Anticoagulants prevent new clots and stop existing clots from growing; they do not dissolve clots (that is the job of thrombolytics). They act on the coagulation cascade, mainly on factor Xa and thrombin (factor IIa).

Because warfarin does not affect factors already in the blood, and because protein C falls first (a brief prothrombotic state), warfarin is overlapped with a parenteral anticoagulant for at least 5 days and until the INR is ≥ 2.0 for 24 hours when treating an acute clot.

Main uses: prevention and treatment of venous thromboembolism (VTE) (deep vein thrombosis, pulmonary embolism), stroke prevention in atrial fibrillation (AF), mechanical heart valves, acute coronary syndrome and PCI (heparin, bivalirudin), dialysis and extracorporeal circuits, and heparin-induced thrombocytopenia (HIT) (argatroban, bivalirudin).

DOACs are preferred over warfarin for most clients with AF or VTE — except those with mechanical heart valves, moderate–severe rheumatic mitral stenosis, or high-risk antiphospholipid syndrome, where warfarin is used.

• Bleeding — the main adverse effect of every anticoagulant (warfarin boxed warning: major or fatal bleeding — regular INR monitoring required). Watch for bleeding gums, nosebleeds, easy bruising, hematuria, black or bloody stools, coffee-ground vomit, heavy menses, and hidden bleeding (sudden headache or neurologic change = possible intracranial hemorrhage; back or flank pain = retroperitoneal bleed; falling hemoglobin, hypotension, tachycardia).
• Heparin-induced thrombocytopenia (HIT): immune reaction, usually 5–10 days after starting heparin (within 24 hours if heparin was given in the past 100 days); platelets fall by more than 50% and new venous or arterial clots form. HIT is prothrombotic, not mainly a bleeding problem. Less common with LMWH; does not occur with fondaparinux.
• Heparin: hyperkalemia (aldosterone suppression), osteoporosis with long-term use, injection-site hematoma.
• Warfarin: skin necrosis (days 3–8, linked to protein C deficiency), purple toe syndrome, teratogenicity (nasal hypoplasia, bone and CNS defects).
• Dabigatran: dyspepsia and GI bleeding.
• Spinal/epidural hematoma (boxed warning for LMWH, fondaparinux, and DOACs) with neuraxial anesthesia or lumbar puncture — can cause permanent paralysis.
• DOAC boxed warning: stopping early without another anticoagulant increases the risk of stroke and clots.

Contraindications (all anticoagulants): active major bleeding, recent intracranial hemorrhage, severe uncontrolled hypertension, recent brain, eye, or spinal surgery, severe thrombocytopenia (per drug), and hypersensitivity.

• UFH and LMWH: history of HIT — enoxaparin is contraindicated with immune-mediated HIT within the past 100 days or circulating HIT antibodies; pork allergy (porcine source).
• Warfarin: pregnancy — contraindicated except in selected clients with mechanical heart valves at high clot risk under specialist care. LMWH is the anticoagulant of choice in pregnancy; DOACs are avoided in pregnancy and breastfeeding. Warfarin and heparin are compatible with breastfeeding.
• DOACs: mechanical valves, moderate–severe mitral stenosis, high-risk antiphospholipid syndrome, severe kidney impairment per product (dabigatran most dependent on kidneys), significant liver disease.

Interactions

• Additive bleeding with any anticoagulant: aspirin, other antiplatelets, NSAIDs, SSRIs/SNRIs, other anticoagulants, thrombolytics, and herbal products (ginkgo, garlic, ginseng, high-dose fish oil, turmeric).
• Warfarin — increased INR (bleeding): amiodarone, azole antifungals (fluconazole), metronidazole, trimethoprim-sulfamethoxazole, macrolides, fluoroquinolones, acetaminophen at regular high doses, alcohol binges, acute illness, hyperthyroidism, reduced intake.
• Warfarin — decreased INR (clotting): rifampin, carbamazepine, phenytoin, St. John's wort, sudden increase in vitamin K–rich foods (leafy greens) or vitamin K supplements, enteral feeding products.
• DOACs: strong CYP3A4 and P-glycoprotein inhibitors (ketoconazole, itraconazole, ritonavir) raise levels; inducers (rifampin, carbamazepine, phenytoin, St. John's wort) lower levels — generally avoid.
• Warfarin genetics: variants in CYP2C9 and VKORC1 change dose requirements; many Asian clients need lower doses.

