DEANDRA’S FORENSICS CORNER “LEGAL BLOOD” VS. “HOSPITAL BLOOD”
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.
Blood drawn in these cases is delivered to a forensic laboratory for analysis. The method of analysis is called headspace gas chromatography with flame ionization detector (or “GC”). GC is a separation science. It separates the volatiles found within a whole blood specimen so that they can be quantified. A visual representation is created during analysis. This is called the chromatogram.
If there is good separation of a properly prepared specimen on a calibrated instrument, the quantity or amount of ethanol found in the sample can be reliably measured and reported. Gas chromatography is often referred to as the “gold standard” for this type of measurement. Blood alcohol results on specimens analyzed in a forensic laboratory using gas chromatography are often referred to as “legal blood”.
Another type of analysis found in alcohol-related criminal cases is enzymatic serum ethanol testing or “hospital blood”. Enzymatic testing is typically performed in hospital laboratories. This is not a forensic method of quantifying blood ethanol. Enzymatic assay testing is quick and inexpensive. The process involves measuring an alcohol concentration indirectly by using the reaction of its antibody to an antigen. It is not a direct measurement of the alcohol molecule itself. Enzymatic testing is a screening test that should be confirmed using gas chromatography.
The typical case where this type of testing will be at issue involves an accident where the client was transported to the hospital, an alcohol level obtained through the hospital lab was noted in the medical records, and these records were later subpoenaed by the State to support prosecution for DWI or a related offense.
Plasma or serum, rather than whole blood, is tested using this method. Plasma results when whole blood is centrifuged (spun) and the red blood cells, white blood cells, and platelets are removed. Serum is the same as plasma minus any coagulation proteins. Measuring plasma or serum for alcohol concentration yields a measured result 16-25% higher than the alcohol concentration in whole blood. Different experts will use different conversion numbers. Most experts will testify to a range between 0.13 and 0.20.
EXAMPLE: 0.10 serum/plasma alcohol concentration conversion to a 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
With forensic testing, contamination may occur before, during or after a blood specimen is drawn. Errors can occur during sample preparation and during the analysis itself. In a hospital lab, many substances can interfere with the results of an enzymatic serum ethanol test leading to erroneous results. The defense attorney defending a DWI case in today’s environment needs to have a working knowledge of blood alcohol testing and the potential errors that may occur during the process.