A blood test can look like the most scientific, hardest-to-argue-with piece of evidence in a DUI case — but the number on the lab report is only part of the story. In short: Maryland blood testing is used mainly when breath testing isn’t possible or drugs are suspected, requires either a warrant or a valid exception under the Fourth Amendment, must be drawn by a qualified medical person using approved equipment, and — for drugs like marijuana — a positive result doesn’t automatically prove impairment at the time of driving. How the sample was obtained, who handled it, what exactly was measured, and whether the substance found was still active in the body all shape what a blood test can actually prove. This article walks through how Maryland blood testing works in a DUI case and where the evidence can be challenged.
When Is a Blood Test Used Instead of a Breath Test?
Breath testing only measures alcohol — it can’t detect marijuana, prescription medication, or other controlled substances. When an officer suspects drug impairment, alone or combined with alcohol, a blood or urine test becomes the tool that can actually identify what’s in a driver’s system. Blood testing also comes into play when a breath test isn’t a practical option: a driver who is unconscious or seriously injured after a crash, a driver who is physically unable to provide an adequate breath sample, or a case where the investigating officer specifically wants a substance-level analysis rather than just a breath-alcohol estimate.
Because a blood draw is more invasive than breathing into a machine, it comes with a higher constitutional bar. In Missouri v. McNeely (2013), the U.S. Supreme Court held that police generally need a warrant before drawing blood from a DUI suspect, and rejected the argument that the natural dissipation of alcohol in the bloodstream, by itself, automatically creates an emergency excusing that requirement — courts instead look at the totality of the circumstances. The Court later clarified in Birchfield v. North Dakota (2016) that this warrant requirement applies specifically to blood tests, distinguishing them from breath tests, which can generally be required as a search incident to a lawful arrest without a separate warrant. A later case, Mitchell v. Wisconsin (2019), recognized that an unconscious driver’s condition can itself support a warrantless blood draw in many circumstances. In practice, that means a blood draw in a Maryland DUI case was typically obtained either with a warrant, with the driver’s consent, or under one of these narrower exceptions — and which of those applies in a given case is a real, fact-specific question.
Alcohol Testing vs. Drug Testing
Blood tests can be used to measure either alcohol concentration or the presence of drugs, and the two are analyzed and evaluated differently.
| Alcohol (Ethanol) | Drugs (Including Marijuana) | |
|---|---|---|
| Legal standard | Per se limit: 0.08 BAC supports a DUI charge; 0.07–0.08 supports a DWI charge | No per se concentration limit under Md. Code, Transp. § 21-902 — the State must prove the driver was actually impaired |
| What the lab measures | Blood alcohol concentration, typically by gas chromatography | Presence and concentration of the specific substance and, often, its metabolites |
| How straightforward the science is | Well-established relationship between BAC and impairment | Highly variable — concentration doesn’t map onto impairment the same way alcohol does |
| Common defense questions | Chain of custody, lab calibration, timing/absorption | All of the above, plus whether the substance detected was active or a leftover metabolite |
Maryland’s drugged-driving framework treats any drug — legal or illegal, prescription or recreational — the same way under Maryland’s drugged driving laws: the question isn’t whether the substance was in the driver’s system, it’s whether it impaired their ability to drive safely. That distinction is far more contested in drug cases than in alcohol cases, and it’s the central issue in most blood-test drug DUI defenses.
How Blood Samples Are Collected
Maryland law is specific about who can draw blood for a DUI case and how it has to be handled. Under Md. Code, Courts and Judicial Proceedings § 10-304, blood must be obtained by a “qualified medical person” — someone legally permitted to withdraw blood from a person — using equipment approved by the toxicologist in the Maryland State Police Forensic Sciences Division, and the sample must be tested in an approved laboratory.
Notably, the statute also allows an officer who witnessed the draw to testify that they reasonably believed the person who drew the blood was qualified to do so, without necessarily requiring that person to testify themselves. That’s a procedural shortcut worth knowing about, because it means the person who actually drew the blood isn’t automatically part of the courtroom proceeding — which is one more reason chain-of-custody documentation matters as much as it does.
Why Chain of Custody Matters
Chain of custody is the documented, unbroken record of who collected a blood sample, how it was stored, who transported it, and who tested it — from the moment the needle comes out of the driver’s arm to the moment a toxicologist reports a number. Every link in that chain is a place where something can go wrong: a mislabeled vial, a sample stored at the wrong temperature, a gap in the documentation showing who had custody of the sample overnight, or a delay between the draw and the lab analysis that raises questions about degradation.
Blood is biological evidence, not a printout from a machine sitting in front of the officer. It has to travel from a collection site to a state lab, sometimes over days, and pass through multiple hands along the way. A defense attorney reviewing a blood-test DUI case typically requests the complete chain-of-custody documentation — collection records, transport logs, storage conditions, and lab intake records — because a genuine break anywhere in that chain can call the reliability of the final result into question, independent of whether the underlying science was done correctly.
