DETROIT -- Every 30 minutes it happens.

Someone is killed in a drinking-related car accident.

Last year, nearly 600 people in Michigan were killed drinking and driving. But could your life be saved by taking a simple test before getting behind the wheel of a vehicle?

These are personal alcohol tests. They promise to give you immediate blood-alcohol-level results.

A test lower than .08 gives you the green light to drive.

Macomb County Sheriff's Department Lt. Brenda Baker: "If I have to check to see am I drinking too much to drive then I am already drinking too much to drive."

Baker helped Local 4 conduct a test Wednesday. These women were picked by police. The drinks are poured. The women start drinking.

An hour later the alcohol is taking effect.

First we focus on this $1.50 Guardian Angel One-Step alcohol test.

Baker: "We're going to tear open the pouch, place tester on end of tongue soak it for 10 seconds. We're going to wait two minutes and look at it. We're going to match the darkest color on the test pad to the meter below."

Two minutes later, we get Cathy's results. According to Guardian Angel, Cathy has a blood-alcohol level of at least .08. Michigan law says .08 is legally impaired.

Now, we compare these results with a preliminary breath test used by police.

Baker: "Ready, take a deep breath. Blow hard, blow hard, keep blowing."

The results -- .04 --- legally able to drive. The PBT shows the Guardian Angel is not accurate.

After about 3 1/2 drinks we have Lynn take the Guardian Angel test.

According to Guardian Angel, Lynn's blood alcohol level is below .04 -- not drunk and able to drive. But the police PBT test tells a different story. It shows Lynn's blood alcohol level is actually .07, just shy of being legally impaired.

So why did Guardian Angel fail?

The Defenders discovered it may be because we bought in April. It actually expired in March. And you can't read the expiration date until after you purchase the product. It is only printed inside the sealed packet.

We showed this problem to Jeff Schult, vice president of Guardian Angel.

Schult tells us because of our investigation, the company will re-design the product's packaging.

We also test this product called Safe-Slim, purchased online for about $5.

Cathy's up first. Legally drunk, according to the police PBT test, she blows into the Safe-Slim.

The red and green lights both appear telling her it's unsafe to drive. The Safe-Slim does not indicate blood-alcohol level.

Next, it's Lynn's turn. The Police PBT says she, too, is legally drunk. She gets the same results from the Safe-Slim, indicating she is too drunk to drive.