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