Judges are more than willing to allow the results of these tests into evidence based on the pseudo-scientific exercise of retrograde extrapolation - more commonly knows as throwing a dart at a board.
But this past December, the Nevada Supreme Court threw a monkey wrench into the works when it held that the results of a blood draw taken over two hours after an arrest weren't relevant.
In The State of Nevada v. The Eighth Judicial District Court and the Honorable Stefany Ann Miley, 127 Nev.Adv.Op. 84 (Nev. 2011), Mr. Bobby Armstrong collided with another car, causing serious bodily injury to the other driver. Almost two-and-a-half hours later a blood draw was conducted. According to the test, Mr. Armstrong had an alcohol concentration of .18. Mr. Armstrong moved to suppress the results of the test on the grounds that the blood was drawn outside the state's two-hour window, only one sample was conducted and that retrograde extrapolation was unreliable, irrelevant and unfairly prejudicial.
The trial court ruled that the test results were inadmissible but allowed prosecutors to introduce other evidence of Mr. Armstrong's intoxication. The state's writ of mandamus (thus accounting for the bizarre style of the case) followed.
The Nevada Supreme Court held that retrograde extrapolation was reliable. But the Court also found that the the relevance of the evidence was far outweighed by its prejudicial effect. In coming to its conclusion, the Court looked to the Mata decision in Texas in which the Court of Criminal Appeals laid out a list of factors to be considered when determining the reliability of retrograde extrapolation. These factors included the length of time between the arrest and the test, the number of samples taken and whether the expert had knowledge of the defendant's individual characteristics when
In this case the Court was concerned that the blood sample was taken more than two hours after the stop and that the state's expert had very little knowledge of any of Mr. Armstrong's individual characteristics. In fact, the state's expert based much of his testimony on the characteristics of an "average" person, not Mr. Armstrong. But the Court's biggest concern was that only one blood sample was taken - giving just one point of reference.
The Court reasoned that if two blood samples had been taken some time apart, then the results of those tests would tell whether Mr. Armstrong was absorbing or eliminating at the time of the accident. With but one sample, Mr. Armstrong's alcohol concentration could have been higher, lower or the same at the time of the accident as it was at the time of the test.