Table 9. Risk of thromboembolism on patients without anticoagulation therapy. (Not during withholding warfarin for procedure/surgery). (Adapted from Kearon C, Hirsh J. Management of Anticoagulation Before and After Elective Surgery. NEJM 1997; 336:1506-1511).

CONDITIONS	RATE* OF THROMBO- EMBOLISM	RATE PER DAY	RISK REDUCTION WITH THERAPY
VENOUS	.
Acute VTE	.
-- Month 0-1	40% / month	1.3	80%
-- Month 1-3	10% / month	0.3	80%
Recurrent VTE	15 % / year	0.04	80%
ARTERIAL	.
Acute arterial embolism	.
-- Month 0-1	15% / month	0.5	66%
NVAF	4.5% (1-20) / year	0.01	66%
NVAF and previous embolism	12% / year	0.03	66%
Mechanical heart valve	8%† / year	0.02	75%

• VTE = Venous thromboembolism. NVAF = Non valvular atrial fibrillation. * = Average
• † = Varies with type, position, and multiple prostheses.

(Rate of thromboembolism may be higher following holding warfarin and during surgical procedures - "rebound phenomenon")