unfractionated heparin within about five minutes by forming an inactive salt with it, but reverses low-molecular-weight heparin only partly and has no useful effect on fondaparinux. (4) (1)
• Direct thrombin inhibitors given intravenously — argatroban and bivalirudin — are the alternatives used when heparin has to be stopped because of heparin-induced thrombocytopenia. (5) (6)
• Beyond bleeding, chronic heparin use is associated with osteopenia and osteoporosis, and heparin can impair aldosterone synthesis and raise potassium. (1)

Overview

The heparins are the parenteral anticoagulants of hospital practice. They are used to prevent and treat venous thrombosis and pulmonary embolism, in atrial fibrillation, during cardiac surgery, extracorporeal circulation and dialysis, in acute coronary syndromes and coronary intervention, in acute limb ischaemia, and in consumptive coagulopathies such as disseminated intravascular coagulation. What unites them is not a chemical structure but a mechanism: none of them attacks a clotting factor itself, and all of them work by making a protein the patient already has — antithrombin — far more effective than it normally is. (1)

Antithrombin is a natural inhibitor that slowly inactivates thrombin and factor Xa. Heparin binds it and changes its shape, and the inactivation that would have taken a long time now happens quickly. The catch is geometry. Neutralising factor Xa needs only the short sugar sequence that binds antithrombin, but neutralising thrombin requires the heparin chain to be long enough to hold antithrombin and thrombin against each other at the same time. Unfractionated heparin, with chains averaging around forty-five sugar units, can do both. Low-molecular-weight heparin, averaging about fifteen, mostly cannot reach thrombin. Fondaparinux, a synthetic chain of exactly five sugars, cannot reach it at all. (1) (2) (3) (7)

The practical differences follow from that. Unfractionated heparin is given intravenously, has a short and dose-dependent half-life, produces a variable response between patients, and therefore needs measurement — usually the activated partial thromboplastin time, or the activated clotting time in the operating theatre and catheter laboratory. Those same properties make it the preferred agent when anticoagulation may have to be switched off at short notice. Low-molecular-weight heparin is injected under the skin, is absorbed almost completely, lasts longer, behaves predictably enough for fixed weight-based dosing without routine monitoring, and is cleared substantially by the kidney, so kidney function matters. Fondaparinux sits further along the same line again, with a much longer half-life and near-complete renal elimination of unchanged drug. (1) (2) (7)

The adverse effect that defines this class is not bleeding, common though bleeding is. It is heparin-induced thrombocytopenia — specifically its immune form, in which IgG antibodies against a complex of heparin and platelet factor 4 activate platelets and produce a severe hypercoagulable state. The platelet count falls, and the danger is clotting rather than haemorrhage. Because chain length governs how readily those complexes form, the risk is greatest with unfractionated heparin, lower with low-molecular-weight heparin, and absent with fondaparinux. When it occurs, every form of heparin is stopped and replaced with an anticoagulant that does not cross-react, in practice an intravenous direct thrombin inhibitor such as argatroban or bivalirudin. (3) (5) (6)

Classification and drug examples

The natural way to divide this group is by how long the sugar chain is, because that determines which clotting factors are reached. The intravenous direct thrombin inhibitors are included because they are the parenteral anticoagulants used when a heparin cannot be, and because contrasting them with heparin is the clearest way to see what indirect inhibition means.

Mechanism of action

Heparins bind antithrombin and change its shape so that it inactivates thrombin and factor Xa far faster than it otherwise would. How much of that dual effect survives depends on chain length: a long chain can hold antithrombin and thrombin together, a short one can only present antithrombin to factor Xa.

Molecular target

Antithrombin III, and through it thrombin (factor IIa) and factor Xa; the direct thrombin inhibitors included for contrast bind thrombin itself

1. Antithrombin is already there, working slowly

   Antithrombin III is a natural regulatory protein circulating in plasma that inactivates several clotting enzymes, most importantly thrombin and factor Xa. On its own it does this slowly enough that clotting proceeds normally. (1)

2. Heparin binds antithrombin and changes its shape

   Heparin binds antithrombin through a specific pentasaccharide sequence, inducing a conformational change that dramatically increases its inhibitory activity. Fondaparinux, which is that sequence and nothing else, is stated to potentiate antithrombin's neutralisation of factor Xa by about three hundredfold. (1) (7)

