Blood Tests


How is blood tested? Are the blood test results infallible? Blood tests involve drawing blood from a vein and not an artery, by a qualified technician. The samples must be properly logged into evidence and stored. Ultimately, police ship the samples to a laboratory that tests them for the presence of alcohol. The laboratory generates a report documenting the test results. Above all, blood tests are not infallible and have several important sources of error.

How is blood tested?

Blood Test in DUI CaseWhen blood is drawn, it is placed into a container that has preservatives and anticoagulants. The preservative, typically sodium fluoride, stops decomposition of the blood and bacterial contamination. Without the correct amount of preservatives, the blood could decompose because it is an organic compound. Blood decomposition would create its own alcohol and thus raise the alcohol level artificially in the sample. If blood clots, the blood alcohol concentration will not be evenly distributed between the solid and liquid matter. The blood alcohol concentration would then become artificially too high.

Three are three methods of analyzing blood: gas chromatography, enzymatic and dichromate. Of the three, gas chromatography is the most common. Blood analysis by gas chromatography uses a machine called a gas chromatograph. Two methods are used by gas chromatographs: head space gas chromatography and direct injection gas chromatography. Head space chromatography involves ethanol being transferred from the liquid blood into vapor above the liquid by addition of a solution and then being heated. The direct injection chromatography involves whole blood or other fluids being injected directly into the machine with blood flaking on a heated area and the alcohol being carried into a column.

How can blood results be disputed?

Blood analysis for alcohol can be more accurate than that of breath. But blood results are not infallible and can be challenged. First, the person performing the blood draw must be properly licensed and trained. Second, the person drawing the blood must perform the blood draw correctly. Venous and not arterial blood must be taken; contamination of the draw must be prevented by not using an alcohol swab on the skin before the draw. Third, the prosecution must show that the “chain of custody” has been properly maintained. The prosecutor must prove that the blood that was drawn was the blood that was tested, and that it was properly logged into evidence, stored and transported to be tested.

Other errors associated with blood are the samples being too small for analysis; persons handling the sample contaminated it; the “sample” was from someone’s blood draw and not the driver in question; the sample was taken during the absorption period (before the alcohol in the driver’s body was equally distributed) and was taken from a portion of the person’s body where the alcohol concentration was higher; the machine analyzing the sample was inaccurate; the person performing the analysis was not qualified; and the person performing the analysis made mistakes.