How do drug test UAs work?

Factual Questions
1

I am prescribed a stimulant medication for treatment of ADD. Apparently, one of the new standards of care at my clinic is to have periodic urinalysis tests for drugs of abuse when being prescribed controlled substances so that my prescriber can check that I don’t have any addictive processes working in me. I submitted my first sample for this the other day and got my results today. Unsurprisingly, it showed a positive result for amphetamines, and negative for everything else.

At the bottom of the test, it says that a spectrometry analysis they ran on my urine also showed evidence of acetaminophen, amphetamine, diphenhydramine, metformin, and ranitidine. None of those is especially surprising, as they’re all either medications I am prescribed or may have taken for symptoms I had in the previous 24 hours. I am curious, though, because that does not represent a sum total of all the medications I take.

So, are my following surmisings correct?

  1. The spectrometry is designed to confirm the results of the primary (presumably reagent-based) tests.
  2. An additional purpose of spectrometry is to specifically test for medications which are known to cause false-positive or otherwise oddball results.
  3. No one really cares that I’m taking Lipitor, because it has no known abuse potential or effect on the results that people do care about. As such, they don’t specifically look for it.

BTW, I am not looking for medical advice or ways to beat UAs here, so would appreciate if people would try to steer away from these hot-buttons.

2

IIRC the actual reason they do those tests is to make sure you are actually taking the medication and not selling/diverting it to someone else.

3

Ah, that make sense as well. I worked at a methadone clinic, and we tested patients for methadone metabolites for the same reason.

4

From what little I know about mass spectrometry, I understand that a given instrument or set of run parameters can only detect molecules in a range of masses.

At first glance, mass sets apart Lipitor (molecular weight 558 g/mol) from the rest of the medications you mention (ranging from 129-314 g/mol). That’s consistent with them looking for molecules of a similar size to amphetamine (135 g/mol).

It may also be that this drug testing lab is usually in the business of detecting illicit drug use. In that case, they might have two methods of detecting amphetamine to reduce the false positive rate. As a general matter of ethics, liability, and I hope some industry or legal standards, they had better be certain of their results since a positive result could get someone fired or sent to jail.