The cannabis plant does not actually produce THC. Instead, the plant produces tetrahydrocannabinolic acid (THCA), a non-psychoactive, acidic cannabinoid.

When the plant is heated, it undergoes a process called decarboxylation, where THCA is converted to THC.

Lab testing of marijuana is now commonplace in areas where medical or recreational marijuana is permitted. Marijuana labs test for a number of things, including total cannabinoid content.

Labs test for THCA, and use a mathematical formula to determine the THC content. This is because the conversion from THCA to THC involves a loss of molecular mass, and strain potencies are measured as a percentage of the total weight.

But the figure produced from these mathematical conversions can often be misleading, explains Alec Dixon, co-founder of SC Laboratories, a large laboratory facility in Santa Cruz, California.

The equation uses a mass weight correction of 12 percent, which would only occur “in a perfect theoretical vacuum,” he says.

Labs have not developed a reliable, consistent method for testing for THC value, so THCA is the more accurate and routine way to discuss cannabis potency.

For this reason, Dixon says it’s the better figure to pay attention to while selecting a strain.

“Looking at the THCA value will give you the best idea of how much THC will be there when you use it,” says Dixon, “without the lab doing any math to see how much will convert, which could potentially be wrong.”