We have a Warfarin Management Program here and we have been coding the reason the patient is on warfarin (i.e. a-fib, history of PE/DVT, etc.) and then we use Z79.01 for long-term anticoagulant use. One of the coders said that we should also use Z51.81 for therapeutic drug level monitoring. I feel like we are not really monitoring the level of a drug but the effect a drug is having so is the Z51.81 code really relevant?