Having a negative urine THC test later turn positive is more common than you think. Here is how to avoid it.

You spend weeks trying to detox yourself from THC and finally become negative. How disappointed would you be if you test yourself the next day (or even a week later) and you are positive again? This happens all of the time!

For some people, this has serious implications. One person told me a story about how his parole officer administered a urine test that showed he had higher THC metabolite concentrations compared to his previous test. He hadn’t actually used cannabis in that time (after all, why would he lie to me about it?) But did the PO believe him? Nope, he was sent back to jail and missed the plans he had to spend the 4th of July weekend with his son. Terrible!

So many people have told me stories like this. Parole officers, judges, employers, and even doctors do not understand that the results of urine THC drug tests can be all over the place. An increase in urine levels does NOT necessarily indicate new use.

How Much Can Urine THC Metabolite Levels Vary?

Let’s take a look at some individuals from a study where every urine sample was measured for an entire week. These subjects were regular cannabis users who spent the entire week inside the study unit to ensure that they didn’t use any more cannabis during the THC washout period. Urine THC metabolite concentrations were plotted relative to urine creatinine levels as a means of controlling for hydration level.

You can see that THC metabolite levels (the solid line) are all over the place in these two subjects (although I will admit that I cherry-picked the two subjects with the most variation). Results went from negative to positive to negative again even within a few hours. The concentration even changed as much as 6-fold. Rapid changes in blood levels (the dashed lines) were not observed, so all that seems to be changing is the rate of renal elimination.

A 2008 study measured THC excretion from 60 abstinent subjects over a period of up to a month. They similarly noted how variable the urine THC-COOH results were (even though this study also normalized the urine levels to creatinine to control for hydration).

They reported that on average, the last time a subject had a positive result was 8 days after the first time they had a negative result. However, the higher their urine levels were to start, the longer this time became. The record in this study was 28 days between the first positive and last negative result!

This study also reported that on average, the urine THC-COOH levels reached a peak of about 100 ng/mL after testing negative at the 50 ng/mL level. Although the highest subject reached 289 ng/mL, about a 6-fold increase!

Why Do Urine THC Metabolite Levels Vary So Much?

The first reason, urine output, is already well known. Your body can quickly change the rate at which it produces urine, depending on how much water it wants to conserve or get rid of. Producing urine at a higher rate does not lead to significantly greater THC metabolite excretion. People use this to their advantage to drink a lot of fluids and dilute urine metabolites.

But what happens if you aren’t paying much attention to how much fluids you drink? You may be adequately hydrated one day and pass a test. But another day you don’t happen to drink as much and fail. Or perhaps you were working outside and lost a lot of fluids through sweating, which can also lead to more concentrated urine and a test failure.

The plots above already control for urine output level by normalizing the THC metabolite values to urine creatinine. This means that there is some additional mystery factor that affects THC metabolite excretion. If we can figure out what it is, then we can use it to our advantage.

I recently wrote about how urine pH may affect excretion of the THC-COOH metabolite. The quick summary is that THC-COOH is filtered from the blood by your kidneys, but then nearly 100% is reabsorbed back into your blood. Making your urine more alkaline can result in almost complete ionization of THC-COOH, which can potentially trap it in the urine. This could (in theory) lead to much higher urine concentrations.

It is a mediocre detox strategy because even if reabsorption is completely blocked, hepatic elimination is still 10 times faster. But after doing the research for this article, urine pH has become my #1 hypothesis for why urine metabolite levels jump around a lot.

Your fasting urine pH is acidic, but generally goes up after meals. However this completely depends on what you eat and what you drink. This can explain why urine metabolite levels jump around a lot in some people and not in others. It would also explain why urine metabolite levels appear to be periodic in some people (i.e. they regularly go up and down). This may be my brain seeing patterns out of noise, but the same thing has been observed for renal excretion of methamphetamine, where it directly correlated with urine pH levels throughout the day:

What You Can Do To Prevent Testing Positive After Testing Negative

The first step is to control hydration prior to each test. I recommend starting with 0.5 liters of fluids per hour for 3 hours before each test as a baseline. You may need to increase this later for greater dilution. Also, don’t test yourself after anything that can cause a lot of sweating, like exercise or being in a sauna.

The effect of urine pH on THC-COOH excretion is still theoretical, but it is a strong enough possibility that I now caution against anything that will alkalinize your urine prior to a test. Try to keep your urine pH below 7 (or even 6 to be sure), which you can monitor with urine pH test strips.

If you are in trouble with the law or your employer for your levels going up when you haven’t smoked, show them this article! You can also download the two scientific studies that I linked to above, since both are open access.