Consider Z51.81, encounter for therapeutic drug level monitoring, as the primary diagnosis plus additional code Z79.01, long-term (current) use of anticoagulants. If the reason for anticoagulation is given/known, you may use that as the primary diagnosis instead of Z51.81 but still use additional code Z79.01. If the contusion is addressed/treated, you may also code that, but your post indicates the reason for the visit is to check the PT/INR.
The above is the information I have gotten from searching this topic on this forum, so you might also try a search for further information or to see how others handle it. I found slightly different ways, but the above is how I will code encounters for PT/INR monitoring.