3. Factor Xa inhibition needs only the binding sequence

   Once antithrombin is activated, it can neutralise factor Xa without any further help from the heparin molecule. This is why every drug in the group, however short, retains anti-factor Xa activity. (2) (7)

4. Thrombin inhibition needs a long chain as well

   To inactivate thrombin, the heparin molecule must provide a binding site for thrombin alongside the one for antithrombin, holding both on the same chain. Only the longer chains of unfractionated heparin can do this, which is why unfractionated heparin inhibits both targets and has a faster onset of anticoagulant action. (2)

5. Blocking either target stops fibrin forming

   Inactivating factor Xa means prothrombin is not converted to thrombin; inactivating thrombin means fibrinogen is not converted to fibrin. Either way the final common pathway is interrupted and no stable clot forms, which prolongs clotting times without affecting bleeding time. (2) (1)

6. The effect is capped by how much antithrombin there is

   Because the drug works through a cofactor, its effect depends on the patient's antithrombin level. Inflammation, major thrombosis, malignancy, the postoperative state and inherited deficiency all reduce the response, a phenomenon described as heparin resistance. (1)

7. Long chains also bind platelet factor 4, which is where the immune problem starts

   Platelet factor 4 is positively charged and binds negatively charged heparin with high affinity. The shorter the chain and the smaller the molecule, the less readily it binds plasma proteins and cell surfaces, which is why the risk of the immune reaction falls from unfractionated heparin to low-molecular-weight heparin and disappears with fondaparinux. (3)

8. Direct thrombin inhibitors bypass the cofactor entirely

   Argatroban binds reversibly at the active site of thrombin, and bivalirudin binds both the catalytic site and the anion-binding exosite, in circulating and clot-bound thrombin alike. Neither needs antithrombin, and bivalirudin does not bind platelet factor 4, which is what makes them usable after heparin-induced thrombocytopenia. (5) (6)

9. Protamine reverses by charge, not by receptor

   Protamine is a positively charged arginine-rich peptide that forms an inactive salt aggregate with strongly negatively charged unfractionated heparin, neutralising it within about five minutes. Because the interaction depends on charge and chain size, it reverses low-molecular-weight heparin only partly. (4)

Major clinical uses

Pharmacokinetics

• The monitoring test follows the mechanism. Unfractionated heparin inhibits thrombin, which the activated partial thromboplastin time detects, while the shorter agents act mainly on factor Xa, so anti-factor Xa activity is the measurement used when a level is needed at all. (1) (2)
• Kidney function matters progressively more as the chain gets shorter: unfractionated heparin is cleared by the liver and the reticuloendothelial system, low-molecular-weight heparins are partly renal, and fondaparinux leaves almost entirely in the urine unchanged. (1) (2) (7)
• Heparin used only to keep an intravenous line open is not therapeutic anticoagulation, and that use has declined because even small exposures carry risk. (1)
• This page gives no dose regimens by design. Doses depend on indication, body weight, kidney function, the monitoring target and local protocol, and belong in a prescribing reference used by the treating clinician.

Adverse effects

Common

• Bleeding: The most significant complication and the one that governs monitoring. Patients are watched for new bleeding such as blood in the urine or stool, and for bruising, a petechial rash and nosebleeds. (1)
• Mild thrombocytopenia: A fall in platelets occurs in a substantial minority of heparin-treated patients and is usually mild and of no clinical consequence; the concern is distinguishing it from the immune form. (1)
• Injection site reactions: Local reactions are common with subcutaneous administration, and patients frequently dislike self-injection because of pain or bleeding at the site — a real determinant of which anticoagulant is chosen for long courses. (1) (2)
• Hyperkalaemia: Heparin can impair aldosterone synthesis, and the resulting rise in potassium matters most in kidney impairment, diabetes, metabolic acidosis, or when a potassium-sparing drug is also being taken. (1)

Serious adverse effects

• Heparin-induced thrombocytopenia, type II: An immune, antibody-mediated reaction that usually appears after about five to fourteen days of exposure, or on the first day if there has been exposure within the preceding few months. It produces an intensely hypercoagulable state, with up to half of affected patients developing a thromboembolic complication and a substantial reported mortality. Assessed with the 4T score at the bedside. Above the action threshold, all heparin exposure — including flushes, coated catheters and dialysate — is stopped and a non-heparin anticoagulant started, with antibody and functional testing sent to confirm the diagnosis. (3) (1)
• Thrombosis complicating heparin-induced thrombocytopenia: Deep vein thrombosis, pulmonary embolism and skin necrosis are the commonest complications, and both venous and arterial territories can be involved. Risk is highest in the first days but the prothrombotic state persists for weeks after heparin is stopped. Treated as an indication for full therapeutic anticoagulation with a non-cross-reacting agent rather than for observation. (3)
• Warfarin-associated skin necrosis in acute heparin-induced thrombocytopenia: Starting or continuing warfarin during the acute phase lowers protein C and S further in a patient who is already hypercoagulable, and skin necrosis is a recognised consequence. Where warfarin has recently been started, the source describes holding it and giving vitamin K to restore protein C and S stores before anticoagulation is transitioned. (3)
• Osteopenia and osteoporosis with prolonged use: Linked to chronic heparin therapy rather than to short courses, and one of the reasons long-term parenteral anticoagulation is avoided where an alternative exists. Considered when parenteral anticoagulation is expected to continue for a long period, particularly in pregnancy where oral options are unsuitable. (1)
• Spinal or epidural haematoma: Bleeding into the spinal canal can follow neuraxial anaesthesia or spinal puncture in a patient anticoagulated with a low-molecular-weight heparin, a heparinoid or fondaparinux, and may cause long-term or permanent paralysis. The risks and benefits are weighed before any neuraxial procedure, and patients are watched afterwards for midline back pain, numbness, tingling or weakness in the legs, and bladder or bowel disturbance. (7)
• Protamine reactions: Anaphylaxis with hypotension, bronchoconstriction and bradycardia is the commonest serious reaction, and pulmonary hypertension from thromboxane A2 release can be catastrophic, raising pulmonary vascular resistance many times over. Given by slow infusion, often after a small test dose, and usually through a peripheral rather than a central line; suspected anaphylaxis is treated as any other perioperative anaphylaxis. (4)

Drug-specific effects

• Unfractionated heparin: The highest risk of heparin-induced thrombocytopenia in the group, because its long heterogeneous chains bind platelet factor 4 most readily; it also carries the hyperkalaemia and long-term bone effects. (3) (1)
• Low-molecular-weight heparins: Lower risk of the immune reaction than unfractionated heparin, but not zero, and accumulation in severe kidney impairment is the practical hazard that distinguishes them. (3) (2)
• Fondaparinux: Does not cause heparin-induced thrombocytopenia and does not react with the antibodies, but its long half-life and near-complete renal elimination mean the effect cannot be quickly withdrawn and it is contraindicated in severe kidney impairment. (3) (7)
• Protamine: In excess it becomes an anticoagulant in its own right, impairing platelet function, interfering with clotting factors and stimulating clot breakdown, so over-dosing worsens rather than corrects the bleeding. (4)

Contraindications, precautions and interactions

Contraindications

• Active major bleeding, which is a stated contraindication for fondaparinux and the situation in which anticoagulation is stopped rather than adjusted. (7)
• Bacterial endocarditis, listed as a contraindication for fondaparinux and as a caution for heparin because of the bleeding risk. (7) (1)
• Severe kidney impairment in a patient being considered for fondaparinux for prophylaxis or treatment of venous thromboembolism. (7)
• Continued heparin of any kind once immune heparin-induced thrombocytopenia is suspected above the action threshold, including line flushes, heparin-coated catheters and heparin in dialysate. (3)

Precautions

• Severe uncontrolled hypertension, and major surgery involving the brain, spinal cord or eye, both of which raise the consequences of any bleeding. (1)
• Recent or ongoing spinal tap or spinal anaesthesia, given the risk of bleeding into the spinal canal. (1) (7)
• Haematological disorders including haemophilia, thrombocytopenia and vascular purpura. (1)
• Hereditary antithrombin III deficiency in a patient also receiving antithrombin III concentrate, where the heparin requirement falls. (1)
• Ulcerative lesions of the gastrointestinal tract, or continuous gastric or intestinal drainage. (1)
• Liver disease and menstruation, both listed among the situations calling for particular care. (1)
• Older age, in whom equivalent doses of unfractionated heparin produce higher plasma concentrations and slower elimination can prolong the activated partial thromboplastin time. (1)

