Presumptive vs Confirmatory Tests in Forensics

Unknown substance

Forensic scientists love running tests. Love, love, love it! It’s how we arrive at, and support, various conclusions. Often times we are asked to determine the identity of a particular substance; be it suspected drugs, blood, semen, explosive residue, etc. To do this we follow various departmentally established protocols utilizing both presumptive and confirmatory tests or examinations. But understanding the difference between the two types of tests is sometimes lost on those outside the laboratory like attorneys, detectives, and jurors.

Presumptive tests are designed to do one of the following:

  • Eliminate the identity of a particular substance (i.e. the stain is not blood)
  • Establish the probability (presumptively positive) of a substance identity (i.e. stain is probably blood)

Presumptive tests are presumptive because there is either a chance of a false positive result (bleach reacts with Luminol) or the result is defined by a larger group (the sample is blood but not necessarily human).  Common presumptive tests include Phenolphthalein, Luminol, Hemastix, and Leuco-crystal Violet (blood); Acid Phosphatase (semen); Duquenois-Levine spot test (Marijuana) as well as many other tests.

Gas Chromatograph Mass Spectrometer

Confirmatory tests on the other hand are used to specify the identity of a substance. With suspected drug or explosive samples the forensic chemist will likely use a Gas Chromatograph-Mass Spectrometer (GCMS) or Fourier Transform Infra-red Spectroscopy (FTIR). For blood, a common field test is the ABAcard HemaTrace test which will specify the blood as human (primate) followed by laboratory DNA analysis. Semen samples collected on glass slides at autopsy or in a rape kit will be examined microscopically for sperm cells.

Stained sperm cells

So when writing scenes in which your characters are conducting forensic tests it would be best to know whether the test is presumptive or confirmatory before they draw any conclusions. Presumptive test results can add a bit of conflict to a story in that they may reveal the presence of blood in which later exams prove it to be non-human. I remember years ago we had a large dumpster that had at least five gallons of liquid blood poured into it (Coroner estimate). We spent hours carefully screening the materials on tarps before tests confirmed it was pig blood. Until that test was complete we had to proceed as if it were human blood just to be safe.

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About forensics4fiction

Hi there. Thank you for visiting my BLOG for crime writers. I hope you will find it interesting. I would love to hear your questions and thoughts regarding forensics and criminal investigations. I hope that the information here will help answer your questions or ignite your imagination. I am a retired senior criminalist with 15 years of forensic experience. I have served as the president of the Association for Crime Scene Reconstruction, Rocky Mountain Association of Bloodstain Pattern Analysts, and the Rocky Mountain Division of the International Association for Identification. I am triple board certified in forensic related fields and one of only 40 board-certified bloodstain pattern analysts and 80 board-certified footwear examiners worldwide In addition to writing over 60 scientific papers, I have worked as the editor of the Journal of the Association for Crime Scene Reconstruction, been interviewed by and consulted for television, books, magazines, and newspaper articles including documentaries on the Discovery Channel and National Geographic.

Posted on May 25, 2011, in General, The Crime Laboratory and tagged , , , , , , , , , , , , . Bookmark the permalink. Comments Off.

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