Blood alcohol content for purposes of DUI prosecutions are often obtained by withdrawing blood from the defendant. Depending on the county, analysis of the blood could take a couple of days or over a month.
It is possible that, at the time the needle is injected into the arm to withdraw blood, sugars and microorganisms located on skin at the injection site can be withdrawn into the vial along with the blood sample. Although the common practice of technicians taking the blood is to swab the injection site with hydrogen peroxide–thankfully, not rubbing alcohol—contamination of the blood sample is still possible. If analysis of the blood takes place a month or more after giving the sample, it is possible that those sugars and microorganisms may ferment in the vial of blood and cause the blood alcohol content to rise. Although the chances of this happening are extremely slim, the theory has been used successfully to create reasonable doubt in DUI cases.