This is our local MAC's guidline:
Diagnosis Coding
When billing for therapy services, include the ICD-9 code that identifies the medical condition necessitating the therapy. It is not necessary to use the V57.1-V57.89 codes as primary or subsequent codes. However, if providers do use them, they must also include the diagnosis code of the specific medical condition for which each therapy service was provided.
Bill the most relevant diagnosis. As always, when billing for therapy services, the ICD-9 code that best relates to the reason for the treatment shall be on the claim, unless there is a compelling reason. For example, when a patient with diabetes is being treated for gait training due to amputation, the preferred diagnosis is abnormality of gait (which characterizes the treatment). Where it is possible in accordance with State and local laws and the contractors Local Coverage Determinations, avoid using vague or general diagnoses. When a claim includes several types of services, or where the physician/NPP must supply the diagnosis, it may not be possible to use the most relevant therapy code in the primary position. In that case, the relevant code should, if possible, be on the claim in another position.
Codes representing the medical condition that caused the treatment are used when there is no code representing the treatment. Complicating conditions are preferably used in non-primary positions on the claim and are billed in the primary position only in the rare circumstance that there is no more relevant code.