Screening prior to CT scan

What diagnosis code would I use when ordering a BUN and Creatinine prior to a CT scan?

I am an imaging center part owner

and I called my billing people- they are checking on what they do. Some they said that they bill with the diagnosis for the test- headache, abdominal pain. I would think that the BUN and Cr will be paid for most diagnoses that require a CT scan and in the office I have never had one denied when ordered whether I do a CT or not. (In other words, checking kidney function in a patient with abdominal pain or headache is medically appropriate as renal failure can cause pain, headache, etc)

Just be sure you call the referring doc to be sure their office did not do one!