Blood Evidence and Retrograde Extrapolation – The 2 Hour Rule and Blood Evidence

A blood sample is often a key piece of evidence in a DUI prosecution and therefore an important point in the preparation of a DUI defense case. While an individual may refuse to supply a blood sample after an arrest, one risks a one year loss of license simply based on that refusal. The license suspension is a civil sanction and therefore outside the scope of a criminal prosecution. Technically, the Commonwealth of Pennsylvania could withdraw charges against you and still suspend your license simply because you refused to submit to a blood test. I therefore don’t recommend refusing a blood test under any circumstance and it’s much better to have your attorney dispute the admissibility of the test with pretrial motions and arguments at trial. This article, however, focuses on the use of blood after a certain period of time (retrograde extrapolation) and not its admissibility under the Fourth and Fourteenth Amendment to the United States Constitution (or Article 1 Section 8 of the Pennsylvania Constitution (illegal searches and seizures.))

The Pennsylvania DUI statute does permit the Court to convict someone without the presentation of blood evidence but if the prosecution is pursuing a case under that statute (3802(a)(1)) the case becomes substantially weaker. Blood evidence is obviously more objective as chemical analysis provides hard data. A prosecution for “general impairment” however forces the prosecution to rely on the testimony of a law enforcement officer and his/her subjective observations at the time of the alleged incident. Subjective observations, unlike objective data, are open to interpretation as there is a variety of reasons to explain the unusual movements of an automobile or a person’s demeanor following a traffic stop on a suspicion of DUI.

The presentation of blood evidence at a DUI trial requires the prosecution to present testimony regarding the analysis of whole blood BAC. It is important to understand that to calculate whole blood BAC, the prosecution must present evidence which mathematically converts blood serum to whole blood BAC utilizing a predetermined conversion factor. Blood serum is that which remains after the blood’s clotting agents are separated from the plasma. The prosecution is not able to use blood serum as it is less dense then whole blood and the alcohol in the serum will exceed that which is in whole blood. This will incorrectly estimate a person’s level of intoxication. Retrograde extrapolation is the scientific method used to arrive at an educated estimate of one’s BAC at the time of driving.

In 2004, Pennsylvania amended the DUI statute to include language of a “two hour window” for impaired driving. Basically, the prosecution must establish that a chemical test showed that the person was intoxicated within two hours of their arrest. While there are exceptions to this two hour rule, that is a subject to a different article. Retrograde extrapolation is not required but where the chemical test provides only a “weak inference” of a defendant’s level of intoxication the prosecution will not be able to meet its burden of proof to achieve a conviction. It is always important to keep in mind that the burden of proof in a criminal prosecution is guilt beyond a reasonable doubt.

As stated earlier, the DUI statute allows for a conviction based on observations alone but the harsher sections of the statute requires the establishment of a specific level of intoxication (3802(b)-.10-.15 and 3802(c).16 or higher). Retrograde extrapolation is only required where chemical analysis reveals that a person’s level of intoxication is reasonably close to the specific percentage amounts in the DUI statute. Retrograde extrapolation is not needed where a person, however, is heavily intoxicated such as where their BAC is two or three times the legal limit. It is important to remember, however, that in many DUI cases a person’s BAC level is just above the percentage cutoff and so a successful defense argument could lead to a full acquittal or, in the alternative, a conviction for a lesser offense under the statute allowing a person to avoid in jail time and a license suspension in some cases.