Blood testing is often viewed as the most dependable way to measure alcohol concentration in a North Carolina DWI case. The science
DWI defense lawyer Bill Powers has spent more than thirty years watching how blood testing evidence develops in courtrooms across Mecklenburg County and throughout North Carolina. During that time, he has cross examined toxicologists, reviewed extensive laboratory documentation, and taught officers and lawyers about breath and blood science, standardized field sobriety tests, and trial strategies and protocols. His experience includes the practical knowledge of how jurors interpret toxicology evidence and how those interpretations can shift once they hear how the underlying science actually works.
If you are dealing with a DWI charge that involves blood testing, or if you are a lawyer looking to sharpen your trial approach to forensic toxicology, please contact the DWI defense lawyers at Powers Law Firm in Charlotte. Call or TEXT 704-342-4357 to discuss how careful preparation and scientific clarity can shape the outcome of a case.
Forensic Blood Alcohol Testing (Headspace GC and GC/MS)
When a DUI investigation turns to blood testing, sometimes considered the gold standard of BAC measurement, a different set of scientific protocols and issues may come into play. Blood tests are generally more reliable than breath tests. Instead of estimating BAC via breath, the blood is drawn from the suspect and analyzed in a laboratory to directly measure alcohol concentration.
However, blood testing involves complex analytical chemistry, typically gas chromatography, and is subject to its own potential errors. A DUI defense attorney should understand how blood samples are handled and analyzed to uncover vulnerabilities with the science and technology.
The most common technique for BAC analysis in forensic labs is Headspace Gas Chromatography (HS-GC), often coupled with a flame-ionization detector (FID) or mass spectrometer (MS) for detection. In HS-GC, a measured volume of the blood sample is sealed in a vial and gently heated. Alcohol (and other volatile substances) in the blood evaporate into the airspace (headspace) above the liquid in the vial. A portion of that vapor (which now contains alcohol proportional to the concentration in the blood) is then injected by the machine into the gas chromatograph.
The gas chromatograph is essentially a long tube (column) coated with a chemical; it is heated in an oven and has an inert carrier gas flowing through it. As the vapor travels through the column, the various components in the vapor separate because they interact with the column or “phase” differently.
Each substance will come out of the column at a characteristic time (called the retention time). Ethanol, for example, will “elute” at a known retention time under the given conditions. The detector at the end (FID or MS) will sense the ethanol as it exits the column.
An FID does this by burning the sample and measuring ions (which, for organic compounds like ethanol, produce a proportional signal), whereas a mass spectrometer would identify substances by their molecular fragmentation pattern. Some state labs use HS-GC with dual columns and dual FID detectors, or one FID and one MS, to enhance reliability.
| Method/Issue | Description | Key Forensic Challenge / Defense Focus |
| Headspace Gas Chromatography (HS-GC) | The primary lab method. Blood is heated to vaporize alcohol into the headspace, which is then injected into the GC column for separation. | Calibration/Controls: Checking if the machine was calibrated correctly and if control samples were within acceptable limits. |
| Retention Time | The specific time it takes for ethanol (or any substance) to exit the GC column, used for identification. | Interfering Peaks: Looking for unusual peaks on the chromatogram that might indicate another volatile substance was misidentified as ethanol. |
| GC with Mass Spectrometry (GC/MS) | GC coupled with a Mass Spectrometer, which identifies compounds by their unique molecular fragmentation pattern. | Specificity: Used for greater specificity than standard GC, reducing the risk of misidentification; defense may argue it should have been used. |
| Chain-of-Custody | The documented, chronological record of the sample’s handling from the moment it was drawn to the time of analysis. | Gaps/Irregularities: Looking for breaks in the record that suggest the sample may have been compromised, mislabeled, or improperly stored. |
| Fermentation (Post-Draw) | Microbial growth in poorly preserved or stored blood that consumes sugar and artificially produces ethanol, raising the BAC result. | Preservative/Storage: Checking if the tube contained adequate sodium fluoride and if the sample was refrigerated “as soon as practicable.” |
| Contamination | Introduction of foreign substances, such as from a non-standard alcohol swab during the draw or other lab solvents. | Swab Use: Confirming if non-alcohol based swabs (e.g., Betadine) were used, and reviewing chromatograms for foreign peaks like isopropanol. |
| Inadequate Preservative | Insufficient or improperly mixed sodium fluoride (preservative) or anticoagulant. | Integrity Issues: Checking for evidence of clotting (due to inadequate anticoagulant) or hemolysis (ruptured red cells). |
The result of the chromatographic analysis is a printout (a chromatogram) showing a peak for ethanol. The height or area of the ethanol peak is compared against peaks from calibration solutions of known alcohol concentration run earlier, to quantify the BAC of the blood sample.
