A five-lab study found that some cannabis calibration vials measured differently from the amounts stated on their labels, a discrepancy that can carry through to potency results. Nandu Sarma of United States Pharmacopeia (USP) presented the findings at the Cannabis Quality Conference.
Before a lab can measure THC in a sample, it needs a benchmark. Labs buy calibration vials from companies that make them specifically for testing, such as a vial labeled “1 milligram of THC per milliliter.”
The lab uses the label number to calibrate its instrument. If the label number is wrong, it can throw off the potency results for the samples the lab tests.
The American Council of Independent Laboratories coordinated the study, which included vials from five suppliers. One of those suppliers was Restek, a company that sells calibration vials to testing labs.
All five labs used Restek vials as a common benchmark to set up their instruments. They then used their own testing methods to see how closely the study vials matched the amounts listed by their suppliers.
For delta-9 THC, CBD, and CBDA, the measured amounts were within 5% of what the suppliers listed across the batches tested. THCA-A showed more variation.
Seven of 16 THCA-A results fell outside the 5% range. The two largest differences came from the same supplier.
The study was designed to examine variability across the vials included in the comparison. Sarma said the manuscript was under peer review at the time of his presentation.
Sarma used a USP test for CBD as a pharmaceutical ingredient to show what is at stake. When a batch is close to the cutoff, a calibration error can be enough to make it pass when it should fail, or fail when it should pass.
A vial labeled as containing 1 milligram of THC per milliliter might actually contain 0.8 milligrams per milliliter. If the lab treats the weaker reading as the signal for 1 milligram per milliliter, it can overstate how much THC is in the sample.
When a new batch of calibration vials arrives, the lab can use its already-calibrated instrument to measure the THC in one of the new vials. If the measurement matches the label, the lab can use the new batch.
If the difference is larger than the lab allows, it can set the new vials aside and double-check its measurements. If the discrepancy persists, Sarma recommended contacting the company that made the vials so it can investigate.
Labs should spell out how they check each new batch before using it to test customer samples. The procedure should state how close the measurement must be to the label amount and what to do if it falls outside that range.
Checking the vials up front can keep a single discrepancy from affecting every potency result measured using that calibration.
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