Interpreting Your Potency Results Part I – An Exact Estimate

The THC level listed on the label simply does not tell a complete story.

A Certificate of Analysis from Lightscale Labs displaying the average levels of various cannabinoids contained in a batch of cannabis flower.

I bet you want what most smokers want: to know your cannabis product is safe, free of dangerous pesticides, and full of THC. You may not, however, be as interested in sorting through the details of the lab test results… but perhaps you should be. While most consumers remain at least partially interested in THC potency, a truly savvy smoker knows there’s a lot of nuance in how that potency can be reported.

The figure many buyers focus on is the average THC potency of a batch of cannabis flower. This Batch Average is the potency that ends up on the jar in the dispensary. If you’re buying a vape cartridge or edible, the Batch Average is the number printed on the label on back of the package. But because it’s nearly impossible to chemically test a cannabis nug or a gram of product without ruining it, and nobody has the time or money to test every single nug that comes off a plant, an entire 15lb batch of cannabis flower will be assigned the same Average THC potency. Meanwhile, a Production Batch of cannabis products can contain tens of thousands of vape cartridges or bottles of tincture or grams of shatter, which will all be assigned the same Average Potency!

So does every nug or vape cart in that one batch possess the same amount of THC? Obviously not.

Which is why I tell you this now, and may you never forget it: the THC level listed on the label simply does not tell a complete story.

The label on this cannabis flower is a perfect example of how cannabinoid reporting can be deceiving.

To truly understand what a potency test result means, you have to consider several different factors, such as: How has your lab chosen to calculate “Total THC”? What is your body doing with these THC molecules, or, how are you metabolizing this chemical? How many samples from this batch were analyzed to get the Average THC Potency? And possibly the most important factor: how much of a gap exists between the Average Potency and the true THC content of the samples collected?

The more you understand how a lab tests for, calculates, and reports THC potency, the better prepared you’ll be to interpret test result data and ask follow-up questions of your testing lab. And it is really in your best interest to ask some follow-up questions, because as of right now, there are few reference methods and no nationally accepted standard for how to test cannabis. This means it's conceivable that you could take your product to 5 different labs and get 5 slightly different results.

What’s more, even if the testing at all 5 labs is done exactly the same way, chances are that the reporting will not be.

That’s why, in this series of articles about “Interpreting Potency,” we’ve laid out some of the ways cannabinoids can be measured, calculated, and reported.

The Best Estimate

The first important point you need to understand about the potency of THC (and all chemicals) in cannabis is this: the test result listed on every product, ever, is an estimation, and not an exact measurement.

Let’s imagine you’re buying oil in a vape pen from a dispensary, and that the cartridge is labeled with a potency of 64% THC. You would be tempted to assume (and rightly so) that 64% is the actual amount of THC in that vape cart. You’d probably also assume that 64% THC figure was the result of a test done on the vape cartridge you’re holding…

But that’s impossible. If that particular cartridge had gone to the testing lab, it wouldn’t be available at the dispensary. It would be in a chemical waste bin.

Maybe you’re a little savvier about the cannabis industry, and you know that 64% is the potency assigned to the entire production batch that the oil in this cartridge came from. You might also know that the 64% Average was calculated from the potencies of several different grams of oil across the production batch. (To learn more about this, check out our article about how cannabis products are sampled for testing).

Individual grams of oil taken from a large production batch will always contain at least slightly different amoutns of THC, and all chemicals.

However, these are not the only complications that stand in the way of sussing out the true potency of your unique vape cart. On top of knowing how the batch of oil was sampled, you also need to know how the ‘Total THC’ was calculated.

The Take Away

It’s clear that precise and accurate test results for cannabis products depend not only on the integrity of the lab performing the analysis, but also the way the data is calculated, and presented. Results can become even more muddled by the fact that the average batch potency may not always accurately depict the true potency of each individual gram of product. When you also factor in that most chemical potency results are actually taken from the average of 2 different samples, each of which is a composite sample from several different sections across the production batch, you start to see just how complicated analytical test results can be to interpret. (This is, of course, a topic for another article.)

