Initiation of Therapy Days 1-4

• Warfarin is generally started on day 1 or 2 of heparin or low molecular weight heparin (LMWH) therapy. Heparin or LMWH administration should overlap with warfarin for a minimum of 6 days and until INR is within the desired therapeutic range on 2 consecutive days at least 24 hours apart when initiating warfarin therapy. In general, warfarin therapy should be initiated with consultation from hematology unless the patient is in a critical care unit or on the cardiology or cardiothoracic surgery service.

• Target INR:

  • 2.5 to 3.5 for patients with mechanical/prosthetic mitral valves or recurrent thrombotic events with a therapeutic INR

  • 2.0 to 3.0 for all other patients including patients with mechanical aortic valves

• Obtain blood for baseline INR/PT, aPTT

• Calculate initial (day 1) warfarin dose based on weight, co-morbidities and baseline INR

  • Patient with a baseline INR < 1.2 and no liver or Fontan co-morbidities or hemorrhagic risk:

    • 0.2 mg/kg PO as a single dose

    • Maximum dose 10 mg

  • Patient with a baseline INR ≥ 1.2 - Consult Hematology

  • Patient with liver dysfunction, Fontan procedure, presence of other hemorrhagic risk (hemodialysis):

    • 0.1 mg/kg PO as a single dose

    • Maximum dose 5 mg

• Obtain daily INR during initiation protocol

• Calculate subsequent (days 2 - 4 only) warfarin initiation doses based on the INR response (see Table 3)
  
  

Table 3. Adjusting Warfarin Dose for Days 2 to 4 ONLY

References

Ansell, J., Hirsh, J., Hylek, E., Jacobson, A., Crowther, M., & Palareti, G. (2008). Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest, 133(6 Suppl), 160s-198s. https://doi.org/10.1378/chest.08-0670



Bolton-Maggs, P., & Brook, L. (2002). The use of vitamin K for reversal of over-warfarinization in children. Br J Haematol, 118(3), 924. https://doi.org/10.1046/j.1365-2141.2002.03631_5.x



David, M., et al. (2004, May). Warfarin Therapy in Children. Thrombosis Interest Group of Canada. Retrieved Oct 21, 2008 from http://www.tigc.org/eguidelines/warfarinchildren04.htm.



Horton, J. D., & Bushwick, B. M. (1999). Warfarin therapy: evolving strategies in anticoagulation. Am Fam Physician, 59(3), 635-646.



Lexicomp Online, Pediatric and Neonatal Lexi-Drugs. Warfarin. Retrieved Oct 2008, from https:online.lexi.com.



Monagle, P., Chan, A. K. C., Goldenberg, N. A., Ichord, R. N., Journeycake, J. M., Nowak-Göttl, U., & Vesely, S. K. (2012). Antithrombotic therapy in neonates and children: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest, 141(2 Suppl), e737S-e801S. https://doi.org/10.1378/chest.11-2308



Monagle, P., Cuello, C. A., Augustine, C., Bonduel, M., Brandão, L. R., Capman, T., Chan, A. K. C., Hanson, S., Male, C., Meerpohl, J., Newall, F., O'Brien, S. H., Raffini, L., van Ommen, H., Wiernikowski, J., Williams, S., Bhatt, M., Riva, J. J., Roldan, Y., . . . Vesely, S. K. (2018). American Society of Hematology 2018 Guidelines for management of venous thromboembolism: treatment of pediatric venous thromboembolism. Blood Adv, 2(22), 3292-3316. https://doi.org/10.1182/bloodadvances.2018024786



Roach, E. S., Golomb, M. R., Adams, R., Biller, J., Daniels, S., Deveber, G., Ferriero, D., Jones, B. V., Kirkham, F. J., Scott, R. M., & Smith, E. R. (2008). Management of stroke in infants and children: a scientific statement from a Special Writing Group of the American Heart Association Stroke Council and the Council on Cardiovascular Disease in the Young. Stroke, 39(9), 2644-2691. https://doi.org/10.1161/strokeaha.108.189696