The results of blood alcohol tests are routinely used as evidence in criminal cases in New Jersey. These tests are most frequently used to document the level of a driver's intoxication in DWI cases. Blood alcohol tests measure the amount of alcohol present in a person's blood at the time of test administration. If the level is above .08 percent, criminal charges may be filed.
It only takes a few minutes after drinking before alcohol becomes detectable in a person's blood. Blood tests are used in DWI cases because they are believed to be more reliable than breath tests. In some cases, a breath alcohol test is used to detect the presence of alcohol in a driver and then a blood test is used to confirm the results. Because blood alcohol concentrations peak about an hour after drinking, a blood test may register a higher level of intoxication than a breath test that was performed earlier.
Although breath alcohol tests can be administered by police officers at the scene of an arrest, blood tests are usually performed by trained medical professionals. A sample of blood is collected in a syringe and sent to a testing facility. Depending on the location of the testing facility, the results of the blood test can take a few days or weeks to be determined.
In New Jersey, drivers consent to a blood test just by operating a motor vehicle on public roads. Even with blood test results, a lawyer might be able to prevent the test from being used as evidence of DWI in court. For example, if the test was administered improperly or if the defendant was taking certain prescription medications, the test results could be deemed inadmissible due to inaccuracy.
Source: Web MD, "Blood Alcohol", November 05, 2014