The take-away is: with so many variables contributing to and altering the final reported cannabinoid potency, it's crucial that the calculations done to find that potency be as transparent and precise as possible. The more we educate consumers on how these factors affect reported THC, the more questions they can ask of the people testing their cannabis, and the greater our potential to move towards a national standard for analytical test results reporting.

Up Next: How does THC potency differ from Total THC?

Interpreting Your Potency Results Part II – Calculating ‘Total’ Cannabinoids

The THC potency at the time of testing is NOT what the potency will be at the time of consumption.

When reviewing your product’s potency results on the Lightscale Labs Certificate of Analysis (COA), you’ll notice there’s more than one value for THC. You’ve got Δ9-THC, THCA, and then a big, bold number called ‘Total THC.’ Each of these numbers is different, and each is important.

A Certificate of Analysis from Lightscale Labs displaying the average levels of various cannabinoids contained in a batch of cannabis flower.

Here’s the COA for a batch of cannabis flower. Let’s evaluate how we arrived at each of those potencies.

By Oregon state law, “Total THC” must measure psychoactive THC (current or future) in all its different and various forms. (As of May 2021, Δ8-THC is not factored in to Total THC, but that will likely change very soon.) Most cannabinoids exist in an acidic and non-acidic form, such as THCA (non- psychoactive) and Δ9-THC (psychoactive).

Sure, the lab can measure the amount of each form of THC in the flower and add them together… but this gets tricky, because the THCA molecule and the Δ9-THC do not weigh the same! This matters because, as you’ve likely learned by now, potency is a description of what percentage of the product, by weight, is comprised of each particular kind of molecule.

What differentiates a THCA molecule from a Δ9-THC is a carbon, 2 oxygens, & a hydrogen atom, otherwise knowns as a Carboxyl Group.

What further complicates things is the fact that, legally, the product must be labeled with what the potency will be at the time of consumption, which is not the same as the potency at the time of testing.

When the consumer heats the vape cartridge, the heat will cause many of the THCA molecules to lose a cluster of atoms known as a ‘carboxyl group.’ (Don’t freak out, it's just a carbon atom, two oxygens, and a hydrogen.) This process is called ‘decarboxylation’ and is what transforms THCA into Δ9-THC.

The consequence of that carboxyl group floating away is that, while these 2 molecules still look very similar, they do not weigh the same. To be precise, without the carbon group, Δ9-THC weighs 87.7% as much as it did when it was THCA.

So here’s the problem: what currently exists in your product as a heavier THCA molecule, will likely become a lighter Δ9-THC molecule when you apply heat. Since potency is a measurement of weight, we have to find a way to account for the missing weight of that Carboxyl group. In other words, we need to know what all that THCA will weigh after it’s been decarboxylated and becomes Δ9-THC.

An Example

Let’s say we test some (unbelievably potent) buds, and find out that each 1 gram of flower contains:

100mg of Δ9-THC (or 10% THC)
342mg of THCA (or 34.2% THCA)

100 mg of THC per 1 gram of flower means that 10% of every 1-gram nug is composed of THC molecules.

We know that when THCA is decarboxylated and becomes Δ9-THC, it will weigh 87.7% as much as it currently does. Therefore, we need to use the equation:

THCA x 0.877 = Future Potency of Δ9-THC

Heat will cause a molecule of THCA to separate into its constituent parts: 1 molecule of CO2 & 1 molecule of Δ9-THC.

Using our analytical results for this particular product, we get:

342mg THCA (now) x 0.877 = 300mg Δ9-THC (future)

Add that to the 100mg of Δ9-THC that already exists in our flower:

100mg Δ9-THC (now) + 300mg Δ9-THC (future)
= 400mg Δ9-THC (per 1 gram of flower)

Once decarboxylated THCA becomes Δ9-THC, it is added to the ‘Total THC’ potency amount.

Which could also be written as:

Total THC Potency = 40%

Be In the Know

Understandably, we often get the question from clients, “why do I need to know all this math? Doesn’t the lab calculate all this ‘Total THC’ stuff for me?” The answer to that question depends on what state you live in, and what lab you use for testing. In Oregon, the THC level tracked by the state (aka listed in METRC) is required to be the fully-calculated Total THC potency. Some states have similar rules.

