Please consider the following hypothetical situation:

Joe is driving in the state of Georgia. He weaving in his land and stopped by a police officer. He is then arrested and his blood is taken. The blood is then tested. Joe has had no alcohol in the last 3 months and Joe has never used any illegal drugs.

What is the probability that the blood test will find that Joe has used illegal drugs?

1 Answer 1


What is the probability that the blood test will find that Joe has used illegal drugs?

Very low.

Errors When The Testing Is Accurate

The probability of finding the Joe has used alcohol are about 1 in a 1,000,000 which is about the likelihood that Joe has a very rare disorder that causes the body to produce alcohol on its own even if person never drinks it.

In the case of illegal drugs, there are some legal prescriptions and exposures to materials that could lead to a positive test.

For example, exposure to second hand marijuana smoke, or legal prescription use of amphetamines or opioids, or possibly even consumption of poppy seeds (opium is usually manufactured using poppy plant extracts).

Laboratory Errors

There are occasional cases of laboratories that test blood samples for illegal drugs faking results or using bad lab protocols. I can recall hearing about several of these results, but there have probably not been more than half a dozen or a dozen of these cases in the United States, in the modern era of Controlled Substances Act and Driving While Intoxicated enforcement. The tricky point there, however, is that any one laboratory that fakes its results or has a systemic flaw in its methods will often taint hundreds or thousands of cases. If one particular law enforcement agency uses the same lab all of the time, this could invalidate almost every single conviction based upon those tests for many years. So, when this happens, it has an intense localized effect.

But, absent that kind of issue, blood tests are extremely accurate. Probably much less than one in a thousand or one in ten thousand false positive cases.

If you do have some blood alcohol content, there are reasons that the exact amount might be wrong. But finding illegal levels of blood alcohol content in the absence of any alcohol consumption in the last three months virtually never happens, absent intentionally faked lab results.

  • 1
    In UK you have the right to request part of the blood or urine sample, so that you can get it tested independently. In the case of a blood sample, two samples are taken, and the suspect chooses which one they will keep themself. If they don't require one, both samples are sent to the police lab and only one is analysed. Feb 10 at 17:45

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