Gas chromatography, when done properly, can prove highly accurate and specific. It’s often called the “gold standard” for BAC. However, “gold standard” doesn’t mean perfect or unassailable. Errors can occur at any step, from how the blood was drawn and stored, to how the lab analyst performed the test, to how the machine was calibrated.
The defense should seek to obtain and scrutinize forensic lab documents such as the chain-of-custody forms, the lab’s standard operating procedures (SOPs), maintenance and calibration records of the GC machine, the analyst’s batch worksheet, and the chromatograms themselves.
These can contain the seeds of a defense or at least points for cross-examination. For instance, upon reviewing chromatograms, a defense expert might notice an unusual peak that could indicate the presence of another volatile compound (maybe acetone or some solvent), which could confuse the analysis if not properly resolved.
It’s also important to note what type of analysis was done. Some labs use GC/MS confirmatory testing, especially for drugs, and sometimes for alcohol in certain cases (GC coupled with mass spectrometry). GC/MS is even more specific (since the mass spectrometer can positively identify ethanol by its mass fragmentation pattern), reducing the chance that an interfering substance was misidentified as ethanol. A niche issue is endogenous alcohol production.
In rare cases, if a blood sample is not preserved correctly, microbes can ferment glucose in the blood to produce ethanol (more on that below). GC with dual column and MS can usually distinguish fermentative byproducts, but a single-column GC might not.
Is the “Rising BAC Defense” Legit?
While jurors (and even lawyers) often consider a blood test result to be unassailable objective truth, the reality is that it relies on a chain of human and machine actions. Each action is a chance for error, and gas chromatography, though powerful, must be done carefully. The role of the defense is to audit the process.
Frequently Asked Questions About DWI Blood Testing in North Carolina
How accurate is North Carolina DWI blood testing compared to breath testing?
North Carolina DWI blood testing is generally more accurate than breath testing because it measures alcohol directly rather than estimating it from breath. Accuracy depends on proper collection, preservation, storage, and gas chromatography procedures. Even small deviations in protocol can affect reliability, which is why defense lawyers review chain-of-custody, calibration data, and chromatograms.
What scientific method is used to measure alcohol in a blood sample?
Most North Carolina crime labs rely on headspace gas chromatography to measure alcohol in a blood sample. The method separates volatile compounds and identifies ethanol by retention time and detector response. It is effective when performed correctly, but errors in sample preparation, calibration, or device operation can affect the result.
Can a blood test in a DWI case ever be wrong?
A blood test can be wrong if contamination, fermentation, mislabeling, improper storage, faulty calibration, or analyst error occurs. BAC errors, while somewhat uncommon, are documented in forensic literature. Reviewing chain-of-custody, SOPs, calibration records, and chromatograms helps determine whether the result is scientifically sound.
What is fermentation and how can it affect a DWI blood alcohol result?
Fermentation occurs when microbes in a poorly preserved or improperly stored blood sample begin producing ethanol after collection. If a tube lacks sodium fluoride or was not refrigerated, the BAC could rise artificially. This defense depends on documented storage conditions, preservative levels, and the timeline before testing.
How does chain-of-custody affect the validity of a DWI blood test in North Carolina?
