Drawing blood as part of a DWI arrest (either by consent of the arrestee or by court order in the form of a search warrant) has increased exponentially in Texas since 2008. It is now a rare circumstance for a DWI case in North Texas not to have a breath or blood alcohol test result. When blood is involved, one of the first questions the defense must answer is: what kind of blood test produced this result, and how reliable is it?

Not all blood alcohol results are created equal. There are two fundamentally different methods of measuring blood alcohol that appear in Texas DWI cases, and they carry very different levels of forensic reliability. Understanding the distinction between “legal blood” and “hospital blood” is foundational to evaluating the evidence in any blood-draw DWI case.

Blood drawn in DWI arrest cases is delivered to a forensic laboratory for analysis. The method of analysis used by Texas forensic laboratories is headspace gas chromatography with flame ionization detection which is commonly referred to as GC-FID, or simply “GC.”

GC-FID is a separation science. It works by separating the volatile compounds found within a whole blood specimen so that each can be individually identified and quantified. During analysis, a visual representation of the separation is generated called the chromatogram. When a specimen is properly prepared, the instrument is correctly calibrated, and the separation is adequate, the quantity of ethanol in the sample can be reliably measured and reported as a whole blood alcohol concentration.

Gas chromatography is the gold standard for forensic blood alcohol measurement not because it is perfect, but because it is an indirect measurement of the ethanol molecule itself, using a validated methodology with established quality control standards. Blood alcohol results generated by GC-FID analysis in a forensic laboratory are what practitioners refer to as “legal blood.”

Even with GC-FID analysis, the defense must examine the complete analytical record, not just the reported number. Contamination can occur before, during, or after the blood specimen is drawn. Errors can occur during sample preparation. Instrument calibration must be verified. The chromatogram itself (the visual output of the separation) must be examined for evidence of peak overlap, baseline drift, or other anomalies that can affect the reliability of the reported result.

“Hospital Blood”: Enzymatic Serum Ethanol Testing

The second type of blood alcohol measurement that appears in DWI cases (and the one that is frequently misunderstood by both defendants and their attorneys) is enzymatic serum ethanol testing, or “hospital blood.” This method is used in hospital laboratories, not forensic laboratories.

Enzymatic testing is quick and inexpensive. Unlike GC-FID, which measures the ethanol molecule, enzymatic testing measures alcohol concentration indirectly by measuring the reaction of alcohol with its specific enzyme (alcohol dehydrogenase), using the resulting change in a cofactor (NAD+) as a proxy for the alcohol concentration. It is not a direct measurement. It is a screening method that, in a clinical context, is entirely appropriate for its intended purpose: rapid assessment of a patient’s alcohol level in an emergency or treatment setting.

Enzymatic testing is not a forensic method of quantifying blood ethanol. It was not designed or validated for use as evidence in criminal proceedings. The appropriate standard is that enzymatic results should be confirmed using gas chromatography before being used for evidentiary purposes. In DWI prosecutions, however, this confirmation step is frequently absent and the hospital lab result is what the State uses to prosecute the case.

The typical case where this type of testing becomes the central issue involves an accident: the driver was transported to a hospital, a blood alcohol level was obtained through the hospital laboratory and noted in the medical records, and those records were later subpoenaed by the State to support prosecution for DWI, intoxication assault, or intoxication manslaughter.

The Critical Difference: Serum vs. Whole Blood

Beyond the methodological differences between GC-FID and enzymatic testing, there is a fundamental specimen difference that makes direct comparison between the two results misleading if not corrected for.

GC-FID forensic analysis is performed on whole blood which is the complete blood specimen including red blood cells, white blood cells, and platelets. Hospital enzymatic testing is performed on plasma or serum, not whole blood.

Plasma results when whole blood is centrifuged (spun in a centrifuge) and the cellular components (red blood cells, white blood cells, and platelets) are removed. Serum is essentially the same as plasma, minus any coagulation proteins that were active at the time of separation.

