Challenging DUI Blood Evidence Part 5: Cross-Examining Forensic Experts

One question young, ambitious lawyers often ask me is, “how do you effectively cross-examine forensic experts?”  Cross-examining the prosecution’s experts is one of the most important stages in a DUI case because if that “expert” testimony is left unchallenged, the blood alcohol reading or the drug concentration result will be accepted and most certainly will lead to a conviction.

It takes a great deal of preparation and knowledge to cross-examine a forensic expert in a DUI case.
It takes a great deal of preparation and knowledge to cross-examine a forensic expert in a DUI case.

Preparing a successful cross-examination takes a lot of planning and requires a lot of knowledge about the forensic science in question.  I have seen many very good criminal defense attorneys in Pennsylvania, who don’t specialize in DUI get totally bogged down while trying to cross-examine a forensic expert in a DUI case. You cannot “wing it.” It cannot be faked.  This is why it takes a true an experienced DUI attorney to defend DUI cases and challenge the experts the prosecution brings to court.  Here are some important things to remember when cross-examining a forensic expert:

Need to have a plan

When you are cross-examining an expert in pharmacology, toxicology or analytical chemistry, a successful DUI attorney will always go in with a detailed plan of attack. The expert is not going to just fumble away information about how they may have messed up.  It takes detailed questioning and establishing the facts and the truth to be able to expose the mistakes of a forensic witness.

Scientific Knowledge

In Pennsylvania, the threshold for being an expert is very low. By definition, an expert is someone who possesses knowledge “beyond the ken of a layperson” meaning someone can literally read a few articles and be considered an expert in Pennsylvania. Just because someone is called an expert, doesn’t mean their opinion is authoritative or even reliable. This is why it is important to have an organized plan, in order to show the court and the Jury that the expert in question is really limited in his or her knowledge and that their opinion is not reliable.

Many times these pseudo-experts offer testimony far beyond their domain of expertise. Most of the prosecution experts are not really scientists, they are lab technicians or button pushers who are experts in performing a particular procedure, but don’t understand the science or the theory of what they are doing. Most of them do not have the training or education in hard sciences and thus do not fully understand “why” they are doing what they are doing.

This is why it is important to show the limitations of their knowledge and refute their opinions with scholarly papers are real experts in the field. To do this, a DUI attorney must have proper scientific training and be able to speak from a position of knowledge and power.  You also need to have a strong network of experts to call upon to testify if needed.

Need to Know the Law

Recently, The Supreme Court of the United States ruling in the case of Bullcoming vs. New Mexico established the “Particular Witness Rule.”  This means that the actual analyst who performed the procedure or test, must be called on to testify to enter the report as evidence.  A supervisor or another analyst from that lab does not satisfy what the law requires.  This is why a good defense attorney needs to be on top of the law and know who they are cross-examining and if the prosecution is satisfying what the law requires of them.

Justin McShane

PA DUI attorney Justin J. McShane is the President/CEO of The McShane Firm, LLC - Pennsylvania's top criminal law and DUI law firm. He is the highest rated DUI attorney in PA as rated by Justin McShane is a double Board certified attorney. He is the first and so far the only Pennsylvania attorney to achieve American Bar Association recognized board certification in DUI defense from the National College for DUI Defense, Inc. He is also a Board Certified Criminal Trial Advocate by the National Board of Trial Advocacy, a Pennsylvania Supreme Court Approved Agency.

2 responses to “Challenging DUI Blood Evidence Part 5: Cross-Examining Forensic Experts”

  • In PA (Northampton County) can the DA use blood evidence that was collected after 2 hours? Is this a case of my expert witness versus the state’s expert witness. I have what I have learned to be a timid Atty that is afraid to challenge an overly aggressive DA who won’t drop charges because he knows it costs a fortune to go to trial and most people will take plea deal. Any interest in working pro bono to put aggressive DA in his place? DA says he would rather lose case than drop charges because of MADD and how he would look. I have your brochure and can’t afford to hire your firm.

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