Chain-of-custody documents every transfer of the blood sample from collection to analysis. If the record contains gaps, inconsistent timestamps, unclear signatures, or questionable storage locations, a lawyer could possibly argue that the reliability of the blood result is weakened (on a case-by-case basis). Even the possibility of mishandling might support reasonable doubt.
What role do preservatives and anticoagulants play in blood alcohol testing?
Blood alcohol tubes are supposed to contain sodium fluoride to inhibit microbial growth and an anticoagulant to prevent clotting. If the preservative was inadequate or not mixed properly, fermentation or chemical degradation can in some circumstances occur. Documentation of tube type, kit lot numbers, and analyst observations may prove important given the specifics of the individual fact pattern and allegations.
Why do defense lawyers request chromatograms and calibration records?
Chromatograms show how the gas chromatograph separated the components of the blood sample and whether the ethanol peak was clean or interfered with by another compound. Calibration records confirm whether control samples tested within acceptable limits. Reviewing both is a good idea to evaluate the scientific reliability of the reported BAC.
Can a lab error change the BAC reported in a DWI case?
A lab error could change the reported BAC if the calibration curve was inaccurate, if quality control samples were out of range, or if the analyst deviated from laboratory protocols. Labs perform hundreds of tests, and even small oversights might improperly influence results. Defense review of the batch file may help uncover BAC testing errors and miscalculations.
Is GC/MS more reliable than basic gas chromatography in DWI cases?
GC/MS is generally more specific than either a single or dual column gas chromatograph because it confirms substances by molecular fragmentation. Some labs use GC/MS only for drug testing, while others use it to confirm alcohol results. When available, GC/MS data can resolve questions about interfering compounds or unexpected peaks.
Can a defense lawyer challenge a DWI blood test in North Carolina?
DWI defense lawyers examine chain-of-custody, review chromatograms, evaluate calibration and control data, confirm preservative adequacy, identify storage issues, and question the analyst’s training or proficiency (obviously when appropriate). The goal is not to attack science itself but to determine whether the scientific method was followed correctly in that specific case.
Chain-of-Custody and Laboratory Procedures
A fundamental aspect of blood evidence is the chain of custody. From the moment the blood is drawn from the suspect, its handling must be carefully documented to ensure that the result reported actually corresponds to that suspect’s blood, uncompromised and unaltered.
Chain-of-custody is essentially the paper (or electronic) trail that records who had possession of the sample, when, and what was done. It usually starts with the phlebotomist or officer who drew the blood, then transfers the materials to an evidence refrigerator, then to a courier or directly to the lab, then within the lab possibly from an accessioning person to an analyst, etc., up to the point of analysis and storage. Each transfer is ideally logged with date, time, and signature. The purpose of chain-of-custody is to prevent tampering or misidentification and to be able to testify that the blood result presented in court belongs to the defendant’s sample to a reasonable degree of certainty.
Defense counsel may be able to obtain the chain-of-custody logs in discovery or voluntary provision of testing documentation. Look for any gaps or irregularities. Was the sample left unaccounted for at any period? Was it mailed via regular mail (a concern in some cases) or handled by many different people? Did the lab receive it in a sealed condition with intact labels? Any break in the chain of custody could be an opportunity to argue the sample may have been mishandled or even mixed up with another. While outright sample swaps are unlikely, even the possibility can instill reasonable doubt in appropriate circumstances.
A sloppy chain of custody can diminish confidence in the evidence and may violate testing protocols and best practices. There have been cases where someone received another person’s results due to mislabeling. These are rare but can be game-changing when they occur.