Measuring plasma or serum for alcohol concentration yields a result 16 to 25% higher than the alcohol concentration in the corresponding whole blood. This is not a flaw in the measurement. It reflects the fact that ethanol distributes differently between the cellular and fluid components of blood. The water content of plasma is higher than that of whole blood, so a given amount of ethanol appears at a higher concentration in plasma than in the same volume of whole blood.

Different experts use different conversion factors. Most will testify to a range between 1.13 and 1.20 (meaning a 13% to 20% reduction from the plasma result to estimate whole blood concentration), though some use factors as high as 1.25.

The practical consequence of this conversion is substantial, particularly at BAC levels near the legal threshold. Consider a hospital lab result of 0.10 serum/plasma alcohol concentration:

Conversion to estimated whole blood alcohol concentration:

0.10 ÷ 1.16 = 0.086

0.10 ÷ 1.18 = 0.084

0.10 ÷ 1.20 = 0.083

0.10 ÷ 1.25 = 0.080

At the upper end of the conversion range, a hospital result of 0.10 may correspond to a whole blood alcohol concentration at or near the legal limit of 0.08%. This conversion argument is available in every case where the prosecution relies on a hospital enzymatic result and it is an argument that requires forensic science expertise to make effectively.

Additional Reliability Concerns with Enzymatic Testing

Beyond the serum-to-whole-blood conversion issue, enzymatic testing has its own reliability vulnerabilities that do not apply to GC-FID:

Interfering substances.  Multiple substances can interfere with the enzymatic reaction and produce falsely elevated results. These include isopropanol (found in certain medications and cleaning products), acetaldehyde, lactate (elevated in patients with liver disease, diabetes, or shock), and certain medications that alter NAD+/NADH ratios. A patient who is seriously ill, in metabolic distress, or who has ingested certain substances may show an elevated enzymatic result that does not accurately reflect their blood alcohol concentration.

No forensic chain of custody.  Hospital blood draws are performed by medical personnel for treatment purposes under different protocols than forensic blood draws. The documentation of the specimen from collection through analysis is generally not maintained to the standards required for forensic evidence. The chain of custody for a hospital blood specimen is typically less rigorous than for a blood draw performed pursuant to a search warrant.

No independent retest specimen.  When blood is drawn pursuant to a DWI arrest, the defendant has the right to have a portion of the specimen independently tested. Hospital specimens are not collected with this right in mind and may not be available for independent analysis.

What This Means for Your Case

If your DWI case involves a blood alcohol result (whether from a forensic laboratory or a hospital) the defense must understand the type of testing performed, the specimen type analyzed, the methodology’s limitations, and whether the result accurately reflects your actual blood alcohol concentration at the time of driving.

In hospital blood cases specifically, the defense should: identify whether the result is a serum or plasma result rather than a whole blood result; apply the appropriate conversion factor; evaluate whether any interfering substances were present; examine the hospital records for evidence of clinical conditions that could affect the result; and challenge the use of an unconfirmed screening test as the evidentiary foundation for a criminal prosecution.

The defense attorney handling a DWI case involving blood alcohol evidence needs more than legal knowledge. They need a working understanding of blood alcohol testing methodology, the potential errors that occur during specimen collection and analysis, and the forensic science of ethanol distribution in biological specimens. This is not general legal knowledge. It is analytical chemistry and pharmacology.

Deandra Grant holds the ACS-CHAL Forensic Lawyer-Scientist designation, a Master’s Degree in Pharmaceutical Science, and a Graduate Certificate in Forensic Toxicology. Partner Douglas Huff holds the same ACS-CHAL designation. When blood alcohol evidence is at issue in a North or Central Texas DWI case, it is reviewed at the chemistry level.

If you are facing DWI charges involving a blood alcohol test result (whether from a forensic laboratory or a hospital) call (214) 225-7117 for a free, confidential consultation. Or schedule online at texasdwisite.com.