Across the country, however, there are few laws about how labs have to report information on their Certificates of Analysis, which is what many dispensaries use to determine whether or not they will buy a batch of cannabis product. There are plenty of COAs that list ‘Total THC’ as simply “THCA% + Δ9-THC%.” It’s not difficult to see why: forgetting to adjust for the weight lost during decarboxylation artificially inflates the perceived THC potency of a product. Unfortunately, reporting Total THC this way is at best misleading, and at worst, completely inaccurate.

Take the batch of flower from our example math problem above. Without adjusting for decarboxylation, you would simply arrive at the wrong ‘Total THC’ level.

When reading a Certificate of Analysis, always look to see that the THCA potency has been multiplied by that magic number, 0.877, before being added into the Total THC. Furthermore, make sure the “Total THC” level written on the COA matches what is being reported to the state (in METRC, or whatever Cannabis Tracking System your state uses.) Perhaps one day analytical test results will have to be reported clearly and transparently across the country. Until then, it is important to understand where your potency numbers are coming from.

Up Next: How Well Does the Average Potency Really Represent the Batch?

Interpreting Your Potency Results Part III – Does the Average Represent the Batch?

Every product in one batch is labeled with the same potency… but no two products are exactly the same.

A laboratory technician testing vape cartridges for consistency in chemical content.

How to Know if a Batch is Uniform

Or: Why every vape cart in this batch has the exact same potency written on the label

As we’ve seen in our series about interpreting THC potency (and really, the potency of all the different kind of chemicals that make up a cannabis product,) knowing the Average Potency of the batch is important… but it’s really only half the story.

If you harken back to junior high math class, you’ll remember that when analyzing a set of numbers, what matters is not just the Average, but the Range of the numbers. In other words, how far do these numbers spread out on a timeline? And how much do they generally differ from the set’s Average?

Let’s assume we have a batch of 1000 vape cartridges, all produced at the same time, from the same container of concentrated cannabis oil. We take a few of the carts back to the lab for analytical testing, and find that, together, they have an average THC potency of 64%.

As the figure below demonstrates, there are several different ways we could have arrived at that Average potency.

There are many different ways to arrive at the same Set Average.

As you can see, not only can we reach a 64% Average in a nearly infinite number of ways, there’s also a large discrepancy in just how well the Average represents each set of numbers. In the first Set, its clear that the potencies of each individual vape cart are pretty close together. (They’re all within ±2% of the average, if we were to calculate it out.) However, in the second set, the closest vape cart potency is 6% different from the Average, while the furthest is 46% away from the Average!

So why does this matter? Because the Average potency is what’s written on the back of every vape cart sold from that batch! Regardless of that cart’s actual potency!

That means, in theory, that the vape cart you just purchased because it boasted a 64% THC level could contain just 24% THC! (Don’t freak out yet, this is just a hypothetical situation. In real life the gap between the label and reality won’t be that big. While Oregon is lucky and has an upper limit on just how much products can differ from the Batch Average, not all states do.)

The THC potencies listed on this Certificate of Analysis is really the average THC content of many different grams shatter from across a large batch of many grams.

The Average Distance From the Average

Luckily, when evaluating different vape carts from the same production batch, we have a way to quantify exactly how much of a gap there is between their potencies. Another way to think about the same concept is that by looking at the Range of potencies across the batch, we’re also measuring the homogeneity, or uniformity, of the batch.

This magical concept we’re referring to is known in the math world as the Relative Standard Deviation (RSD). Essentially, the RSD is the average distance between each sample potency and the Batch Average. The reason this figure is so important is because it tells us just how confident we should be about applying that Average Potency (64% in our example) to the entire batch.

To put it another way, the RSD tells you the likelihood that the vape cart you just purchased actually contains the amount of THC that’s listed on the package.

Does this THC Sauce have the same potency as these THC Diamonds? Probably not.

