What is the difference between oral fluid and urine drug testing?

By Work Place | Last updated: January 16, 2019

The important thing to understand about urine and oral fluid drug testing is that they are both very accurate and the sensitivity at the lab is essentially equal (they are both very reliable and valid testing methods).

The major difference between the two is the time frame that a drug will test a positive in the specimen for. What I mean by this is oral fluid is a fluid that is made from blood serum, where urine is a fluid that has waste that your body doesn’t need anymore (this fluid has been processed through the body). This is a very simplified version of the difference, but for the time being it should suffice for the explanation for why a substance would show up in one sample but not the other (I will release something more in depth and much more scientific in the coming weeks). To better explain this, I will describe a drug going through the body and where it will likely be in which drug specimen.

Let's say a person ingests cocaine only once with no previous use. No matter the method of ingestion, within minutes the drug is present in the blood and circulatory system. The circulatory system takes the blood around the body with the cocaine in it, taking it to the brain where the drug has its effect on the body. In the path around the body, some of the blood serum is converted into saliva or oral fluid. What this means is that any drug that is in the blood will be present in that person’s oral fluid. Within minutes of ingestion, the drug will show positive on an oral fluid drug test, and will continue to show positive in the oral fluid until the drug is out of the blood (12-24 hours depending on the drug and concentration).

After a few hours, the body starts to process the drug out of the blood and out of the body. To do this the blood has two cleaners that it uses: the liver and the kidneys. The cocaine drug is cleaned out of the blood over time and converted into cocaine metabolites. This process in the kidneys and liver is very slow and takes several hours to completely process the drug out. The body then gets rid of these metabolites in a few ways: you sweat it out, it grows out in your hair, you urinate it out, and eliminate it through defecation as well. From the time our subject ingested the cocaine until it shows up in his urine is roughly 4 hours, and will continue to show positive for approximately 48-72 hours (remember this example is for a single use, with recurrent use these times being longer, and for THC these time frames are very different). Looking at these time frames, we know that the drug is out of the blood, and therefore the oral fluid, within 24 hours. The process for the “cleaners” takes roughly 48-72 hours.

Looking at the example above we know we can know the following:

Positive Level Time Frames:

  • Oral Fluid: 0 hours - 24 hours
  • Urine: 4 hours - 72 hours

If a test is done at 2 hours after ingestion of the drug, an oral fluid drug test would be positive for the drug, but a urine test would be negative as the body has not processed the drug out yet. On the other side of the coin, if a drug test is performed at 30 hours after ingestion, an oral fluid drug test would be negative as the drug is completely out of the blood and therefore oral fluid, but a urine test would show positive as the body is still busy processing the drug through the liver and kidneys and excreting it out in the urine. It is easy to see there are ways for either an oral fluid test to be positive and a urine test to be negative, and visa-versa. Because of this it is never recommended to use an oral fluid and a urine drug test to verify each other.

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