Indeed. From the article . . . "And it shocked everyone, including ourselves, that we could measure, in some of these individuals, THC in the blood for 30 days," says Marilyn Huestis, a toxicologist with the University of Maryland School of Medicine who recently retired from leading a lab at the National Institute on Drug Abuse. The participants' bodies had built up stores of THC that were continuing to slowly leech out, even though they had abstained from using marijuana for a full month. In some of those who regularly smoked large amounts of pot, researchers could measure blood THC above the 5-nanogram level for several days after they had stopped smoking. Conversely, another study showed that people who weren't regular consumers could smoke a joint right in front of researchers and yet show no evidence of cannabis in their blood. So, in addition to being invasive and cumbersome, the blood test can be misleading and a poor indicator of whatever is happening in the brain.In one study, researchers had 30 frequent marijuana users stay at a research facility for a month without any access to drugs of any sort and repeatedly tested their blood for evidence of cannabis.
Marilyn Huestis doesn't know shit if she is stating that they are measuring 'THC in the blood.' When you test people for cannabis, you test their blood for cannabinoid metabolites, not psychoactive delta-9 THC. I don't actually know if it's possible to test for cannabinoids, considering the body makes them naturally. It's possible that she's 'translating' things a bit for laymen.