Drug interactions

• Warfarin during the acute phase of heparin-induced thrombocytopenia: Lowering protein C and S in an already hypercoagulable patient risks skin necrosis, so warfarin recently started is held and vitamin K given to replete those stores. (3)
• Other drugs affecting haemostasis: Antiplatelet agents and other anticoagulants add bleeding risk, which is why parenteral anticoagulation alongside dual antiplatelet therapy in acute coronary syndrome is a deliberate, time-limited decision. (1)
• Potassium-sparing medicines: Compound heparin's suppression of aldosterone synthesis, raising the risk of hyperkalaemia, particularly alongside kidney impairment, diabetes or metabolic acidosis. (1)
• Antithrombin III concentrate: Restores the cofactor heparin depends on, so the heparin requirement falls and the dose is reduced in patients with hereditary deficiency receiving it. (1)
• Protamine: Neutralises unfractionated heparin by charge within about five minutes; the amount needed is calculated from how much heparin was given in the preceding hours, because heparin's short half-life means the requirement falls quickly with time. (4) (1)
• Aspirin before cardiac surgery: Suggested in some studies to reduce protamine-induced pulmonary hypertension, since the mechanism is attributed to thromboxane A2 release and aspirin suppresses thromboxane production irreversibly. (4)
• Transition from low-molecular-weight heparin to unfractionated heparin before delivery: Guidance for pregnant patients with mechanical valve prostheses directs a switch to unfractionated heparin well before a scheduled delivery, so that anticoagulation can be turned off quickly if needed. (2)

Comparison tables

High-yield exam pearls

• Chain length explains almost everything in this class. (3) Unfractionated heparin averages around forty-five sugar units, low-molecular-weight heparin around fifteen, and fondaparinux exactly five. That single number predicts the target profile, the predictability of the dose response, and the risk of the immune reaction.
• Heparin does not dissolve anything and does not affect bleeding time. (1) (7) It prolongs the time blood takes to clot, but it has no fibrinolytic action and does not act on platelets. The distinction separates it cleanly from the thrombolytics and from the antiplatelet drugs.
• Heparin-induced thrombocytopenia presents as clotting, not bleeding. (3) Up to half of affected patients develop a thromboembolic complication and the reported mortality is substantial, so a falling platelet count in a heparin-treated patient is a thrombotic emergency rather than a reason to worry mainly about haemorrhage.
• The prothrombotic state of heparin-induced thrombocytopenia outlasts the heparin. (3) Risk of complications is highest in the first days, but the hypercoagulable state persists for weeks after heparin is stopped, which is why an alternative anticoagulant is started rather than simply withdrawing the drug.
• Warfarin must not be the immediate substitute in acute heparin-induced thrombocytopenia. (3) Warfarin lowers protein C and S first, and in an already hypercoagulable patient that precipitates skin necrosis; where warfarin has just been started, the source describes holding it and giving vitamin K to restore those stores.
• The 4T score comes before the laboratory test, not after it. (3) (1) It scores thrombocytopenia, timing, thrombosis and other causes, and a score at or above the action threshold triggers immediate withdrawal of all heparin and a non-heparin anticoagulant while antibody testing is sent.
• The platelet factor 4 immunoassay rules out; the serotonin release assay rules in. (3) The immunoassay is highly sensitive with a strong negative predictive value but poor specificity because it detects antibody classes not involved in the disease. The functional serotonin release assay measures actual platelet activation and is the confirmatory test.
• Heparin resistance is usually about antithrombin, not about the heparin. (1) Since heparin works only through antithrombin, states that consume or lack it — systemic inflammation, major thrombosis, malignancy, the postoperative state, inherited deficiency — blunt the response, and anti-factor Xa monitoring is used when the activated partial thromboplastin time becomes unreliable.
• Protamine is not a benign antidote. (4) Given too fast or in excess it causes hypotension, pulmonary vasoconstriction, pulmonary hypertension and anaphylaxis, and excess protamine is itself anticoagulant — impairing platelet function, interfering with clotting factors and promoting clot breakdown.
• Bivalirudin does not bind platelet factor 4. (6) That is the mechanistic reason it does not cross-react with heparin-induced thrombocytopenia antibodies, which is exactly why a direct thrombin inhibitor is the substitute rather than another heparin.

