Anticoagulants: Heparins, Warfarin & DOACs

Overview: target, test, antidote, toxicity

Where they act. Every anticoagulant ultimately limits thrombin generation or activity. Heparins work indirectly by amplifying the natural inhibitor antithrombin; direct thrombin inhibitors (DTIs) and direct factor Xa inhibitors bind their target enzyme with no cofactor; warfarin acts upstream in the liver by preventing the γ-carboxylation that vitamin K-dependent factors need. For each drug, learn four facts: target, monitoring test, reversal agent, and signature toxicity.

Heparins: UFH, LMWH, and fondaparinux

Mechanism. Heparin is a sulfated glycosaminoglycan whose unique pentasaccharide sequence binds antithrombin and changes its conformation, speeding inactivation of thrombin (IIa) and factor Xa, and to a lesser degree IXa, XIa, and XIIa, about 1000-fold. Xa inhibition needs only the pentasaccharide, but thrombin inhibition needs a chain of at least 18 saccharides long enough to bind antithrombin and thrombin at once. Chain length therefore sets each heparin's profile, and hereditary antithrombin deficiency blunts heparin's effect (heparin resistance).

Unfractionated heparin (UFH). Long chains (mean ~15 kDa) give roughly equal anti-IIa and anti-Xa activity. IV UFH acts immediately, has a 1–2 h half-life, and is titrated to an aPTT of about 1.5–2.5× control or to an anti-Xa level. It is cleared mainly by the reticuloendothelial system and endothelial binding rather than the kidney, so it is preferred in severe renal failure and whenever rapid on/off control matters (surgery, procedures). Bleeding is the main risk. Months of use, as in pregnancy, cause osteoporosis by suppressing bone formation and increasing resorption; heparin also suppresses aldosterone synthesis (hyperkalemia) and mildly raises transaminases.

LMWH. Enoxaparin and dalteparin are depolymerized heparins (~4–5 kDa) that keep the pentasaccharide, but most chains are shorter than 18 saccharides and cannot bridge thrombin, so anti-Xa activity exceeds anti-IIa. Predictable weight-based SC dosing needs no routine monitoring and allows outpatient VTE treatment. Because LMWH is renally cleared, peak anti-Xa activity (about 4 h after a dose) is measured with CrCl <30 mL/min, morbid obesity, pregnancy, or low body weight; the dose is reduced in severe renal impairment, and UFH is simpler in dialysis. The aPTT is unreliable for LMWH. HIT and osteoporosis are less frequent than with UFH.

Fondaparinux. This synthetic pentasaccharide produces antithrombin-mediated, purely anti-Xa activity with no thrombin effect. It is given once daily SC, is renally cleared (avoid if CrCl <30 mL/min), and barely forms PF4 complexes, so HIT is very unlikely and it serves as an HIT alternative.

Protamine. This cationic, arginine-rich protein binds anionic heparin: about 1 mg neutralizes 100 units of circulating UFH (full reversal), it neutralizes only ~60% of LMWH anti-Xa activity, and it does not reverse fondaparinux. Hypotension and anaphylactoid reactions are more likely with fish allergy or prior protamine exposure, such as NPH insulin.

Heparin-induced thrombocytopenia

Mechanism. In type 2 (immune) HIT, heparin binds platelet factor 4 (PF4) to create a neoantigen. IgG against the PF4-heparin complex forms immune complexes that cross-link platelet FcγRIIa receptors, activating platelets, shedding procoagulant microparticles, and generating thrombin. Platelets are consumed, yet the net effect is thrombotic: venous thrombosis (DVT, PE) is most common, but arterial occlusion and skin necrosis at injection sites occur. This differs from ITP, where autoantibodies usually target GPIIb/IIIa.

Timing and diagnosis. Platelets typically fall by more than 50% 5–10 days after heparin begins, or within a day if heparin was given in the prior 100 days (preformed antibodies). The 4Ts score estimates pretest probability; an anti-PF4/heparin immunoassay screens and the functional serotonin release assay confirms.

Warfarin started during acute HIT depletes protein C before prothrombin and can cause venous limb gangrene; prophylactic platelet transfusion is avoided because it can fuel thrombosis.

