Warfarin is generally started on day 1 or 2 of heparin or low molecular weight (LMW) heparin therapy. Heparin or LMW heparin 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 1st 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.
Ansell J, et al. Pharmacology and Management of the Vitamin K Antagonists: American College of Cheset Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest. 133 (6Suppl): 160S-198S, 2008 Jun.
Bolton-Maggs P , Brook L. The use of vitamin K for reversal of over-warfarinization in children. Br J Haematol 2002; 118:924.
David, M., et al. Warfarin Therapy in Children. Thrombosis Interest Group of Canada. May 2004. http://www.tigc.org/eguidelines/warfarinchildren04.htm. Accessed 10/21/08.
Horton, JD, et al. Warfarin Therapy: Evolving Strategies in Anticoagulation. American Family Physician; 1999 Feb; 59(3):635-646. http://www.aafp.org/afp/AFPprinter/990201ap/635.html
Lexi-Drugs Online/Pediatric Lexi-Drugs Online, Warfarin, http://online.lexi.com/crlsql/servlet/crlonline, Copyright © 1978-2008 Lexi-Comp, Inc, Hudson, OH 44236
Monagle P. Chalmers E. Chan A. DeVeber G. Kirkham F. Massicotte P. Michelson AD. Antithrombotic therapy in neonates and children: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). [Journal Article] Chest. 133(6 Suppl):887S-968S, 2008 Jun.
Monagle P. Chan A. Massicotte P. Chalmers E. Michelson AD. Antithrombotic therapy in children: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. [Review] [487 refs] [Guideline. Journal Article. Practice Guideline. Review] Chest. 126(3 Suppl):645S-687S, 2004 Sep.
Roach ES. Golomb MR. Adams R. Biller J. Daniels S. Deveber G. Ferriero D. Jones BV. Kirkham FJ. Scott RM. Smith ER. American Heart Association Stroke Council. Council on Cardiovascular Disease in the Young. 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. [Journal Article] Stroke. 39(9):2644-91, 2008 Sep.