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What is the difference between the threshold cutoff of <20…

Customer Question
What is the difference...

What is the difference between the threshold cutoff of <20 and <50 for a urine drug test

Doctor's Assistant: Is this a time sensitive issue? What type of specimen are they testing?

They tested my son's urine which came out negative. the school asked for a threshold of <20 and the Lab tested at <50

Doctor's Assistant: Anything else in your medical history you think the doctor should know?

No other than we just need clarification or an explanation why the school would not be happy with the <50 thank you . I just need a clear explanation of what the difference is in a threshold of <20 and <50

Submitted: 24 days ago.Category: Medical
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Answered in 9 minutes by:
6/1/2018
Doctor: Dr. Muneeb Ali, Doctor replied 24 days ago
Dr. Muneeb Ali
Category: Medical
Satisfied Customers: 9,665
Experience: MBBS, MD, MCCM. Currently working in Critical Care Medicine with 10 year experience in Medicine
Verified

Hi,
Welcome to JA and thanks for this question. Actually, the threshold cutoffs indicate the minimum amount of drug which can be detected by the test in a patient's system. So basically when we are talking about a cut off of <50, we usually a referring to a level of less than 50 ng/ml (nanograms per milliliter). This means that if the level of the drug is less than 50 ng/ml then the test would be considered as being NEGATIVE for the drug. In other words, a test with a cut off <50 ng/ml means that the test is able to detect the drug ONLY when its concentration in the body is ABOVE 50 ng/ml. if the concentration is less than 50, the test would come back as negative.

Similarly, with a cutoff value of <20 ng/ml, the test would be more sensitive. This means that this test would be able to detect a drug in the body if its concentration is above 20 ng/ml. This means that if a person had ingested a very small amount of the drug a few days back, then this test would be able to detect even that small quantity in his system when the 50 ng/ml cutoff test would have come back as negative. So a 20 ng/ml test would come back as POSITIVE if the concentration of the drug was greater than 20 ng/ml in a patient's system and NEGATIVE if the concentration was less than 20 ng/ml.

So simply put, the lower the cutoff value is, the greater the chances are for the test to pick up on even the tiniest quantity of drugs in a patient's system. <50 threshold tests may miss some drugs in a patients system because they cannot pick up on extremely small quantities of drugs whereas <20 cutoff tests would be able to pick up on them easily.

In your son's case, the school is asking for the <20 ng/ml test in order to be absolutely sure that he does not have any drugs in his system.

Does this explain the difference clearly and to your satisfaction? If not let me know and we can discuss this further.

Do you have any more questions?

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Customer reply replied 24 days ago
the test is still legitimate with negative findings with a reading of Final. the school is on a witchhunt. the first test came back as a negative dilute tested at <20. we gave the lab the same cut off values this time for the "retest" and the same lab used <50 as the cutoff for some reason. we are 100% certain that the test would have come back negative regardless of the threshold because the school has an agenda for random drug testing and they wanted him to test positive to prove a point
Doctor: Dr. Muneeb Ali, Doctor replied 24 days ago

I would agree with you as it is less common that a patient who tests negative at 50, would test positive on the 20 threshold test. Yes, it is a more sensitive test but usually, we don't go for a lower cutoff than 50 when screening for drug use. It is in special situations where the suspicion of drug use is very high that we consider going for a 20 threshold test.

I agree with you that additional testing is not necessary once two tests have come back as negative regardless of the testing threshold, the only problem here is that if the first test result indicated that the sample was diluted (regardless of the reason - which could be increased intake of water or an underlying medical condition etc) then they would become suspicious and ask for retests. So if the initial result showed a diluted sample that does raise suspicion and retests are ordered.

But I agree with you, that even if you get him retested again with a 20 ng/ml threshold it is going to come back as negative.

Do you have any more questions?

I sincerely ***** ***** I have helped you and that I have earned my 5-star rating today! Please remember to rate my service by selecting the 5 stars at the top of the screen (rating me now does not close your question). We can continue here until you are satisfied, simply use the reply box and let me know. Thank you!. You can also ask questions from me directly by going to the link below,

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Doctor: Dr. Muneeb Ali, Doctor replied 21 days ago

Would you like to know anything else? Any clarifications or additional questions that you might have, i would be happy to answer them for you. If done for now, please remember to leave a 5-star rating as that is the only way we are compensated for our time spent helping you. You can continue to ask follow-up questions even after the rating.

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