DBI refers to "Dirt Bag Index", and multiplies the number of tattoos with the number of missing teeth to give an estimate of the number of days since the patient last bathed.
I heard another variation on this one from a cop recently: Multiplly the number of tattoos with the number of missing teeth, then subtract the number of times the guy says "fuck" in the first 15 seconds of contact to get the estimated number of baton strikes it will take to get him into handcuffs.