The Cannabinoid Content of Legal Cannabis in Washington State Varies Systematically Across Testing Facilities and Popular Consumer Products

Abstract

The majority of adults in the U.S. now have state-legal access to medical or recreational cannabis products, despite their federal prohibition. Given the wide array of pharmacologically active compounds in these products, it is essential that their biochemical profile is measured and reported to consumers, which requires accurate laboratory testing. However, no universal standards for laboratory testing protocols currently exist, and there is controversy as to whether all reported results are legitimate. To investigate these concerns, we analyzed a publicly available seed-to-sale traceability dataset from Washington state containing measurements of the cannabinoid content of legal cannabis products from state-certified laboratories. Consistent with previous work, we found that commercial Cannabis strains fall into three broad chemotypes defined by the THC:CBD ratio. Moreover, we documented systematic differences in the cannabinoid content reported by different laboratories, relative stability in cannabinoid levels of commercial flower and concentrates over time, and differences between popular commercial strains. Importantly, interlab differences in cannabinoid reporting persisted even after controlling for plausible confounds. Our results underscore the need for standardized laboratory methodologies in the legal cannabis industry and provide a framework for quantitatively assessing laboratory quality.

Figure 1

The THC:CBD ratio defines three broad chemotypes of commercial cannabis flower measured by testing labs in Washington. Left column: Scatterplots of total THC vs. total CBD levels for cannabis flower. Right column: Histograms showing the THC:CBD ratio on a log scale and indicating the proportion of flower samples for each chemotype. Data are displayed for measurements batched across all Labs A-F (panels a-b; n = 175,136), for the lab reporting the lowest mean total THC levels (Lab A; panels c-d; n = 62,719), and the lab reporting the highest mean total THC levels (Lab F; panels e-f; n=26,664). Histograms for each of the six labs contributing to batched data in panels a-b are shown in Figure S1. Panels a and c were subsampled to n=50,000 for visualization purposes.

Figure 2

Total THC and CBD Measurements Differ Between Labs Across Chemotypes and Product Categories. Left column: Violin plots showing the distribution of total THC or CBD levels across labs A-F. Black lines denote median values, which are printed below the x-axis for each lab. Right column: Effect size matrices displaying the effect size of pairwise differences in distributions between labs. Matrices are color-coded according to one measure of effect size (Cohen’s d), and a second measure (Common Language) is printed for each comparison.

Figure 3

THC and CBD Levels Vary Between Labs After Controlling for Plausible Confounds. Average predicted values (+/− 99% confidence intervals) are shown, by lab, for (a) THC levels in chemotype I flower products (n = 161,933); (b) THC levels in chemotype I concentrate products (n = 33,888); (c) CBD levels in chemotype II and III flower products (n = 4,661); and (d) CBD levels in chemotype II and III concentrate products (n = 2,156) after adjusting for grower, strain-name, and time of measurement. Predicted values were generated from fixed-effects regressions with cluster-robust standard errors (see Methods).

Figure 4

Labs differ in the propensity to detect low levels of CBD in chemotype I flower. (a) Kernel density plots of each lab’s distribution of total CBD levels below 1.0% dry weight for chemotype I flower (y-axis scaled to one). Most labs show a local maximum near 0.1% total CBD, which is a commonly reported LOQ. (b) Fraction of chemotype I flower with total CBD levels below 0.1% dry weight. Bars indicate proportions +/− 95% CI for a binomial proportion. (c) Effect size matrix indicating the magnitude of interlab differences shown in panel B. Effect size is quantified as Cohen’s h (see Methods).

Figure 5

Mean THC Levels for Chemotype I Flower Products Over Time. (a) Total THC levels over time averaged across all labs or those reporting the highest or lowest mean THC levels. (b) Distribution of THC levels for each year on record for low THC reporting (LTR) labs. (c) Effect size matrix quantifying the mean difference in THC levels across years for LTR labs. (d) Distribution of THC levels for each year for high THC reporting (HTR) labs, and (e) the effect size matrix quantifying the magnitude of yearly differences.

Figure 6

Total THC and CBD Levels Across Popular Consumer Strain Categories for Flower Products. (a) Distribution of THC levels across popular strain categories for chemotype I flower and (b) effect size matrix quantifying the magnitude of differences between them. (c) Distribution of CBD levels across the same categories for chemotype II and III flower and (d) effect size matrix quantifying the magnitude of differences between them.

Figure 7

Distribution of THC-to-CBD Ratios Vary Across Popular Commercial Strain Names and Between Labs. THC-to-CBD ratios plotted on a logarithmic scale for cannabis flower samples across twenty-three popular commercial strain names for the single lab (Lab A) reporting the lowest (a) and the single lab (Lab F) reporting the highest (b) overall THC levels for cannabis flower.

Figure 8

Popular Strain Names as Signal for THC and CBD Content. (a) Proportion of variation in log₁₀ THC:CBD ratio explained by popular strain names (Intraclass Correlation Coefficient). 99% CIs are shown, by lab, before (black circles) and after (gray squares) filtering test results by the modal chemotype of each strain name. The ICC is shown both for dropping values for which 0% CBD or 0% THC is reported (left) and coercing cannabinoid ratios for these tests (see Methods). (b) Mean THC level of popular chemotype I strains. 99% CIs are shown after filtering by modal chemotype, for the lab reporting the lowest THC levels. (c) Mean CBD levels for popular chemotype II (above dotted line) and chemotype III (below dotted line) strain names. Results shown for the lab reporting the lowest mean THC levels.