DRUG TEST GUIDE

blood test Cannabis

Cannabis blood tests are the most direct measure of recent cannabis use, detecting active THC in plasma rather than stored metabolites. They are the preferred method in DUI investi

Test Type
LC/MS or GC/MS
Window (Casual)
3–4 hours
Window (Daily)
Up to 7 days
Cutoff / Accuracy
1–5 ng/mL (active THC)
99%+ (LC/MS)
Cannabis drug test laboratory analysis

How Cannabis Blood Tests Detect THC

Blood tests measure active delta-9 THC and its primary psychoactive metabolite 11-OH-THC directly in plasma. Unlike urine tests that detect the stored metabolite THC-COOH, blood levels reflect recent consumption far more accurately. THC enters the bloodstream within minutes of inhalation and peaks within 10–30 minutes. Understanding how THC works in the body is essential context for interpreting blood test results. Confirmatory analysis uses LC/MS (Liquid Chromatography-Mass Spectrometry) for precise quantification.

Detection Windows for Cannabis in Blood

Active THC clears from blood rapidly. In occasional users, plasma THC falls below 1 ng/mL within 3–4 hours of smoking. In daily users, THC-COOH may remain detectable in blood for up to 7 days due to redistribution from fat stores. Some forensic labs test for both active THC and THC-COOH ratios to estimate time since last use. The role of body fat in prolonged THC storage explains why chronic users show persistent low-level blood THC. Blood testing is rarely used for employment screening due to invasiveness and rapid metabolite clearance.

Legal Cutoffs and DUI Blood Testing

Blood THC cutoffs for DUI prosecution vary by jurisdiction. Five US states (Washington, Colorado, Montana, Nevada, Pennsylvania) use a per se limit of 5 ng/mL active THC. Germany uses 1 ng/mL (recently lowered from 1.0 to 3.5 ng/mL for drivers with no prior offences under the 2024 reform). The UK has no per se cannabis limit and relies on impairment evidence. Importantly, delta-8 THC produces nearly identical blood metabolites to delta-9 THC. NHTSA research on high-THC impairment consistently shows poor correlation between blood THC levels and driving impairment.

Blood Test vs Urine Test: When Each Is Used

Blood tests are used in post-accident investigations, DUI stops, and legal proceedings where recency of use matters. Urine tests dominate workplace screening because they detect a longer window of past use without requiring venipuncture. Saliva tests are increasingly used for roadside screening. Blood tests cannot determine impairment level — only that THC was present at a specific concentration. For a full comparison of methods and what each detects, see the urine drug test guide and the saliva test guide.

Sources & References

Frequently Asked Questions

How quickly does THC leave the blood?+
In occasional users, active THC typically falls below standard cutoff levels within 3–4 hours of smoking. Edible consumption may extend this slightly due to delayed absorption peaks.
Can a blood test prove you were impaired while driving?+
No. Blood THC concentration does not reliably indicate impairment level. Research consistently shows poor correlation between ng/mL THC and driving performance, especially in regular users who develop tolerance.
What is a normal blood THC level after smoking?+
Peak plasma THC in occasional users ranges from 50–200 ng/mL immediately after smoking, dropping to 1–5 ng/mL within 2–4 hours. Chronic daily users may show 1–10 ng/mL baseline levels even without recent use.
Will a blood test detect CBD?+
Standard blood drug tests do not screen for CBD. However, if you use full-spectrum products containing trace THC, the active THC may appear in a blood test at low concentrations, particularly shortly after use.
How long after using edibles can a blood test detect THC?+
Edibles produce delayed peaks — typically 2–4 hours after consumption — due to hepatic first-pass metabolism. Peak blood THC from edibles may be lower than from smoking but lasts longer, remaining detectable for 6–8 hours in occasional users.

Related Guides