The Difference Between Serum And Whole Blood In DUI Blood Testing

Posted on: 16 August 2016

In addition to breath testing, law enforcement officials may also use blood samples in order to measure blood alcohol content (BAC). Blood samples obtained during DUI investigations are generally analyzed in one of two ways -- as a whole blood sample or serum sample. The following explains the differences between the two and how both testing methods could affect your DUI case.

How Blood Samples Are Used in DUI Cases

For law enforcement officials, collecting a blood sample is a controlled process that requires several careful steps to be taken. For starters, the police officers at the scene must first either obtain consent or have probable cause to obtain a blood sample. The sample itself must be collected by licensed and trained medical personnel (including certified paramedics, registered nurses and trained phlebotomists) in the presence of a sworn law enforcement official.

Once a sample of your blood is taken, the sample is documented and sent off to a forensic laboratory to be analyzed on behalf of law enforcement. At the laboratory, the sample is analyzed through the use of gas chromatography, where the sample is vaporized and its components separated and analyzed for alcohol content.

What analysts usually look for is the concentration of alcohol in terms of grams per 100 milliliters of blood (for whole blood samples) or milligrams per deciliter (for serum blood). A person tested for BAC may be considered under the influence if the concentration of alcohol in their blood sample is equal to or exceeds 0.08 percent (or 80 milligrams per deciliter).

Defining Serum and Whole Blood

The majority of blood samples drawn in DUI cases are tested as whole blood samples. These samples are named such because they contain all of the things normally found in blood, including the plasma and platelets. Whole blood samples are so commonly used in legal cases that they're often referred to as "legal blood."

Serum samples, on the other hand, are blood samples that have undergone a fractionation process to separate the whole blood sample into its component parts. The end result is a clear solution of blood plasma that doesn't contain red or white blood cells. Since these samples are taken under hospital conditions, they're often known as "medical blood."

One crucial difference between serum blood and whole blood is that serum blood's higher water content can also result in a higher concentration of alcohol due to alcohol's affinity towards water. As a result, testing a typical serum blood sample could result in a BAC that's 15 to 20 percent higher than a BAC obtained from a whole blood sample. For this reason, serum blood results must be converted into an equivalent whole blood reading before being used as evidence.

The Effects of Whole and Serum Blood on Your DUI Case

The court's decision to use whole blood or serum blood as evidence may have a sizable impact on your DUI case, especially if the results of the test show a BAC that's at or close to 0.08 percent. A BAC of 0.08 percent produced by a serum blood sample may be closer to a BAC of 0.07 percent if converted into an equivalent whole blood result. This distinction could easily mean the difference between a DUI conviction and a dismissed case.

When using serum blood, the state may be compelled to divulge how it determined the conversion factor used to convert serum blood results into an equivalent whole blood result. To eliminate ambiguities, some states require that a specific conversion factor be used to obtain a whole blood equivalent.

Other factors may also impact the integrity of the blood sample itself. For instance, improper storage of the blood sample could cause the sample to undergo fermentation, dramatically increasing the amount of alcohol in the sample and leading to a false reading. 

For more information about how blood and how its used can affect your DUI case, contact a DUI attorney, such as those at Chichester Law Office.