Remember that diabetes screenings are now payable by Medicare. You can bill Medicare twice per year for "pre-diabetic" patients and you can bill once a year for patients who do not meet the "pre-diabetic" criteria for the twice yearly screening.
The following are the billing codes you would use to bill the screening(s):
82947: Glucose, quantitative, blood
82950: Post-glucose does
82951: Glucose Tolerance Test, three specimens