THC Blood Testing in Georgia: What the GBI Measures and What It Does Not

Almost every contested marijuana DUI in Georgia eventually arrives at a single sheet of paper: the toxicology certificate issued by the Georgia Bureau of Investigation Division of Forensic Sciences. The number on that page is offered to the jury as a measurement. It is. The difficulty is that it is a measurement of something the State has not actually charged.

I · The SampleBlood draw, preservation, and chain of custody

A DUI-drugs blood draw in Georgia is typically collected by a trained phlebotomist after an implied-consent advisement or, increasingly, pursuant to a search warrant. The draw is collected into gray-top tubes containing sodium fluoride and potassium oxalate — a preservative and an anticoagulant designed to inhibit microbial activity and prevent clotting during transport and storage. The condition of those tubes, their refrigeration history, and the time between collection and analysis are all data the defense is entitled to. They are also data the case file frequently contains but the certificate does not summarize.

II · ScreeningImmunoassay and what a positive screen actually means

Initial screening for cannabinoids is performed by immunoassay — an antibody-based test that flags samples above a defined cutoff. The screen is sensitive but not specific. It can cross-react with related compounds, and it does not distinguish between the psychoactive delta-9-THC parent compound and inactive metabolites. A positive screen is a flag for further work. It is not a result the State should be asking a jury to weigh.

III · ConfirmationLiquid chromatography–tandem mass spectrometry

Confirmatory testing in the GBI's cannabinoid panel is performed by liquid chromatography coupled to tandem mass spectrometry — LC-MS/MS. The instrument is reliable; the method is well established in forensic toxicology. The confirmation typically reports three separate analytes:

• Delta-9-THC — the psychoactive parent compound. Peaks within minutes of inhalation and declines rapidly thereafter. Often present at low single-digit nanograms per milliliter, or below quantitation, by the time of a roadside-followed blood draw.
• 11-hydroxy-THC — an active metabolite formed primarily after oral ingestion. Its presence and ratio to delta-9 provide pharmacokinetic information the State's witness is qualified to discuss and frequently does not.
• 11-nor-9-carboxy-THC — the inactive metabolite. Has no psychoactive effect. Persists in blood for hours to days, and in urine for substantially longer in regular users.

IV · LOD and LOQThe number on the certificate is not always a number

A defense lawyer reading a GBI cannabinoid certificate has to read it for two distinct thresholds. The limit of detection is the lowest concentration the instrument can distinguish from noise. The limit of quantitation is the lowest concentration at which a defined-precision number may be reported. Results below the LOQ are sometimes reported as "present" without a numerical value — which is an honest scientific statement but is, in front of a jury, frequently treated as if it carried the weight of a quantified result. It does not.

The case file, on request, will include validation data, reference standards, internal QC charts, and the analyst's bench notes. Those materials are where the defense work in a chemistry case actually begins.

V · ExtrapolationWhy backward calculation does not work for cannabis

A blood-alcohol concentration can, with conservative assumptions, be extrapolated backward to the time of driving. Cannabis is not alcohol. Delta-9-THC pharmacokinetics are multi-compartmental: a rapid peak, rapid redistribution into adipose tissue, and continued slow release over hours and days. The same blood draw can correspond to dramatically different time-of-use windows depending on dose, route, frequency, and the individual. The peer-reviewed literature — including work published or commissioned by NHTSA, NIDA, and the AAA Foundation for Traffic Safety — is consistent on the point: there is no scientifically validated blood concentration of THC that correlates reliably with driving impairment, and backward extrapolation is not defensible.

VI · What the Certificate Does Not SayThe list, in order

1. It does not say whether the person was impaired.
2. It does not say whether the person was impaired at the time of driving.
3. It does not say when the cannabis was used.
4. It does not say how much.
5. It does not say by what route.
6. It does not say what the person's tolerance is.
7. It does not, standing alone, satisfy the State's burden under O.C.G.A. § 40-6-391(a)(2).

A laboratory measures a sample. A jury decides a case. The defense work is to keep the two clearly distinct on the record.

VII · Related Reading

• Defending Marijuana DUI Allegations in Georgia
• Cross-Examining a Toxicologist in a Georgia DUI-Drugs Trial
• The Limits of SFSTs in a Drug DUI Prosecution
• Georgia DUI-Drugs Defense — Practice Overview

This article is general information and does not constitute legal advice. Prior results do not guarantee a similar outcome.