photo © 2011 Francis Storr | more info (via: Wylio)I see a lot of drug screens. As the National Medical Review Officer for 36,000 companies, I spend a lot of time answering questions about them. The most frequent category of question involves the numbers seen on positive urine drug screens results. I will review what these numbers mean and what they can and cannot tell you. I will try to clear up the cloudy urine, so to speak.
We’ll start with cut off levels. There is a simple concentration level that determines the “line drawn in the sand.” Any value equal or above the limit is positive, and those below the limit are negative, kind of like Black Jack.
Now to complicate matters, every positive drug screen goes through two different testing procedures. The first is called an immunoassay – this test is highly sensitive and not very specific. They can detect several variations of a drug (drugs break down into several parts in the body). In the case of marijuana, the cut off of the immunoassay is 50 ng/ml. Either the test is positive (above 50 ng/ml) and goes on to further testing, or it’s below 50 ng and is reported as negative. There is never an immunoassay numeric result.
Any positive initial test (immunoassay) will undergo a second testing of a different type-GCMS confirmation. This stands for Gas Chromatography Mass Spectrogram (don’t worry – there won’t be a test later). This is an extremely specific test that identifies one single drug variant in the urine. Because it identifies a single molecule type, the cut off level is low, 15 ng/ml in the case of marijuana. The GCMS result is the only number reported. The lab reports the test as negative if it is below 15 ng/ml. If it is above 15 ng, a quantitative number is given, like 38 or 300.
That is a lot to write down, but we are just talking a cut off level. We see cut off levels everywhere in life – 90% in school gets you an “A,” 89% a “B.” Driving 75 mph on the highway gets you there, while 80 mph gets you a ticket.
So, let’s say we have a drug screen that is positive for cocaine at 300 ng/ml (the cut off cocaine is 100 ng/ml on GCMS). Every day a company wants to know if the donor was under the influence at the time of the drug screen. The simple answer is: “I have no idea.”
That information isn’t available with a urine drug screen. The reasons are several. The most important is the concentration of drug in the urine is not the same as the concentration of drugs in the brain. Drugs in the urine don’t affect you; drugs in the brain do. The other reason a drug screen won’t reveal impairment is there are too many variables (unknowns). Among these are the donor’s dose, timing, body weight, food and drink intake, kidney and liver function. All affect the level of the drug in the system. So for a given value in urine, the donor could have arrived at that by taking hundreds of different combinations of drug amount and timing before the drug screen. So we can say the cocaine was definitely present in the urine, and it was a positive drug screen. The positive result cannot tell you if the donor is a long-term cocaine user or used it once in his life right before the drug screen. And it can’t tell you whether he was impaired (high) at the time of the drug screen.
So a urine drug screen only places the person into a user or a non-user group. We unfortunately don’t have chronic users groups and tried-it-only-once user groups to pick from.
– Dr. B