A breathalyzer measures the percentage of alcohol in a person's bloodstream by analyzing a person's breath.  The higher the percentage, the more intoxicated the person.  While a breathalyzer is the most commonly known test, it is also possible to test the driver's blood or urine.  Under current South Carolina law, a driver with a blood alcohol concentration of 0.08% or higher is guilty of driving with an unlawful alcohol concentration, which is basically a DUI. 

Keep in mind, that because people under the age of 21 are not allowed to consume alcohol, an underage person can get a DUI for having a blood alcohol percentage of 0.02% or higher. 

S.C. Code Sections 56-5-2933 and 56-1-286.