How to Challenge a Blood Test - You don't always have to plead guilty

How testing is done – the gas chromatograph

Blood testing is done with a machine called a gas chomatograph. Most people – including jurors – are surprised to learn that the lab doesn’t actually test the blood. Instead, the machine tests the gas in the head space of the vial. There is a scientific principle which establishes that there is a relationship between the gas emitted by a submitted a substance, and the amount of material in a substance. In other words, the machine is testing the gas emitted by the blood. The gas is created by heating the vial.

The chromatograph pushes the case through a column, and a machine at the end of the column called a flame ionization detector at the end takes a measurement. The results are then printed out on graph.

It is important to recognize that the chromatograph is just a machine, and by itself, can’t distinguish substances. Instead, it has to be “trained”. That is done through calibration and test runs. A series of known substances are pushed through the column, and the machine notes when the exit. That is the baseline used for measurement. For instance, if the known ethanol standard exits at 5.5 seconds, then any substance exiting at the same time will be considered ethanol. The machine uses a computer program to convert the reading into a blood alcohol reading – the machine doesn’t tell you how much is there; that must be done by the computer program. As with most things in breath testing, how the program works – i.e. the code – is a secret.

To review a blood test, the lawyer must get a number of records – which are normally not sent to the district attorney’s office. Those records include not only the ones associate with the particular client, but also calibration records, as well as records concerning the other tests done at the same time. Those must be carefully examined to determine whether are any issues that might have an impact on the reliability of the results.

Results are not exact – the margin of error

As with all scientific measurements, there is a margin of error. Most labs will note that on the report. It is important to look at that in cases where the reading is only slightly over the legal limit. In some cases, when the margin of error is applied, the actual result could be in a range that is both below and above the legal limit.

Thorough investigation and review is critical

If you have a blood test it is critical to get a lawyer who understands the science, and knows what to look for. While there aren’t problems with every test, there is no way to know that without a thorough review. If that is not done you may be pleading guilty to a case you could win.

If you want more information on this subject, get a copy of our DWI Survial Guide. If you need help with a DWI case, and want to talk with an experienced attorney who knows the issues, give us a call at (254) 781-3588.


Walter Reaves
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Criminal Defense Attorney Walter Reaves has been practicing law for over 30 years.