From Cheboygan, Michigan, USA:
My 14 year old daughter, who has type 1 diabetes, had been throwing up for over 6 hours, and had large ketones. I thought she was in ketoacidosis -- her insulin pump infusion site had come out at a friends house where she had been swimming all day - she thought since she was swimming that her sugar would be okay -- but she was at her friend's for over 7 hours and had also eaten dinner without insulin.
Anyway, when she was seen in our local emergency room, the doctor asked her for a urine test which I assumed was to check ketones, but later he came in and told me she tested positive for amphetamines and began giving her a lecture on taking drugs. This made absolutely no sense to me whatsoever. My daughter is a straight A, physically fit teen who is into dance, gymnastics, volleyball, etc. So, I asked that the sample be sent in for further analysis and a second test which were negative. She had taken zero medications (Tylenol, cough medicine, allergy medicine, no pills from her friends) -- nothing! Why did the initial test come back as a false positive? Could it have anything to do with her diabetes?
I don't know if I can address your questions best. Certainly, there are falsely positive test results (a positive test result even when the condition being sought is not really there), and there are falsely negative test results (a negative result when in fact the condition is present). This is why is often is a good idea to confirm a surprisingly positive or negative lab result -- when that result is not what was expected. Different tests require different machines and methods and testing agents. Indeed, sometimes a test can be done several different ways. Each test may have conditions that interfere with the measurement of the test being done.
A toxicology colleague of mine responded that often with cheap toxicology kits, you "get what you pay for" so he cannot explain a false positive without knowing the type of methodology used. However, in general, ketones, insulin, and DKA [diabetic ketoacidosis] should not interfere with a good amphetamine assay.
He pointed out that medicines like Sudafed (pseudoephedrine) and herbs like Ephedra will be positive in the assay, and the presence of acids (as in ketoacidosis) causes a higher excretion of those material into the urine. So, if the patient had received such a drug (or amphetamines themselves), ketoacidosis could make a sample more positive.
Additional comments from Dr. Larry Deeb:I don't confess to be an expert in drug abuse or drug testing. Nevertheless, the ER doctor has seen his/her share of kids on drugs. So forgive the doctor. Why did this happen? I don't know.
Original posting 17 Sep 2002
Posted to Other
Last Updated: Tuesday April 06, 2010 15:09:36
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