<qa>  What is the blood test done during blood thinning therapy?Blood tests are done during blood thinning therapy to monitor and adjust the dosage. Additional blood samples are taken before and at timed intervals after each rtPA treatment to measure the response to therapy. Patients who benefit from rtPA treatment remain on blood thinners for 5 to 7 weeks and then return to NIH for a follow-up venogram to see if the vein is still open. During warfarin therapy, blood tests are done every few days during the first week or two and every 2 weeks thereafter to ensure the optimal drug dose is being administered. If the repeat venogram at 5 to 7 weeks shows that the vein has closed, the blood thinners warfarin or heparin will be stopped and the patients participation in this study will end. If the vein has remained open, the patients doctor will decide whether or not to continue anti-clotting therapy. 