Investigating CSI – Background material Table of Contents I ...

Specificity refers to how many other substances the test will react to other than
blood. So far, a true positive reaction has been observed to occur only in the
presence of hemoglobin. False-positives may occur in the presence of chemical
oxidants, and it is possible for vegetable-peroxidases to react with the test too,
but these reactions would occur after phenolphthalein was applied and before the
addition of hydrogen peroxide. A true positive result develops only after the
hydrogen peroxide is applied. For this reason, it can be said that the KM test is
highly specific for blood.
III. THE KM TEST: UP CLOSE AND PERSONAL
Let’s examine the items contained in a typical test kit to see what they’re for.
1.) Positive Control: This is a small swatch of cloth that is stained with
dried, non-human blood. (Animal blood is used to stain the control in order
to prevent the possible transmission of diseases.) It is included for the
purpose of serving as a known sample of blood so that the reagents can
be checked before beginning a series of tests at a scene. It would be very
unprofessional if you were to test stains at a crime scene without checking
your reagents first. What if your tests indicated that there was no blood
present only because your phenolphthalein had become oxidized in the
bottle (maybe because the cap wasn’t on very tight, for instance)? Always
check your reagents with the positive control before doing any actual
testing!
2.) Transfer Materials: These may include cotton swabs, filter papers, or, if
you need to get into really small spaces, cotton string or thread. You’ll be
transferring some of the suspect stain onto these media for testing, NOT
applying the test directly onto the evidential stain!! (This is why KM is
considered an indirect test.)
3.) The Reagents: There are three reagents included with the kit. These
are:
a. Alcohol: Methyl or Ethyl alcohol is used to increase the
sensitivity of the test. It does this by “cleaning up” the area in and
around the bloodstain to better expose the hemoglobin.
b. Phenolphthalein: This is a solution which acts as a color
indicator. When prepared, the solution is boiled for several hours to
help remove most of the oxygen trapped in it. It should appear as a
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