Pathology/Lab Coding Alert

Question: A patient comes for a prothrombin time test, and the ordering physician states that the test is to monitor the patient's Coumadin response, and the patient's underlying condition is atrial fibrillation. How should I assign diagnosis code(s) for this scenario?

Ohio Subscriber

Answer: First code the reason for the encounter, which is V58.83 (Encounter for therapeutic drug monitoring). ICD-9-CM instructs you to "Use additional code for any associated long-term (current) drug use." In the case of Coumadin, you should report V58.61 (Long-term [current] use of anticoagulants). Finally, you should report the underlying condition, which in this case is 427.31 (Atrial fibrillation).
 
Had the physician not mentioned the underlying condition, simply reporting V58.83 and V58.61 would be adequate. Code V58.61 is a listed diagnosis that indicates medical necessity for prothrombin time (85610, Prothrombin time), according to the National Coverage Determination for that test.