Active Drug vs. Metabolite
This distinction is one of the most important — and most misunderstood — issues in a drug-related blood test, and it’s especially significant in marijuana cases. When the body processes THC (the psychoactive compound in marijuana), it breaks it down into metabolites, including one commonly tested for called THC-COOH. The distinction between the active compound and its metabolites matters enormously:
| Active THC (Delta-9-THC) | THC Metabolites (e.g., THC-COOH) | |
|---|---|---|
| What it indicates | Recent use, more likely to correlate with present psychoactive effects | Past use — the body has already broken the THC down |
| How long it’s detectable | Hours, in most cases | Days to weeks, especially in regular users, since THC is fat-soluble and released gradually |
| Relevance to impairment at the time of driving | More directly relevant | Far less relevant — a positive metabolite result doesn’t establish the driver was impaired, or even used marijuana, on the day of the stop |
Our detailed article on marijuana DWI in Maryland goes deeper into why Maryland has no per se THC limit and how prosecutors build impairment cases around Drug Recognition Expert testimony instead of a simple number. For a broader overview of how Maryland’s marijuana DUI laws work, including penalties and the license consequences of a conviction, see Maryland marijuana DUI laws. The active-versus-metabolite distinction is exactly why a positive marijuana blood test, on its own, proves far less than most people assume.
Does a Positive Test Automatically Prove Impairment?
No — and this is true for drug cases in a way it generally isn’t for alcohol cases. Because Maryland has an established per se limit for alcohol (0.08 BAC), a qualifying alcohol test result does most of the legal work on its own. Drugs are different. Under § 21-902, the State has to prove the driver was actually impaired to a degree that affected their ability to drive safely — the presence of a drug, or even an active-compound concentration, is evidence toward that conclusion, not proof of it by itself.
This matters most with marijuana, where tolerance varies enormously between occasional and regular users, and where — as covered above — a positive result can reflect use from days or weeks earlier rather than impairment behind the wheel. It can also matter with prescription medications, where a person legally taking a prescribed drug as directed can still test positive without necessarily being impaired, though Maryland law does allow a defense based on being unaware that a legally prescribed medication would impair driving ability. The blood test result is one piece of the State’s case — not a substitute for proving actual impairment.
Laboratory and Expert Evidence
Blood test results don’t get admitted into evidence on their own; they typically come in through the testimony of a forensic toxicologist or lab analyst who can explain how the sample was tested, what the result means, and whether standard procedures were followed. Under the U.S. Supreme Court’s decision in Melendez-Diaz v. Massachusetts (2009), a forensic lab report is “testimonial” evidence under the Sixth Amendment’s Confrontation Clause — meaning a defendant generally has the right to cross-examine the analyst who actually performed the testing, rather than having the State simply submit a certificate or report without that person appearing.
This creates a real opportunity for scrutiny. A lab analyst’s methodology, qualifications, testing protocols, and even their record of errors on other cases can all become fair questions in cross-examination. Expert testimony — from a toxicologist called by either side — is often where a blood-test DUI case is actually won or lost, since jurors are typically relying on the expert to translate a lab number into a conclusion about impairment.
Potential Challenges to Blood-Test Evidence
Blood-test evidence can be challenged at several distinct points, and a defense attorney typically works through each of them rather than assuming the lab result is the end of the analysis:
- Whether the blood draw itself was lawful — obtained with a valid warrant, valid consent, or a recognized exception under McNeely, Birchfield, and Mitchell
- Whether the person who drew the blood was actually qualified, and whether approved equipment was used, under § 10-304
- Chain-of-custody gaps between collection, transport, storage, and lab analysis
- Testing methodology and lab accreditation, including whether the analyst followed standard protocols
- Active compound vs. metabolite issues, especially in marijuana and other drug cases
- Whether the result actually establishes impairment, as opposed to mere presence of a substance in the body
These cases often overlap with the same evidence discussed in our related articles on DUI drug cases generally and on Maryland’s broader drugged driving laws, since blood testing is frequently the evidentiary centerpiece in both alcohol and drug prosecutions. A careful review of how the sample was obtained, handled, and tested is usually the starting point for evaluating whether a blood-test result actually supports the charge.
Frequently Asked Questions
Does a blood test always require a warrant in Maryland? Generally, yes, unless the driver consents or a recognized exception applies (such as an unconscious driver). Under Missouri v. McNeely, police can’t rely on the natural dissipation of alcohol alone to justify skipping the warrant requirement — courts look at the full circumstances of each case.
Can I be convicted of a marijuana DUI just because THC showed up in my blood? Not automatically. Maryland has no per se THC limit, so the State has to prove actual impairment, not just the presence of THC or its metabolites. A positive result — especially for inactive metabolites rather than active THC — doesn’t by itself prove you were impaired while driving.
Who is allowed to draw blood for a DUI test in Maryland? Maryland law requires blood to be drawn by a “qualified medical person” using equipment approved by the state toxicologist, and tested at an approved laboratory. If that requirement wasn’t met, or the equipment wasn’t properly approved, that can become a basis to challenge the result.
What’s the difference between active THC and a THC metabolite in a blood test? Active THC (Delta-9-THC) is the compound directly associated with recent use and potential impairment. Metabolites like THC-COOH are what’s left after the body processes THC, and they can remain detectable for days or weeks after any impairment has worn off — long after someone could reasonably be considered “under the influence.”
Can I cross-examine the lab analyst who tested my blood sample? Generally, yes. Under Melendez-Diaz v. Massachusetts, forensic lab reports are considered testimonial evidence, and a defendant has the right to confront and cross-examine the analyst who performed the testing rather than having a report simply submitted without that testimony.
