What does a urine tox screen test for? (And how do you interpret the results?)

The urine tox screen is one of the most superficially helpful tests we have. At first glance it seems like a simple “yes/no” answer. I mean, either someone did or didn’t take heroin…or did they? It turns out there is a lot more gray in interpreting the results of a urine tox screen, and it’s important to take your time thinking about the results lest you condemn someone unfairly–or let them get away with abuse.

When should someone be screened? 

It’s possible to stratify patients into high-risk and low-risk abusers but you really should screen everyone who is on a potential drug of abuse. Definitely at the first appointment, and then 2-3 times/year has been shown to be effective. If you are changing a patient’s dose substantially, or if they have more complaints about decline in function, that may be a good time as well.

Which is better, the urine or serum tox screen?

Actually, it’s urine. According to Path Group:

Drugs of abuse can be detected in urine for days to weeks after exposure, in contrast to blood detection which is generally in hours. For example, heroin has a half-life of 6 to 15 minutes in blood, but opiate metabolites may be detected in urine for 2 to 3 days.

Is it possible to get false positives on a urine tox screen?

Yes. One factor is individual pharmacokinetics. There are genetic differences in CYP450 enzyme activity, and if patients are on other CYP450-metabolized medications, they could have different levels of processing medications like opiates. Other factors include whether the patient is obese, what dose they are taking and how frequently, and more.

If you are concerned that there may be a false positive, order confirmatory testing with GC/MS (gas chromatography/mass spectrometry). This is highly specific and reliable.

Opioid Risk has a great summary of what could cause different “false positive” or “false negative” scenarios. (Go to “Interpreting Test Results” on page 5)

Here is a list of some caveats from HealthPartners:

Screen Shot 2016-04-04 at 7.41.54 PM.png

If a substance is detected, can you tell how long ago it was taken? (Reference)

Substances are detectable for a certain range of time, and the specific times may depend on your institution’s lab.

  • Alcohol 7-12 h
  • Amphetamine 48 h
  • Methamphetamine 48 h
  • Barbiturate Short-acting (eg, pentobarbital) 24 h
  • Long-acting (eg, phenobarbitol) 3 wk
  • Benzodiazepine Short-acting (eg, lorazepam) 3 d
  • Long-acting (eg, diazepam) 30 d
  • Cocaine metabolites 2-4 d
  • Marijuana Single use 3 d, Moderate use (4 times/wk) 5-7 d, Daily use 10-15 d, Long-term heavy smoker 30 d
  • Codeine 48 h
  • Heroin (detected as morphine) 48 h
  • Hydromorphone 2-4 d
  • Methadone 3 d
  • Morphine 48-72 h
  • Oxycodone 2-4 d
  • Propoxyphene 6-48 h
  • Phencyclidine 8 d

What substances will NOT be detected by urine or serum tox screening? [I’ll try to update this as I get more info] 

  • fentanyl

What are some ways that people try to thwart tox screening? 

The length of time a drug can be detected in the urine varies due to several factors, including hydration, dosing, metabolism, body mass, urine pH, duration of use, and a drug’s particular pharmacokinetics.

  • Someone may try to make a drug level “undetectable” by drinking tons of water to dilute their urine. Conversely, someone may try to make it seem like they do have an appropriate level of a prescription drug by dehydrating themselves to concentrate the urine.
  • This has actually happened: someone buys “clean” urine off the street and passes it off as their own during drug testing. This is why some institutions require supervised urination. There are ways to check for this, like temperature-sensitive cups, measuring the specific gravity of the sample, or creatinine concentration (these will all be different in stagnant urine that has not recently exited the body).
  • This is not “thwarting” per se, but if a patient is on a low dose or has long intervals between doses, they may have a false negative because the concentration of drug is below the cutoff on the assay.



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