Todd Subritzky, PhD

Understanding Cannabis Lab Data and Total THC values

Like everything in the medical cannabis world, it’s complicated.  In order to get the heart of the matter, it’s necessary to explain some fundamentals regarding how cannabis is tested in laboratories and why differences in testing methodologies, equipment, sampling, calibration of machines, and staff training can and does impact results.

However, there is little guidance from the academic literature on this topic, so this segment is primarily informed by testing labs and other authority sources.

The cannabis testing world is new and still developing, however, it is an essential component of legal markets where quality control is paramount.  Cannabis products sold in legal markets in the US must comply with standards regarding potency and contamination.

Accurate potency information is fundamental to developing safe patient and clinician dosing strategies and this aspect is the focus of the rest of this segment.

Cannabis testing process

The testing of cannabis products involves a complex analytical procedure either with gas chromatography (GC) or liquid chromatography (LC) techniques (Subritzky, Pettigrew, & Lenton, 2017). This matters because different procedures produce different results, however, the lack of methodological standardization across the industry can be confusing for those trying to decipher lab results as variation in cannabis lab testing techniques produce markedly different values for potency percentages or ratios of cannabinoids such as THC and THCA.

Understanding Total THC values – a combination of THC and THCA

THCA is a naturally occurring non-intoxicating cannabinoid within the cannabis plant.  In order to transform into THC, the cannabinoid responsible intoxication,  THCA must go through a process known as decarboxylation (ElSohly & Waseem, 2014). Adding heat triggers decarboxylation by removing the carboxy group of molecules (Steep Hill Lab, 2010).

Now here is the problem, GC and LC function at different temperatures (Steep Hill Labs, 2017). On the one hand GC involves high temperatures (thereby triggering the decarboxylation process of THCA) before it passes on to the detector, which then only picks up the THC, that is, not the Total THC value, which includes both THC and THCA.

Furthermore, the decarboxylation process is not completed at those high temperatures with estimates commonly reporting up to 70% of THCA being conserved.  Thus, clearly, cannabis lab testing data produced from GC techniques are little use for understanding total THC values.

On the other hand, LC happens at room temperature and does not decarboxylate any cannabinoids. This provides separate values for THCA and THC, both of which are always present. This disparity between the two results produced by the differing cannabis testing methodological techniques is highlighted in results expressed as a percentage of mass.

As noted above, during the process of decarboxylation THCA loses carboxy molecules, which results in THC weighing less than THCA – about 12% less in fact.

In order to calculate real potency value, it is essential to consider both THCA and THC. To achieve this accurately, a correcting formula for THCA is applied before adding THC values. Use the following formula for Total THC values as provided by (Aurum Labs, 2019):

THCtotal = (%THCA) x 0.877 + (%THC)


Now using 0.877 may seem rather arbitrary but there is a method to the madness. .877 is the molecular mass of THC divided by the molecular mass of THCA. This can be boiled down to a very straightforward formula – take 87.7% of the value for THCA and add the value for THC. Hey presto we have got our total THC value.



Aurum Labs. (2019). WHAT IS “TOTAL THC”? Retrieved 6/6/19 from

ElSohly, M., & Waseem, G. (2014). Constituents of cannabis sativa. In R. Pertwee (Ed.), Handbook of Cannabis. New York, NY: Oxford University Press.

Steep Hill Lab. (2010). Quality assurance for medical cannabis. Retrieved 06/08/2015 from Steep Hill

Steep Hill Labs. (2017). Why Does Each Cannabis Lab Report Total THC Differently? Retrieved 5/19/20129 from

Subritzky, T., Pettigrew, S., & Lenton, S. (2017). Into the void: Regulating pesticide use in Colorado’s commercial cannabis markets. International Journal of Drug Policy, 42(4), 86-96. doi: