Does tetrahydrocannabinol (THC) (marijuana’s primary psychoactive ingredient) concentration correlate to impairment from marijuana/cannabis consumption?

The National Highway Transportation Safety Administration (“NHTSA”) of the United Stated Transportation Department concedes:  “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects.”

THC and other drug concentration in blood does not correlate to impairment for DUI cases

In the broader universe of all drugs, NHTSA similarly concedes difficulty correlating drug concentration in one’s blood to impairment:

“In terms of attempting to link drug concentrations to behavioral impairment, blood is probably the specimen of choice. However, forensic toxicologists generally have failed to agree on specific plasma concentrations that could be designated as evidence of impairment (Consensus Development Panel, No Date). The lack of consensus about per se levels of drugs where impairment could be deemed makes it difficult to identify, prosecute or convict drugged drivers in most states.”

NHTSA’s foregoing concessions underlining the importance of attacking any effort by prosecutors to correlate drug concentration levels in blood to impaired driving.

Of course,  the Virginia Code’s DUI statutory scheme removes the necessity of such a correlation to obtain a conviction for cocaine, methamphetamine, PCP and MDMA/ecstasy so long as their concentration in the blood by milligrams per liter of blood is, respectively, 0.02, 0.1, 0.01 milligrams of phencyclidine per liter of blood, and 0.1. Va. Code § 18.2-266

Rising THC blood concentration levels do not automatically establish non-abstinence

Does rising THC concentration in one’s blood over a course of days prove the person has been continuing to consume marijuana/cannabis? A 2015 report by forensic medical experts in Australia finds: “THC levels have also been found to not decrease monotonically during abstinence,  i.e. levels can rise and fall while maintaining a downward trend.” “Residual cannabis levels in blood, urine and oral fluid following heavy cannabis use,” Odell, Frei, et al., Forensic Science International 249 (2015) 173–180.

The foregoing finding about rising THC levels is relevant to arguing that such rises do not automatically amount to violations of probation and pretrial release conditions to remain drug-free, because THC levels can rise even during early periods of abstinence.

Consequently, the relevance of blood analyses for drug concentrations must always be challenged.

Fairfax marijuana lawyer/ Virginia criminal/drug lawyer Jon Katz has been successfully defending hundreds of clients charged with drug and driving under the influence (DUI) offenses since 1991. To discuss your case in confidence with Jon, please call his staff at 703-383-1100.