Patients with hemophilia are usually seen by the hemophilia doctor every year at the annual comprehensive clinic visit. As part of this visit, the doctor usually orders routine blood tests, including the factor level and inhibitor titer.The factor level and inhibitor blood tests are always taken through a "peripheral stick." A peripheral stick means that a small needle is inserted into a vein in order to obtain a blood sample. The CVL is not used for these blood draws. This is because the investigators don't know if the infusion of factor replacement product and heparin (a blood thinning drug used to flush the line after a factor infusion) can cause the blood sample results to be incorrect. The current practice is to take blood through a needle stick in the vein because the investigators can be sure that the results are accurate.