Fast Facts

• 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 (International Normalized Ratio) 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.

• Warfarin loading period is 4-7 days for most patients before a stable maintenance phase is achieved. Anticoagulation may be seen within 24 hours due to inhibition of factor VII but peak anticoagulation activity is not achieved for 72-96 hours due to factor II inhibition (2-3 days after 1^st therapeutic INR is achieved).

• Warfarin inhibits thrombin formation by interfering with vitamin K metabolism. Age affects the degree of inhibition. Warfarin is rarely recommended for children < 2 months of age.

• Available products at CMH:

  • Warfarin 1 mg, 2 mg, 4 mg and 5 mg tablets.

  • Doses should be rounded to the nearest 0.5mg wherever possible to allow for dispensing in tablet form.

  • Parents may be instructed to crush tablets, mix with water, measure dose and administer immediately if smaller increments are essential.

  • INR is calculated from the measured PT. If PT is ordered, INR will be calculated.

• INR/PT is used to monitor the effects of warfarin.

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. Kcentra. Retrieved Oct 2008, from https:online.lexi.com. 

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