Answer: If the patient has no gastrointestinal signs or symptoms, you should use diagnosis code V76.51 (special screening for malignant neoplasms, colon). Don't expect to get reimbursed for the consultation, however, even though it appears you are doing a thorough workup of the patient.
In general, Medicare does not pay for preventive services. Neither do most commercial insurers. Medicare's coverage of screening colonoscopies and flexible sigmoid-oscopies is the exception to that rule, but it will not cover the office visit that most gastroenterologists require before they perform the screening procedure.
Many gastroenterologists have the patient sign a waiver acknowledging financial responsibility for the consultation. When the patient calls to set up the office visit, he or she is informed that consultation is not a covered service, and that he or she must pay for it.