Diagnosis Code for Billing Drug Testing

Hello, 

We have a drug testing lab that does screenings. We have been billing drug testing codes 80307 and 80305 for our own treatment clients for a few years now, and we are just now gearing up to do drug testing for clients referred to us from outside of our own organization. My question has to do with entering a diagnosis code for a client who is coming to us from outside of our own organization. With our own treatment clients, they already have a diagnosis assigned by the clinical staff. With external clients, we won't know the diagnosis or they may not have one. Is there a catch-all diagnosis for drug testing that can be used? I found Z03.89. Does a clinical staff person have to assign the diagnosis? I'm doing the billing for service provided by our lab and there would be no clinical staff to individually assign a diagnosis to each client. We do, however, have a medical director who writes standing orders. I'm hoping someone with experience doing this can give some guidance on my diagnosis questions. Thanks.