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:

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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.


True or false: “I had the BCG vaccination so I need to get a chest x-ray instead of the PPD”

False! Employee health workers may say that people who have gotten the BCG vaccination–usually from Asia, Africa, or the Caribbean–do not have reliable PPD readings and therefore need chest x-rays. One magazine article reports: “Current Occupational Safety and Health Administration (OSHA) regulations do not require periodic chest x-rays for a health-care worker who is PPD-positive unless symptoms develop.”

Furthermore, according to Ethnomed:

“Prior vaccination with BCG is not a contraindication to TB skin testing, and the CDC guidelines recommend ignoring BCG status when interpreting skin test results and selecting candidates for latent TB treatment. Although BCG vaccination can turn a skin test positive, reactivity due to BCG vaccination wanes over time. If it has been more than 5 years since vaccination, a positive skin test is more likely due to TB infection than vaccination. Furthermore, the larger the size of the PPD reaction, the less likely it is due to BCG. A recent meta-analysis found that reactive skin tests more than 15 years since vaccination or with more than 15 mm of induration were unlikely to be due to prior BCG vaccination.

Interferon-based blood tests such as the QuantiFERON® -TB Gold avoid the possibility of false-positives occurring from BCG vaccination, since cross-reactivity does not occur.”

Be warned that the Quant Gold may come back as “indeterminate” or “borderline” and then you may be in a bit of a pickle and have to get a chest x-ray anyway.

Questions you should make sure to ask someone who has had a past positive PPD:
1. How long ago was it?
2. Have you ever received the BCG vaccine?
3. Did you ever receive treatment for latent TB? (isoniazid)
4. Have you ever had a chest x-ray previously?


How long should I prescribe oseltamivir (Tamiflu) for?

“If I have the flu, shouldn’t I be taking Tamiflu?” is a question that you will hear over and over in the winter months in your primary care practice. But it’s important not to prescribe Tamiflu willy-nilly because (1) why would you go through 4 years of med school to become a prescribing bot (2) oseltamivir-resistant influenza is not a huge thing, but it has the potential to become one…

So who SHOULD get Tamiflu, and for how long?

Naturally, it depends. Uptodate has a lot of data/learning points that I summarize:

If a patient is exposed to someone with the flu and needs Tamiflu for prophylaxis, they would ideally start it 75 mg BID <48 h after exposure and take it for 7-10 days.

If a patient is diagnosed with flu and you prescribe it for treatment, realize that studies have only shown this medication to reduce symptoms for 1-2 days ONLY if the patient starts taking Tamiflu <48 h of being diagnosed. Evidence for whether Tamiflu reduces hospital stay, severity of symptoms, or mortality is mixed at best. People being treated for flu should have a 5-day course.

That being said, high-risk groups SHOULD be treated:

  • Immunocompromised, such as HIV or bone marrow transplant (a 1-2 week course is the longest that a patient should be on; Tamiflu is safe for at least a 6-week course but there’s no studies showing benefit to giving it longer)
  • “immunocompromised” patients with severe COPD/asthma, liver disease, renal disease, diabetes, active cancer (basically the same people who would qualify for an early Prevnar)
  • Pregnant women
  • Nursing home residents
  • ANYONE who needs to be hospitalized because of their symptoms


How to treat seasonal allergies

These treatment strategies work for allergic rhinitis AS WELL AS non-allergic rhinitis.

First-line therapy for nasal symptoms: steroid nasal spray.

  • Fluticasone propionate=Flonase
  • Fluticasone furoate=Veramyst
  • Mometasone=Nasonex
  • Budesonide=Rhinocort
  • Triamcinolone=Nasacort Allergy

Relief should occur within several hrs, but maximal effect may take days-weeks. Start with the max dose and taper down. May combine with nasal decongestants.

Oral steroids should be used only for severe allergic rhinitis that prevents the pt from sleeping or working, for a few days at a time. Avoid repeated courses if possible.

Itching, sneezing, and mild, intermittent sx: oral antihistamines.

First-generation: diphendyramine, hydroxyzine, chlorpheniramine. May cause sedation or paradoxical agitation, or anticholinergic effects (dry eyes, mouth, urinary retention)

Second-generation: onset of action within 1 hour

  • Loratadine=Claritin
  • Cetirizine=Zyrtec
  • Olopatadine=Patanol
  • Azelastine=Astelin, Optivar
  • Fexofenadine=Allegra

Antihistamines are also available as nasal sprays, and can have onset in 15 mins. They may be combined with steroid sprays (Dymista).

They can be combined with decongestants, like pseudoephedrine, which provides better sx relief than antihistamines alone. Side effects: HTN, insomnia, headache, and due to abuse potential, many have been replaced with phenylephrine, which is less effective than pseudoephedrine.

Cromolyn: Less effective than antihistamines, but better than placebo. Requires 4x/day dosing.

Leukotriene inhibitors: only montelukast approved for allergic rhinitis. ?May increase anxiety, dream abnormalities

Ipratropium bromide (Atrovent): useful as an adjunctive therapy for more severe sx, or nasal congestion associated with a cold

Other therapies: Netipot devices can be mildly helpful. Use boiled or distilled water. Complementary/alternative treatments include acupuncture, dietary therapy, herbal mixtures, which appear better than placebo although long-term safety data is lacking.

Can the influenza vaccine and PCV13 be given at the same time?


There were reports that when influenza and PCV13 vaccines were administered at the same time in adults, antibody response was decreased. However it has now been determined that those differences were clinically insignificant.

There were also concerns that giving influenza and PCV13 vaccines at the same time increased the risk of febrile seizure in children. The AAP recently released a statement that the benefits of these vaccines outweighed the potential risk of febrile seizures (which, remember, are not associated with further risk of epilepsy or seizure disorder), and that both should be given in the same visit if indicated.

What’s the difference between the PCV13 and PPSV23 vaccines, and who do I give them to?

PCV13 and PPSV23 are two Pneumococcus vaccines. They are important for helping prevent pneumonia, as well as other diseases caused by Pneumococcus, like bacteremia, meningitis, etc.

To clarify: PCV13 is called Prevnar and is made by Pfizer. PPSV23 is called Pneumovax and is made by Merck.

It is worth noting that babies get the PCV13 vaccine. If they are mistakenly given Pneumovax, they should get Prevnar as soon as possible because Pneumovax is not effective in kids <24 months.

ALL adults >65 should get BOTH pneumonia vaccines–just not at the same visit. Evidence shows that it’s best to give the PCV13 first and then the PPSV23 6-12 months later for better immunity (2 months if they have HIV or asplenia). PPSV23 is only 60-80% effective…but that’s still better than nothing.

Adults ages 19-64 should get the PCV13 if they are at risk for pneumococcal infection:

  • immunocompromised patients (HIV, malignancy, on steroids, etc)
  • Cochlear implant
  • CSF leak
  • asplenia

Human beings ages 2-64 should get the PPSV23 if they are at risk for pneumococcal disease:

  • patient groups listed above
  • chronic cardiovascular disease like CHF, cardiomyopathy
  • chronic respiratory disease like COPD, asthma
  • chronic liver disease, like cirrhosis
  • chronic renal failure
  • smokers >19 years of age
  • significant ETOH use
  • organ transplant patients

Here’s where it gets tricky: if someone got their first PPSV23 before age 65, they should get a SECOND dose 5 years after the first one. They should get a THIRD dose once they turn 65. But once they are 65, they don’t need any additional doses even if they have a risk factor and didn’t get one when they were younger.