Common exam traps

• Trap: Calling heparin a direct factor Xa inhibitor. Actually: Heparin has no anticoagulant activity of its own. It accelerates antithrombin, which is the molecule that actually inactivates thrombin and factor Xa, and that is why it is described as indirect. (1) (5)
• Trap: Assuming low-molecular-weight heparin is simply a smaller dose of the same drug. Actually: The shorter chain cannot bridge antithrombin to thrombin, so the effect is directed at factor Xa. That changes the monitoring test, the predictability of the response, the reversibility with protamine and the risk of the immune reaction. (2) (4)
• Trap: Treating every fall in platelets on heparin as heparin-induced thrombocytopenia. Actually: Mild thrombocytopenia is common and often clinically unimportant, and type I is a non-immune early drop that resolves on its own. The dangerous form is the immune type II, which is what the 4T score is designed to identify. (1) (3)
• Trap: Stopping heparin and considering the problem solved. Actually: Withdrawal alone leaves the patient in a hypercoagulable state that persists for weeks, so an alternative anticoagulant that does not cross-react with the antibodies is started at the same time. (3)
• Trap: Forgetting that line flushes and coated catheters count as heparin exposure. Actually: No amount is too small to trigger the reaction, and management explicitly requires stopping all forms of heparin, including flushes, heparin-coated catheters and heparin in dialysate. (3)
• Trap: Expecting protamine to reverse anything with heparin in its description. Actually: It fully neutralises unfractionated heparin, gives only partial and uncertain reversal of low-molecular-weight heparin, and that low-molecular-weight use is not an approved indication. It is not a general antidote for the class. (4)
• Trap: Assuming the activated clotting time and the activated partial thromboplastin time are interchangeable. Actually: The activated clotting time is a less sensitive, point-of-care test reserved for settings such as bypass, extracorporeal support and coronary intervention, whereas the activated partial thromboplastin time is the routine measure for therapeutic infusions. (1)

Self-test questions

Answers are hidden until you open them. These questions are written from this page's cited content and are for study only — they are not clinical guidance.

1. How does heparin produce its anticoagulant effect?

   • By binding thrombin directly and blocking its active site
   • By binding antithrombin III and greatly enhancing its inactivation of thrombin and factor Xa
   • By blocking the synthesis of clotting factors in the liver
   • By converting plasminogen to plasmin
   Show answer

   Answer: By binding antithrombin III and greatly enhancing its inactivation of thrombin and factor Xa

   Heparin binds antithrombin III and induces a conformational change that greatly increases its ability to inactivate clotting factors, most importantly thrombin and factor Xa. The drug itself has no direct action on those enzymes. (1)

2. Why does low-molecular-weight heparin act mainly on factor Xa rather than on thrombin?

   • It binds a different site on antithrombin
   • Its chain is too short to hold antithrombin and thrombin together simultaneously
   • It is destroyed by thrombin before it can act
   • It works without antithrombin altogether
   Show answer

   Answer: Its chain is too short to hold antithrombin and thrombin together simultaneously

   Inhibiting thrombin requires the heparin molecule to bridge antithrombin and thrombin on the same chain, which needs a long saccharide chain. Factor Xa inhibition requires only the antithrombin binding sequence, so a shorter chain retains it. (2)

3. Which anticoagulant is a synthetic pentasaccharide that does not cause heparin-induced thrombocytopenia?

   • Enoxaparin
   • Dalteparin
   • Fondaparinux
   • Unfractionated heparin
   Show answer

   Answer: Fondaparinux

   Fondaparinux consists of only the five sugars that make up the antithrombin binding sequence. Because it is too small to form the complexes that provoke the immune response, it does not cause the reaction and does not react with heparin-induced antibodies. (3) (7)

4. What is the immunological basis of type II heparin-induced thrombocytopenia?

   • IgE antibodies against heparin causing mast cell degranulation
   • IgG antibodies against a heparin-platelet factor 4 complex that activate platelets via the Fc receptor
   • Direct toxicity of heparin to megakaryocytes
   • Complement-mediated lysis of platelets without antibody involvement
   Show answer

   Answer: IgG antibodies against a heparin-platelet factor 4 complex that activate platelets via the Fc receptor

