Dr. Clive Kearon,from McMaster University, has suggested the concept of a two-phase anticoagulant treatment of of vein thrombosis and pulmonary embolism.See here. A full text version can be accessed from J Thromb Haemostis 2012;19: 507-511 entitled A conceptual Framework for two phases of anticoagulant treatment of venous thromboembolism.
Kearon , and his coworkers at McMaster University in Canada are accomplished researchers in the field of thromboembolism and his thoughts deserve serious attention.
Kearon proposes that four observations provide strong support for this two-phase concept:
1.Treatment for less than 3 months is associated with a higher recurrence rate
2.Treatment for 6 months has the same recurrence risk as treatment for 3 months.
3.The recurrence after too short a treatment predominately occurs at the site of the original thrombus.
4.The recurrence after too short a treatments typically occurs immediately after stopping treatment.
These observations suggest to the author that at first the anticoagulant therapy treats or somehow"turns off" the acute thrombotic process and prevents extension of the clot and reduces the risk of a pulmonary embolus. This theory is consistent with his observations listed above as 3 and 4.
Kearon's two phases are :
1. active treatment phase during which there is a rapid decrease in the risk of recurrence
2.secondary prevention phase, which is as long as the anticoagulation is continued.
Obviously the two phases overlap.
In some patients there are recurrence DVTs when anticoagulant therapy is stopped, whether it be at 3 months or 6 months or several years.So now the way of thinking is changing from should we treat for 3 months or 6 months to three months or extended anticoagulation with periodic reassessment of risks versus benefits, a process that is at best often a semi-educated guess with with broad error terms and is euphemistically described as "determining". Maybe followup D-dimer testing has some role here.