Largely because it directly measures blood alcohol level, a blood test is by far the more accurate of the two commonly-used chemical test in DWI cases. That accuracy is reflected in the conviction rate for blood test cases. In many jurisdiction, this rate is over 95 percent.
But the United States Supreme Court recently held that police officers had to obtain search warrants prior to invasive blood draws. Many police departments do not want to take this extra step. That is, unless the driver happens to get pulled over during a so-called “no refusal weekend.”
Some Scientific and Legal Blood Test Basics
After an officer detains a motorist and determines that there is sufficient probable cause, the officer requests permission to administer a breath test. The driver has theoretically already consented to the breath test, but the driver can revoke that consent at any time.
Typically, if the driver refuses to submit to a breath test, that’s the end of the chemical test portion. But in a few cases, officers may contact judges to obtain search warrants.
Officers need warrants for two reasons. First, a blood test is a lot more invasive than a breath test. In the former, a stranger forcibly pulls blood from the body; in the latter, the defendant voluntarily blows into a tube. Second, the Breathalyzer discards the sample and the blood test preserves the sample.
Typically, officers can obtain search warrants over the phone and in a few minutes. But they usually only do so during high-enforcement periods. These are the aforementioned “no-refusal” weekends. If the driver refuses the Breathalyzer, officers will get a warrant and strap the driver to a hospital gurney.
Sometimes, there is an interesting twist in these cases. In some situations, a driver may refuse a breath test but give consent for a blood test. If that happens, and the officers do not administer the blood test, the jury may conclude that the officer thought the driver would pass the test.
Challenging Blood Test Results
As mentioned earlier, juries rightfully believe that these tests are highly accurate. Therefore, the results are difficult to challenge.
Difficult, but not impossible. Since the blood test preserves the blood sample, the defendant can retest it in another lab and with another doctor. These independent tests often come up with different results than the ones obtained from a police-affiliated doctor at a police-affiliated facility.