Therapeutic Drug Test

What is the correct PROCEDURE CODE for this scenario:

A physician prescribes digoxin for treating a patient diagnoses with congestive hear failure. After six months,the physician performs a therapeutic drug test to monitor the level of the drug on the patient.

re: Therapeutic Drug Test

80162 and 428.1- but are you sure you are allowed to bill for the lab? Are you actually running the assay in the office? If its a medicare patient, the lab must bill it.