Laboratory procedures themselves are governed by protocols and often by accreditation standards (e.g., ISO 17025 or ASCLD-LAB for forensic labs). The lab should have an SOP for blood alcohol analysis. Helpful materials to review include: the lab’s SOP manual (at least the sections on blood alcohol and quality control), validation studies for their method, and the bench notes/records of the analyst. These can be dense technical documents. It can help to consult a forensic toxicologist or chemist consultant. The goal is to ensure the lab followed its own procedures and that those procedures were sound. It’s sometimes helpful to review and consider:
- Proper preservatives and anticoagulants: Blood samples for alcohol are usually collected in vacuum tubes containing preservatives (like sodium fluoride, which inhibits microbial growth) and anticoagulants (like potassium oxalate or EDTA, to prevent clotting). These chemicals maintain sample integrity. If a blood kit lacks preservatives, fermentation can occur (bacteria or yeast in the blood producing ethanol artificially). Check the lab report to see if it notes the type of kit or if any clots (evidence of hemolysis) were observed. A clotted sample might indicate inadequate mixing with anticoagulant, potentially leading to unreliable results. If evidence suggests no preservative or a delayed analysis, the defense might raise a “fermentation defense,” arguing the BAC was falsely elevated due to microbial action. nacdl.org. Without sodium fluoride, blood can create alcohol over time (especially if stored without proper refrigeration). Even with fluoride, given enough time, some fermentation is possible (fluoride greatly slows it but may not always fully prevent it). If the blood was tested weeks or months later (due to crime lab backlogs), this argument may become more salient.
- Calibration and controls in the lab: Just like breathalyzers, the lab instruments must be calibrated. Typically, a series of standard solutions (e.g., 0.02%, 0.08%, 0.20% BAC standards) are run to make a calibration curve. Also, quality control samples, one perhaps near the legal limit (0.08%) and one higher or lower, may be run with each batch. The defense should check if those controls were within acceptable range. Most labs have criteria (e.g., the 0.08% control must test between 0.075–0.085 or similar). If a control was out of range and the batch was still reported, that could prove serious. It may indicate the machine wasn’t performing ideally. Any such anomaly could become cross-exam material.
- Duplicate analysis: Many labs do the test in duplicate (two separate aliquots from the same sample analyzed). If results disagree beyond a certain tolerance (e.g., >0.01 g/dL difference), they may run a third time or investigate. The defense should confirm if duplicates were done and what the results were. If only a single analysis was done, it may be a point to criticize. If duplicates varied, press the analyst on what the variation was and whether it could indicate an issue.
- Analyst proficiency and bias: Laboratory analysts sometimes undergo proficiency testing (blind samples) annually. If allowed, obtain those records to see if the analyst ever flubbed a proficiency test. You may also be able to cross-examine mundane but revealing details. Inquire if the lab is truly independent or part of law enforcement. Some jurors may perceive a police lab as less neutral.
- Results reporting and truncation: Some labs truncate BAC results (drop the thousandths). For instance, a true 0.089 might be reported as 0.08 if truncating. This can cut both ways. Sometimes the lab will report 0.08 when the analysis was 0.084, but they truncate the reported numerical BAC; that could benefit a defendant as the number looks just at the limit.
By examining chain-of-custody and lab procedures, the defense might not always find a “smoking gun” that results in suppression. It could provide the opportunity for an effective cross-examination of the lab analyst. Again, it’s about undermining absolute certainty. Jurors without scientific background might otherwise assume a blood test number is infallible. But if the defense can introduce terms like “contamination,” “fermentation,” “chain-of-custody gap,” or “lab error” supported by evidence, the veneer of infallibility fades.
Errors in Blood Sample Handling: Contamination, Fermentation, and Storage Issues
While laboratory analysis is an important aspect of blood testing, the handling of the blood sample before it gets to the lab can be important. There can be several “weak links” in this chain that a defense attorney should consider: contamination during the draw, preservation failures, and storage conditions. Any of these can lead to an inaccurately high BAC result or at least open the door to doubt about the sample integrity.