Laboratory monitoring

Nursing interventions

1. Before starting: baseline CBC, platelets, PT/INR, aPTT, creatinine, liver tests; assess bleeding risk (recent surgery, trauma, falls, GI ulcer, pregnancy).
2. Heparin is a high-alert medication: verify weight-based dose, use an infusion pump, and independent double-check by a second nurse; distinguish heparin concentrations (e.g., 1,000 vs. 5,000 vs. 10,000 units/mL).
3. SC heparin and enoxaparin: abdomen at least 5 cm (2 inches) from the umbilicus; do not aspirate, do not rub; for enoxaparin prefilled syringes, do not expel the air bubble; pinch a skin fold and insert the full needle length at 90°; rotate sides.
4. Bleeding precautions: soft toothbrush, electric razor, avoid IM injections, hold pressure longer after punctures, fall prevention, test stools and urine for blood as ordered.
5. Neuraxial catheters: coordinate anticoagulant timing with anesthesia; perform frequent neurologic checks of the legs (numbness, weakness) and report at once.
6. Suspected HIT: stop all heparin (including flushes and heparin-coated catheters), notify the provider, send HIT antibody testing; a non-heparin anticoagulant (argatroban, bivalirudin, fondaparinux, or a DOAC) is started; do not start warfarin until platelets recover (limb gangrene risk); avoid platelet transfusion unless bleeding.
7. Warfarin dosing: give at the same time daily (often evening) so the dose can be adjusted after the daily INR.
8. Procedures: warfarin is usually stopped about 5 days before elective surgery (with or without bridging, per provider); DOACs are held about 1–4 days depending on the drug, bleeding risk, and kidney function.

• Take the dose at the same time every day; do not skip or double doses; do not stop without talking to the provider — stopping increases stroke and clot risk
• Report bleeding: gums, nose, urine (pink, red, brown), black or bloody stools, vomit that looks like coffee grounds, unusual bruising, heavy menses; seek emergency care for a severe headache, fall with a head strike, or sudden weakness
• Warfarin and vitamin K: keep intake consistent — do not suddenly start or stop eating green leafy vegetables; tell the provider before changing your diet or starting supplements
• Keep INR appointments; carry medical identification and an anticoagulant card
• Avoid aspirin and NSAIDs (ibuprofen, naproxen) unless prescribed; use acetaminophen/paracetamol for pain within dose limits; avoid herbal products without advice; limit alcohol
• Tell every health care provider, including dentists, that you take an anticoagulant
• Use a soft toothbrush and electric razor; avoid contact sports
• Warfarin: use reliable contraception; report pregnancy at once
• Dabigatran: swallow capsules whole; keep them in the original container
• Rivaroxaban 15 mg and 20 mg: take with the evening meal (or as directed with food)
• Enoxaparin at home: injection technique, sharps disposal; small bruises are expected

Elevated INR without bleeding (common approach): INR above target but below about 10 → hold or reduce warfarin and recheck; INR ≥ 10 → hold warfarin and give oral vitamin K as ordered. Any serious bleeding → hold the anticoagulant, apply pressure, maintain circulation, prepare reversal agents and blood products, notify the provider urgently.

• Anticoagulants prevent clot growth; they do not dissolve clots
• Heparin → aPTT (1.5–2.5 × control) or anti-Xa → protamine
• Warfarin → PT/INR (2.0–3.0) → vitamin K ± 4F-PCC
• Warfarin overlaps heparin ≥ 5 days and until INR ≥ 2.0 for 24 hours
• HIT: platelets fall > 50% at days 5–10 with new clots → stop all heparin; no warfarin until platelets recover
• Enoxaparin: keep the air bubble, no rubbing, adjust for creatinine clearance < 30; spinal hematoma boxed warning
• DOACs preferred for AF and VTE except mechanical valves and moderate–severe mitral stenosis
• Dabigatran → idarucizumab; factor Xa inhibitors → 4F-PCC in the US (andexanet withdrawn)
• Warfarin is teratogenic; LMWH is preferred in pregnancy
• Consistent (not zero) vitamin K intake with warfarin
• Amiodarone, metronidazole, TMP-SMX, fluconazole raise INR; rifampin lowers it