Expert: Dr. Ravindra Bhaskar Ghooi - 4/29/2009

Question
Okay, I am curious for an explanation or even just an estimate or guess of what the value of 80 ng/ml means on a urine THC test. The test result page states the test ordered was a 4846N Marijuan Metab GCMS Quantitative, and says 80 ng/ml which is circled then next to it says GC/MS Reporting level 5ng/ml. The test is my fiance's and he got the test done 20 days after he had used (or that's what he said)... and I purposely had the test done to see if it could tell me roughly how long it had been since he had smoked marijuana. If he is being honest with me, he says he was smoking approx. four puffs off of a pipe per day - about 4 times a week for six weeks. Then he said he stopped and I had the test done 20 days after the day of his last use. I want to know if there is anything you can tell me about this level on his test and if it indicates he was still using for the 20 days he said he wasn't or if it could mean he was truthful and hadn't been using for a while. Please help??!! THanks

Answer
Hi Cherry,
The sensitvty of a test is measured by two different parameters:
1. LOD: Level of detection
2. LOQ: Level of quantification.
As per Beer's law, the absorbance response is proportional to the concentration, but this holds good between certain concentrations only. As a result the instrument may detect 5 ng of the minor metabolites of marijuana, it cannot accurately quantitate them.
Now coming to your question, let me be frank, it is impossible to answer it one way or the other.
After consuming marijuana in any form, the drug enters the blood achieving a concentration that depends upon the amount consumed. Over time it gets excreted from the body and the half life is over 8 days, that means the level of the drug or its metabolites will drop by 50% every 8 days. Let us say the initial level was 100 ng/ml in body fluids, after 8 days it will be 50 ng/ml and after 16 days it will be 25 ng/ml and so on. If we knew what the level was on the day he used the drug we would be able calculate the time when he took the drug. Alternately if we are sure when he took the drug we can work back to find out what amount he consumed. If we know neither, we cannot work out the details.
I know this is a bit complicated, and if I have not been clear, I am sorry. I am used to teaching post graduate and doctoral students, and often find it difficult to put things in simpler terms. But believe me the issue is complicated.
Ravi Ghooi