What is the Margin of Error in a Blood Test?

Some drivers assume that a blood draw test will prove that they are not driving drunk; however, this can be a false assumption. There is little room to argue that the blood alcohol instrument (Gas Chromatograph) has a significant error, sufficient to create reasonable doubt that the defendant is guilty.

When it comes to testing for blood alcohol concentration (BAC) in a driver’s system, blood testing is a relatively reliable method. Defendants can still attempt to challenge blood test results, but they should have an understanding of the margin of error in a blood test.

One such claim from defendants is that the blood testing instrument measured and reported something other than ethyl alcohol, in effect artificially increasing the reported BAC. Gas chromatography happens to be the method used by most, if not all labs to test alcohol in the blood, and it is a universally accepted separation science.

With this method, an instrument separates the sample in the column, and measures the amount of the substances it tests; each substance comes out of the column at different, yet specific times. Because this method separates volatile substances such as ethyl alcohol and isopropyl alcohol before the analysis, gas chromatography is highly specific and does not allow the interference of other substances.

Gas chromatography has been proven to differentiate other volatile substances from ethyl alcohol, and this validation has been conducted extensively by the scientific community. Further, the lab that conducted the analysis should be able to provide supporting testimony in court.

In an effort to ensure accuracy when testing a defendant’s blood for alcohol, state labs are in the practice of conducting duplicate tests on DUI blood samples. This methodology analyzes a subject’s blood twice by using separate portions of the sample. Additionally, dual columns are often used, thereby testing each portion of a duplicate twice.

In the scientific and legal communities, dual column gas chromatography is considered the “gold standard” for analyzing a defendant’s blood alcohol.

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