How do breath tests differ from blood tests in a DUI case?

Although officers use breath tests to measure drivers’ blood alcohol levels, the results can vary from actual blood tests.

People in Pennsylvania who have been pulled over on suspicion of drinking and driving will often be asked to submit to a roadside breath test. Law enforcement officers in the state use these devices to help them determine whether the driver of the vehicle is actually intoxicated. The machines give officers a blood alcohol content reading that is supposed to be comparable to the BAC reading officers would get from an actual blood test. Yet the accuracy of breath tests has come under fire as multiple studies show that the blood alcohol content levels calculated from breath test machines are not equivalent to those obtained from blood tests.

In fact, researchers at the State University of New York at Potsdam found that the BAC results obtained from a breath test device can vary by as much as 15 percent when compared to the BAC reading from a blood test.

How they work

Blood tests measure the concentration of ethyl alcohol contained in the blood directly from a blood sample. Breath tests, on the other hand, determine the driver's blood alcohol level by detecting the presence of alcohol found in the driver's exhaled breath sample. The machine then converts the breath alcohol content to a blood alcohol content. Anyone found driving with a BAC level of 0.08 percent or higher in Pennsylvania runs the risk of getting charged with a DUI.

Factors that affect breath test devices

The problem stems from the fact that breath test devices detect more than just ethyl alcohol groups. There are other substances in the environment and in the body that can alter breath test readings. These include the following:

  • Gasoline, paint remover and cleaning fluid fumes.
  • Dirt, tobacco smoke and moisture in the air.
  • Residual vomit, food or blood in the subject's mouth.
  • Electrical interference from police scanners, radios and cellphones.

In one study, a subject spent 20 minutes spray painting a room and was then given both a blood test and a breath test to determine the BAC level. Surprisingly, the breath test showed that the subject's BAC level was .075 percent, which is close to the legal limit. The blood test, however, indicated that there was no alcohol present in the test subject's system.

Upholding your rights

If you have been charged with a DUI, then you may feel overwhelmed at the thought of paying multiple fines and losing your driver's license. Furthermore, you may be required to use an ignition interlock device on your vehicle and you might have problems finding a job in certain industries. A criminal attorney in Pennsylvania who has experience in DUI cases may be able to help you explore your legal options and defend your rights in court.