Anticoagulant

Heparin

Also called UFH

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

9:40

Watch the explainer

Heparin vs Warfarin: aPTT vs INR, HIT, Bridging, Antidotes and DOAC Reversal

9:40 video · Chapters and full transcript

Indications

DVT and PE, especially with instability or CrCl under 30 (AHA/ACC 2026 prefers LMWH even when lysis or a procedure may follow)
VTE prophylaxis (subcutaneous), including in severe kidney failure
Acute coronary syndromes and PCI (off-label)
Anticoagulation of bypass, ECMO, CRRT and dialysis circuits
Atrial fibrillation with embolization; peripheral arterial embolism
DIC in which thrombosis predominates

Dosing

Heparin drip card and mL/h converterStart: VTE: 80 units/kg bolus, then 18 units/kg/h. ACS: 60 units/kg bolus (max 4,000), then 12 units/kg/h (max 1,000 units/h)

Where	Dose
Daily ICU prophylaxis	Heparin (UFH) SC: 5,000 units every 8–12 h; for severe kidney failure or when a procedure is near

Mechanism of action

Heparin acts through antithrombin, a serine protease inhibitor. A pentasaccharide on about one-third of heparin chains binds antithrombin and bends its reactive loop, speeding protease inhibition about 1,000-fold. That shape change alone blocks factor Xa, but blocking thrombin needs a chain of at least 18 saccharides to bridge antithrombin and thrombin; heparin then lets go and is reused. Fibrin-bound thrombin and Xa in prothrombinase are shielded, so heparin stops clot growth without dissolving clot.

Pharmacology

Hematologic: inhibits Xa and thrombin equally (1:1), plus IXa, XIa and XIIa; prolongs the aPTT, ACT and thrombin time
Hematologic: chains of about 12 or more saccharides bind platelet factor 4, forming the complexes behind HIT
Metabolic: releases lipoprotein lipase (triglycerides fall, then rebound when heparin stops); suppresses aldosterone, so potassium can rise
Other: osteoporosis after months of use; transaminases often rise without liver injury

Pharmacokinetics

Onset	IV immediate; SC slower and variable
Peak	SC 2–4 h
Duration	Hours; stop 4–6 h before procedures
Half-life	Dose-dependent: about 30 min after 25 units/kg, 60 min after 100 and 150 min after 400 (ACCP 2012)
Distribution	Vd 0.07 L/kg (plasma volume); does not cross the placenta
Protein binding	Extensive and variable (fibrinogen, lipoproteins, PF4), so the dose response is unpredictable
Metabolism	Endothelial cells and macrophages take up and depolymerize it: fast but saturable
Elimination	Renal, a slower route that dominates once the cellular route saturates; little change in kidney failure
Routes	IV or deep SC (reduced bioavailability); avoid IM

Chemistry

A densely sulfated glycosaminoglycan (iduronic acid and glucosamine units), the most negatively charged natural molecule: fully ionized, water-soluble, not absorbed orally
Chains of 3,000–30,000 Da (mean 15,000, about 45 saccharides); the antithrombin-binding pentasaccharide needs a rare 3-O-sulfated glucosamine
Extracted from porcine intestinal mucosa, the only US source since the 1990s: contraindicated in pork allergy, and some patients decline it on religious grounds
Vials of 1,000–20,000 units/mL (pH 5.0–7.5), some preserved with benzyl alcohol (avoid in neonates); premixed dextrose bags contain sodium metabisulfite (sulfite allergy)

Adverse effects and cautions

Safety

Bleeding: titrate by aPTT (range matched to anti-Xa 0.3–0.7 IU/mL; label 1.5–2 × normal) or by anti-Xa, 6 h after each change
ACT guides the high doses of PCI (about 250–300 s) and bypass (usually 400–480 s or more)
HIT: platelets fall over 50% on day 5–10 (sooner after recent heparin) and clots form; screen counts from day 4 to 14 when HIT risk is 0.1% or more (ASH 2018). Stop all heparin, flushes included, and start argatroban or bivalirudin
High-alert drug: mix-ups with flush vials have been fatal; double-check strength, pump and bag
Resistance: low antithrombin (bypass, ECMO) or high factor VIII and fibrinogen; anti-Xa avoids overdosing
Contraindicated after HIT (unless remote, with negative tests, for cardiac surgery) or with uncontrolled bleeding
Reversal: protamine 1 mg per 100 units given in the last 2–3 h (max 50 mg), slowly

In the one-pagers

Practice Bleeding, Reversal, HIT and DIC questions free34 exam-style questions with rationales on the topic where Heparin comes up most. No account or download needed.

In the algorithms

Practice free in your browser3,800+ exam-style questions with rationales. No account, no ads. Also free on the App Store and Google Play.

Sources

FDA label: Heparin Sodium, Fresenius Kabi (2021)
Garcia, ACCP Parenteral Anticoagulants (Chest 2012)
ASH HIT guideline (2018)

For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow the order, the label and your institution's protocols.