   Platelet factor 4 binds heparin with high affinity, and IgG raised against that complex engages the platelet Fc receptor. Activated platelets release more platelet factor 4 and prothrombotic mediators, driving a self-amplifying hypercoagulable cycle. (3)

5. A patient develops type II heparin-induced thrombocytopenia. What is the correct immediate management approach?

   • Stop heparin and observe, since the platelet count will recover
   • Stop all forms of heparin and start a non-heparin anticoagulant
   • Switch to a lower dose of the same heparin
   • Start warfarin immediately as the sole replacement
   Show answer

   Answer: Stop all forms of heparin and start a non-heparin anticoagulant

   All heparin exposure is stopped, including flushes and coated catheters, and an alternative anticoagulant is started because the prothrombotic state persists for weeks. Warfarin alone is not appropriate acutely, since falling protein C risks skin necrosis. (3)

6. Which laboratory test is the confirmatory, functional test for heparin-induced thrombocytopenia?

   • Platelet factor 4 immunoassay
   • Activated partial thromboplastin time
   • Serotonin release assay
   • Anti-factor Xa activity
   Show answer

   Answer: Serotonin release assay

   The immunoassay detects antibodies and is sensitive but poorly specific, so it is best at ruling the diagnosis out. The serotonin release assay measures whether the patient's antibodies actually activate platelets in the presence of heparin, and is the gold standard for confirmation. (3)

7. Which statement about protamine is correct?

   • It reverses fondaparinux completely
   • It neutralises unfractionated heparin rapidly by forming an inactive salt complex
   • It has no adverse effects of clinical importance
   • It works by displacing heparin from antithrombin
   Show answer

   Answer: It neutralises unfractionated heparin rapidly by forming an inactive salt complex

   Protamine is a positively charged peptide that forms an inactive salt aggregate with the strongly negatively charged heparin molecule, neutralising it within about five minutes. Reversal of low-molecular-weight heparin is partial only, and protamine carries a real risk of anaphylaxis and pulmonary hypertension. (4)

8. Why is an intravenous direct thrombin inhibitor chosen after heparin-induced thrombocytopenia?

   • It works faster than any heparin
   • It does not bind platelet factor 4 and so does not cross-react with the causative antibodies
   • It has no bleeding risk
   • It can be given by mouth
   Show answer

   Answer: It does not bind platelet factor 4 and so does not cross-react with the causative antibodies

   Bivalirudin binds thrombin at its catalytic site and anion-binding exosite and does not bind platelet factor 4, so there is no cross-reactivity with heparin-induced antibodies. Argatroban is likewise a direct, non-immune inhibitor and is licensed for this setting. (6) (5)

9. A patient on unfractionated heparin is not achieving the expected effect despite escalating infusion rates. What is the most likely explanation?

   • Antithrombin deficiency or consumption, so-called heparin resistance
   • The patient has developed antibodies that destroy heparin
   • Heparin is being cleared by the kidney too quickly
   • The patient has an inherited excess of antithrombin
   Show answer

   Answer: Antithrombin deficiency or consumption, so-called heparin resistance

   Heparin acts only through antithrombin, so inflammation, major thrombosis, malignancy, the postoperative state and inherited antithrombin deficiency all blunt the response. In these situations anti-factor Xa monitoring is used because the activated partial thromboplastin time becomes unreliable. (1)

Frequently asked questions

References

1. Heparin (StatPearls) StatPearls Publishing / NCBI Bookshelf, 2025
2. Low-Molecular-Weight Heparin (LMWH) (StatPearls) StatPearls Publishing / NCBI Bookshelf, 2025
3. Heparin-Induced Thrombocytopenia (StatPearls) StatPearls Publishing / NCBI Bookshelf, 2023
4. Protamine (StatPearls) StatPearls Publishing / NCBI Bookshelf, 2023
5. Argatroban (StatPearls) StatPearls Publishing / NCBI Bookshelf, 2023
6. Bivalirudin (StatPearls) StatPearls Publishing / NCBI Bookshelf, 2024
7. Fondaparinux sodium injection — prescribing information (DailyMed) DailyMed, US National Library of Medicine, 2020

Related drug classes

• Antiplatelet agents
• Vitamin K antagonists
• Direct oral anticoagulants
• Thrombolytics

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