Dear Dr. H,
I was recently thrown off probation because of a dirty urine test. Let me tell you the story. When I started probation, I was still smoking marijuana. So of course, my first urine test came up positive for THC. My PO told me that this was okay as long as I stopped using right then. He said that as long as the concentrations on my subsequent tests went down, he would not violate me.
Things went well at first. The second test was indeed lower than the first. The third test was negative. I figured I was out of the woods but the fourth test was positive, although at a level lower than the second test. As a result, my PO violated me and I am now doing a year in the county jail.
Please help me. I didn't use any drugs. Why did my fourth test come back positive?
Rotting in Jail
Before I start, I want to point out that you marijuana smokers have been none too kind to us heroin users. You have been as bad or worse as the general public at vilifying our drug of choice. NORML, in particular, is very keen on making arguments like, "if marijuana were legal, law enforcement could concentrate on stomping out dangerous drugs like heroin." But since you are in jail and you bring up an interesting issue, I will take pity on you and explain.
Drug testing is a billion dollar a year business in this country and given that its use is probably the single greatest threat to the little freedom we have left, it may come as a surprise that as a scientific enterprise, it is poorly run. Those who do the actual testing understand the uncertainties with the whole enterprise but those who use the results, do not. The worst offenders are judges, prosecutors, and probation officers, because they have no one to whom they answer.
When a urine test comes back negative for THC (or any other substance) it does not mean that there was no THC in the urine. All it means is that the THC concentration was below some set limit determined by the test. This is called the "detection limit" of the test. Think of it this way: when you look at an eye chart, there are some letters which are large enough for you to identify and others that are just a blur. The blurry letters are below your visual detection limit. The letters are still there, just like the THC.
So if the detection limit of this particular test is 90 ng THC per ml of urine, a negative test means only that the THC level was less than 90 ng/ml. For example, 89 ng/ml would be called negative. Note that this is not 0 ng/ml even though a negative test is normally assumed so.
Everyone notices that sometimes their urine is a dark yellow whereas other times it is colorless. The darker the urine, the more impurities are present. As a result of this, a person's urine THC will vary throughout a day being higher when the urine is dark and lower when the urine is light.
So what happened with you and your PO? I can't say for sure without having the numbers from the tests. But I believe that you are telling the truth. When you took your third test your urine may have been fairly dilute and so the THC concentration fell below the detection limits of the test. When you went for your fourth test, your urine may have been much more concentrated and so the THC concentration was back above the detection limits even though the amount of THC in your body had declined between the two tests.
There are two things to be learned here. First, always drink a lot of water when you are subject to urine testing, even when you are not using any drugs; doing this will lessen the chances of a false positive. Second, people in the legal systemand even those working in the testing industryare often very ignorant of the errors inherent in drug testing and their ignorance can and will be used against you in a court of law.