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Imagine the following hypothetical situation. Officers Jones has arrested Joe Average for driving under the influence of an illegal drugs. The case looks very promising for the prosecution and the state wants a conviction. There is very strong video evidence but there is no confession. Joe's blood was taken and sent to a lab for analyzed. No drugs were found. Officer Jones feels that the lab did not test for the right drugs.

Officer Jones strongly feels that Joe Average is guilty. How difficult would it be for the attorney for the state to keep the blood test results out of evidence?

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    Given that Joe Average knows there was a blood test done, so their lawyer knows - well, really really hard...
    – Jon Custer
    Mar 26 at 14:09
  • @JonCuster I thought that blood tests could be challenged by attacking the chain of evidence. Would that work? Suppose the person who processed the blood test was not available?
    – Paul59
    Mar 26 at 14:32
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    Joe Average does not want to attack the result - they want the null result reported in court. Should the prosecutor try to hide the null result that will be brought up by Joe's lawyer.
    – Jon Custer
    Mar 26 at 14:44
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    @Paul59 - Define "not available". The lab tech can't decline to testify merely because it would potentially help the defense. If the lab tech happens to be out of the country for his long-planned wedding, that would be brought up when scheduling the trial. On the other hand, if the lab tech passed away before the trial, that could create issues. Mar 26 at 15:06
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    I just want to point out that I lived in a county in the US where pseudo-scientific "who is on drugs" eye tests were used to prosecute people for DUI even when the blood tests came back negative. I do not think that they got any convictions.
    – Tiger Guy
    Mar 26 at 15:06

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Effectively, it would be impossible barring some exceedingly exceptional circumstances.

In every state, the prosecution is obligated to turn over any exculpatory evidence they have to the defense. Should they overlook the blood test results, the defendant knows blood was taken so the defense attorney would be asking for the results. Unless there are some exceptional extenuating circumstances that cause the judge to believe that the blood test was unreliable-- it turns out the technician that did the test was the defendant's cousin or the chain of custody was broken and the lab is not certain that the test was done on the defendant's blood-- the judge would have no reason to exclude it.

The prosecutor, of course, can point out that the lab only tests for the presence of a small number of common drugs not every possible drug. The jury is then free to weigh the evidence. Were I a betting man, "strong video evidence" is going to make a much stronger impression on the average juror than a dry discussion of which drugs were tested for and which drugs were not tested for so I'd expect that introducing the blood test results wouldn't affect the strength of the case too much.

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