NIST Report Highlights Cross-Laboratory Differences in Cannabis Testing

Accurate cannabis testing depends on more than sophisticated instruments. Results can also vary with sample preparation, calibration, dilution, and the way laboratories identify closely related compounds. Those challenges are documented in the National Institute of Standards and Technology’s 2024 final report on cannabinoid testing, produced through its Cannabis Laboratory Quality Assurance Program, or CannaQAP.

Launched in 2020, CannaQAP gives cannabis and forensic laboratories a way to compare their measurements with those of other laboratories and with reference or target values. It is designed as an educational interlaboratory program rather than a pass-or-fail test. The program supports laboratories working with both hemp and marijuana plant materials and cannabis-derived products.

What Exercise 2 examined

Exercise 2 involved six cannabis plant samples: three hemp samples and three marijuana samples. In total, 226 laboratories registered to measure cannabinoids, toxic elements, moisture, or some combination of the three. Participation varied by analyte. About 90% of the laboratories that registered for cannabinoid testing selected the hemp samples, while roughly 22% registered to analyze the marijuana samples, which required additional documentation for shipment and handling.

The cannabinoid portion covered major compounds including delta-9 THC, THCA, total delta-9 THC, CBD, CBDA, and total CBD. Laboratories also reported results for a range of minor cannabinoids, including CBC, CBDV, CBG, CBN, THCV, delta-8 THC, and their acidic forms where applicable.

The samples were prepared with different cannabinoid concentrations. Three were hemp materials with total delta-9 THC concentrations at or below 0.3% by mass, while three marijuana materials were prepared with approximate delta-9 THC concentrations of 0.5%, 1%, and 2%. NIST established target values for most of the samples using liquid chromatography with photodiode array detection and/or tandem mass spectrometry.

Calibration and dilution affected accuracy

The report found that laboratories generally produced more accurate results when cannabinoid concentrations were above their methods’ limits of quantitation but did not require sample dilution. Lower-concentration measurements were more vulnerable to calibration bias, chromatographic interference, and mistaken identification of compounds.

NIST recommends using calibration standards that meet applicable ISO requirements, reviewing purity information carefully, preparing calibrants independently and routinely, and establishing traceability to the International System of Units when possible. For samples requiring dilution, the report recommends gravimetric dilution—measuring by mass—because it can be more accurate than relying solely on volumetric measurements.

High-concentration samples presented a different challenge. THCA and CBDA results were less accurate in some cases when concentrations were at least 1%, potentially because of calibration bias near the upper end of a calibration curve or errors in dilution procedures.

Closely related compounds can interfere with identification

The report also shows why retention time alone may not be sufficient to identify cannabinoids. Approximately 80% of laboratories analyzing the certified hemp reference material used liquid chromatography with absorbance detection, a method that is generally less selective than mass spectrometry.

In one example, a compound identified as CBNA appeared to elute with delta-9 THC in the reference material. That coelution likely caused laboratories to report a delta-9 THC value higher than the certified target. NIST also found that several minor cannabinoids could be misidentified when analysts relied primarily on retention times rather than additional confirmation, such as spectral or mass-spectrometric information.

To reduce these errors, the report advises laboratories to periodically assess their chromatographic methods and verify baseline separation for cannabinoids with similar retention behavior.

Laboratories were more consistent with themselves than with one another

A central finding concerned the difference between repeatability and reproducibility. Repeatability measures how closely a laboratory’s own repeated measurements agree. Reproducibility measures how closely results agree across different laboratories.

Most laboratories met the report’s cited repeatability criteria for hemp samples. However, almost none met the corresponding between-laboratory reproducibility criteria for most samples. In practical terms, individual laboratories could often obtain consistent results using their own procedures, while laboratories using different procedures produced results that were not consistently comparable.

That distinction is important for regulatory testing, product labeling, commerce, and forensic decisions. A result can be precise within one laboratory without being accurate or directly comparable to a result from another laboratory.

Moisture and toxic elements were evaluated separately

Exercise 2 also included measurements of moisture and 13 toxic elements, including arsenic, cadmium, lead, mercury, chromium, nickel, selenium, and uranium. NIST published separate reports covering those portions of the exercise, including the Exercise 2 moisture report and the Exercise 2 toxic-elements report.

Moisture measurement matters because cannabinoid concentrations may be reported on a dry-mass basis, making the water content of a plant sample part of the calculation. Testing for toxic elements is also relevant because cannabis plants can absorb contaminants from soil and other environmental sources.

NIST’s CannaQAP program overview now states that the program will not offer additional exercises. It lists the Exercise 3 report as still in preparation. The broader value of the program, however, is already clear: standardized reference materials and interlaboratory comparisons can reveal sources of variation that may not be apparent when a laboratory evaluates its methods in isolation.

The Exercise 2 findings do not show that cannabis testing laboratories are uniformly unreliable. Instead, they identify specific technical weaknesses—especially calibration, dilution, compound identification, and cross-laboratory consistency—that laboratories can address to make cannabinoid, moisture, and contaminant measurements more comparable.