- Contamination during blood draw: The blood draw process itself can introduce foreign substances if not done properly. A classic (and frankly outdated) issue is the use of an alcohol swab on the skin to avoid infection before inserting the needle. The logic was, that if an alcohol-based disinfectant is used (testing protocols now include use of non-alcohol iodine or betadine swabs instead), then there could be a risk that isopropyl alcohol from the swab could get into the blood sample, potentially registering on certain tests. Most modern GC methods differentiate isopropanol from ethanol, but that wasn’t always the case. Apart from swab contamination, improper phlebotomy technique could theoretically result in hemolysis.
- Fermentation (microbial contamination); Fermentation may become a concern if a blood sample is not preserved or stored correctly. The fermentation defense is old-school. It posits that the blood sample’s BAC increased after the draw due to microbial action producing ethanol. nacdl.org. Key facts to support this defense include lack or insufficiency of preservative (sodium fluoride), evidence of bacterial contamination (some labs actually note if a sample had clots or signs of hemolysis/bacteria), a long delay before analysis, or storage at warm (non-refrigerated) temperature. If the lab result shows the presence of unexpected byproducts, those could be markers of fermentation. Most labs won’t check for that unless the sample looks suspicious.
- Improper storage and handling: Besides fermentation, even just mishandling can adversely affect a sample. If tubes were stored in a hot car trunk for hours, or if they were frozen (which can break blood cells), these can become relevant issues to consider. Chemical changes can occur. Look at chain-of-custody for where the sample went. Ask if samples are refrigerated at each step. Many police procedures require refrigeration “as soon as practicable.” If an officer left the sample in his patrol car for the rest of his shift on a summer day, that could be a problem. If mailing, was it overnight with cold packs or regular mail?
- Mix-ups and mislabeling: While rare, human error like mislabeling vials or switching samples is the nightmare scenario for labs. If the defense finds any evidence or even insinuation of this (for example, if two DUI arrestees’ samples came in the same batch and there’s confusion in the paperwork), it might be a worthwhile area of inquiry. Even the hint that the lab could have swapped samples could result in reasonable doubt. “How can we be sure the 0.16 result wasn’t actually from John Doe’s sample and not my client’s?” Check if the lab has a two-person verification on labeling. Check if the sample ID on the report matches the police report ID. It usually will, but diligence can make a difference. Labs process a large number of cases, and mistakes have happened. We all make mistakes.
- Laboratory error: Sometimes, lab mistakes occur beyond handling. The defense might benefit from taking a look at the lab packet for anomalies and, if possible, speak with the lab technicians before trial.
In challenging blood evidence, attention to detail can be your friend. Even if the science seems intimidating, remember the core theme: reasonable doubt can arise from any break in proper procedure, best practices, or any plausible alternative explanation for the BAC result.
When is Video Evidence Admissible?
A juror (or lawyer, prosecutor, or judge) might not ever fully understand how headspace GC works. The attorney’s job is to translate the science into commonsense terms that support the theory of the case.
Why Toxicology Evidence Requires More Than a Number
Blood results carry weight because they feel definitive. A printed BAC value looks objective, precise, and grounded in scientific truth. Yet every number in a toxicology report represents a chain of events that occurred long before a juror ever sees the exhibit. Those events include how the blood was drawn, how it was preserved, how it was stored, how the lab followed its own protocols, and how the analyst interpreted the chromatograms. Each link matters, and each can create space for reasonable doubt when the documentation or testimony reveals something unexpected.
Understanding the difference between laboratory theory and real-world practice can reshape a DWI case. A lawyer who knows how headspace gas chromatography works, who can read and interpret calibration curves, who understands chain-of-custody weaknesses, and who can explain fermentation or contamination to a jury has a very different path to trial than someone who relies solely on the printed BAC value. Blood testing is powerful, but it is not automatic. It requires scrutiny, context, and the willingness to review the materials with patience and precision.
If you are facing a DWI charge involving toxicology evidence, or if you want to understand how forensic science influences the strength of a case, you can reach the DUI defense lawyers at Powers Law Firm in Charlotte. Call or TEXT Bill Powers at 704-342-4357 to discuss how scientific accuracy and thorough preparation can influence the defense of a North Carolina DWI case.