Presumptive vs Confirmatory Tests in Forensics
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.
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.
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.
Posted on May 25, 2011, in General, The Crime Laboratory and tagged BSPA, confirmatory test, Crime Scene, csi, detective, fiction, forensics, murder, mystery, police, presumptive test, thriller, tom adair. Bookmark the permalink. Comments Off.