Direct thrombin and factor Xa inhibitors

Parenteral DTIs. Argatroban and bivalirudin bind thrombin directly, inhibiting free and clot-bound thrombin without antithrombin. Argatroban (IV) is hepatically metabolized, making it the HIT agent of choice in renal failure; it is monitored by aPTT and falsely raises the INR, complicating the switch to warfarin. Bivalirudin, a synthetic analog of the leech anticoagulant hirudin, has a ~25-minute half-life with proteolytic and renal clearance and is used during PCI, including in patients with HIT.

Dabigatran. Dabigatran etexilate is an oral prodrug converted by esterases to a direct thrombin inhibitor. About 80% is renally excreted, so it accumulates in CKD; dyspepsia and GI bleeding are characteristic. Idarucizumab, a humanized monoclonal antibody fragment, binds and reverses it within minutes, and low protein binding allows removal by hemodialysis.

Oral Xa inhibitors. Rivaroxaban, apixaban, and edoxaban bind free and prothrombinase-bound Xa without antithrombin. Fixed dosing, no routine monitoring, onset within hours (no heparin bridge), and about half the intracranial hemorrhage rate of warfarin make DOACs first line for most VTE and nonvalvular atrial fibrillation. Apixaban relies least on the kidney; strong CYP3A4/P-glycoprotein inhibitors or inducers still alter levels. Routine PT/aPTT do not quantify these drugs; drug-calibrated anti-Xa assays do. Andexanet alfa, a recombinant, catalytically inactive factor Xa decoy, sequesters rivaroxaban and apixaban so native Xa recovers; it was withdrawn from the US market in December 2025 (thromboembolic risk), so 4F-PCC is used instead.

Stem decoder: -xaban drugs block factor Xa; -gatran drugs block thrombin.

When a DOAC is wrong. Warfarin remains the choice with mechanical heart valves (a dabigatran trial showed more valve thrombosis and bleeding), moderate-to-severe mitral stenosis, and high-risk triple-positive antiphospholipid syndrome. DOACs are also avoided in pregnancy and in advanced renal failure (thresholds vary by agent; apixaban is the usual exception, used even in dialysis).

Warfarin

Mechanism. Reduced vitamin K is the cofactor for γ-glutamyl carboxylase, which adds γ-carboxyglutamate (Gla) residues to factors II, VII, IX, and X and to proteins C and S, oxidizing vitamin K to its epoxide. Warfarin inhibits vitamin K epoxide reductase (VKORC1), so the epoxide is not recycled. Undercarboxylated factors cannot bind calcium and phospholipid membranes. Factors already circulating are unaffected, so onset is delayed.

Monitoring. PT/INR reflects the factor VII-dependent extrinsic pathway. Because factor VII disappears first, the INR climbs within 1–2 days, but real antithrombotic effect requires prothrombin depletion at about 5 days. The usual target INR is 2–3, and 2.5–3.5 for a mechanical mitral valve. Pairing to remember: warfarin → extrinsic → PT/INR; heparin → intrinsic → aPTT.

Skin necrosis. Protein C falls faster than prothrombin, so early warfarin is transiently procoagulant. Dermal microvascular thrombosis produces painful purpura that turns necrotic over fatty areas (breasts, thighs, buttocks), typically on days 3–5, especially with hereditary protein C deficiency. Prevention is heparin or LMWH overlap for at least 5 days and until the INR is ≥2 for 24 hours. Purple toe syndrome, by contrast, reflects cholesterol microemboli.

Pharmacogenetics and interactions. Warfarin is racemic; the potent S-enantiomer is cleared by CYP2C9. Reduced-function CYP2C9 alleles (*2, *3) and the low-expression VKORC1 haplotype lower dose requirements and raise bleeding risk. The drug is ~99% albumin-bound with a narrow therapeutic index.

Pregnancy. Warfarin crosses the placenta: exposure in weeks 6–12 causes fetal warfarin syndrome (nasal hypoplasia, stippled epiphyses), and later exposure can cause CNS abnormalities and fetal hemorrhage. Heparins are too large and charged to cross, so LMWH is preferred in pregnancy, with UFH near delivery; DOACs are avoided. Warfarin, UFH, and LMWH are compatible with breastfeeding.

Reversal. Major bleeding calls for IV vitamin K (restores factor synthesis over 12–24 h) plus 4F-PCC (factors II, VII, IX, X with proteins C and S) for immediate correction; FFP is the fallback when PCC is unavailable, at the cost of volume and slower effect. A supratherapeutic INR without bleeding is handled by holding doses, adding low-dose oral vitamin K when the INR is markedly high.