That’s something you as a consumer might, ya know, be interested in. One could, if one were so inclined, make the argument that the RSD is the single most important testing statistic that is excluded from the product label… and perhaps that’s something that should be reconsidered.

That’s because, as you may recall, that gram of cannabis oil you’re about to smoke has never actually been tested. If it had been, its remains would be in the laboratory trash can.

All we at the lab can do is test some of the oil that was adjacent to that bit that ended up in your vape cart. We do that a bunch of times, and then calculate the Average potency. The entire batch of oil gets assigned that Average potency, and every vape cart made from the oil gets labeled with it. The lower the RSD of all those samples we take, the higher the chance that the THC% advertised is actually the potency of the gram of product you’re holding.

I’ll say that again:

The lower the Relative Standard Deviation, the higher the chances that the THC% advertised is actually the potency of the gram of product you’re holding.

The Mathematical (or Nerdy, take your pick) way to say this is - the Standard Deviation of any set of numbers tells you how precisely that Average represents the whole data set. Do most of the numbers in this set agree with each other, or are the data points all over the place?

Potencies, Visually Mapped

In the example potency sets from our graphic a few paragraphs ago, the second set of numbers would have a very high RSD, because none of the samples tested were, numerically, anywhere close to the batch Average of 64% THC.

Of course, the second set was also a very over-exaggerated example, meant to make a point. Let’s see how RSD plays out in real life.

Here are the potencies for some samples pulled from 2 different batches of cannabis oil. (The story you are about to hear is true. Only the names have been changed to protect the innocent.) Each 1-gram sample of oil was taken from a different location in the batch, and tested for THC content separately. Then those potencies were used to calculate the batch average. Note that both batches have the same average potency: 53% THC.

The batch of OG Kush oil was clearly well homogenized. Every gram of oil we pulled for sampling had a potency very close to the 53% Batch Average.

Samples from across this well homogenized batch of oil are very similar in potency, meaning they have a small Relative Standard Deviation.

Then there’s the Stardawg Oil. This batch was clearly not well homogenized, as the THC concentration was very different depending on where in the batch we pulled a sample from. The range of potencies in this batch of oil is huge, which means the Relative Standard Deviation is also huge.

A less homogenized batch of oil produces samples with a massive range, most of which are very distant from the Set Average.

This gives us a visual idea of Relative Standard Deviation. Because all of the potencies in the OG Kush were fairly close together, and fairly close to the 53% Batch Average, the RSD for that batch is very small.

Alternatively, the Stardawg shatter has a very high RSD, because the data points are all over the place. The Average is still 53% THC, but none of the samples taken had anything even close to potency.

Disclaimer: These graphics provide a very oversimplified explanation of RSD. If you’re a real stickler for doing the math yourself instead of trusting what you read in online blog posts, (and good for you if you are) you can calculate the precise Relative Standard Deviation for each batch, using the following equation:

Here’s the important point to remember - both the Stardawg and the OG Kush batches have the same Average Potency: 53% THC. This should drive the point home that the quality of a batch cannot be determined by Average Potency alone.

In fact, the RSD almost tells us more about the batch as a whole than the Average Potency does. Mainly, it tells us just how much chemical variation we can expect in any given gram of this product.

Cautionary Measures

While this information is extremely useful… we all know that most consumers are not going to waltz into dispensaries and casually ask their Budtenders for the Relative Standard Deviation of a vape cartridge. Which is why the state of Oregon put a 20% RSD cutoff in place. This limit is a way to say, ‘a batch must be a certain level of uniform before we can call it safe.’

On top of that, dispensaries can educate their Intake Managers about RSD, and how to read the lab Certificates of Analysis for each batch. The more the dispensaries understand about how the RSD affects the quality of a batch, the greater chance you, as a consumer, have of getting a product whose actual potency matches what’s on the label.

While we at Lightscale Labs can’t promise that the average potency we assign to the batch will be the true potency of every gram of shatter from that batch, we can offer the assurance that every gram is extremely chemically similar to every other gram, by ensuring the batch has a low Relative Standard Deviation.

Armed with this knowledge, you can feel confident in your understanding of the potency test results on your Certificate of Analysis.