Clotted while on Warfarin

I have being on warfarin for 6 months do to a blood clot behind my right knee and the lower part of my femoral vein.  Have being therapeutic the entire time on warfarin and side effects were not to bad.  I was due to come off and had another doppler just to check how much of the clot was still there.  I still had residual clot and they say 2 more smaller ones behind my knee but lower than the original.  How can this be?  Is it possible these new clots were missed on the first doppler?  They only went from thigh to knee on my first doppler and on the second they went thigh to ankle.  Also can they tell the difference between old and knee clots? Is it possible that the clot my have spread right after my first doppler before I got on medication?  My first clot was found about 1:30 in the afternoon and I wasn't given Lovenox until about 8:00 that night.  Partly because they didn't know what dose to give.  Could it have gotten bigger in those few hours?  I guess I am really hoping that I didn't clot on the warfarin.  Haven't being getting too many answers from my doctor.She is sending me to a specialist. So now I am off Warfarin and on daily injections of Tinzaparin for at least 3 months.

[Dr. Kokil Mathur]

Hi!
Yes, it is possible that clots were missed on the first dopplar. Also, the radiologist can differentiate between old and new clots. If you are on any blood thinner then your prothrombin time and INR values should be monitored regularly either weekly or every fortnight or once a month depending on the need and risk. If your INR values remain below 2, then there are great chances of clot formation despite being on blood thinner. Please consult a specialist as soon as you can. Meanwhile please ask your doctor to monitor your INR. Take care!

The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.