Page 14 - Stained With Suspicion Blood Detection

level of precision is not possible with blood typing, presumptive tests must be recorded immediately
but must be performed in a lab once blood has before the sample is oxidized by air exposure, as
been detected. this may result in a false-positive reading.

When an investigator is confronted by a stain that A phenolphthalein test, better known as the
looks like blood at a crime scene, it is difficult KastleMeyer test, is now one of the most frequently
to know for certain that the stain is blood. After used presumptive color tests. In a positive reaction,
careful documentation, the investigator may reduced phenolphthalein will turn bright pink
quickly identify blood through a presumptive test in an alkaline solution. This occurs because the
at the scene. These tests are called presumptive phenolphthalein is oxidized by hydrogen peroxide
because if a test result is negative, blood is absent, in the presence of hemoglobin. Phenolphthalein
but if a test result is positive, blood is presumed to reagents, however, have been known to give false
be present. As numerous compounds may cause positives when vegetable materials are present.
false positive reactions, a confirmatory test must As a result, after the evidence is collected and
be performed following a positive presumptive test. transported to the lab, a confirmatory test is
Confirmatory tests provide much more accurate performed.
results, but take longer to perform and require
samples to be sent to a lab for analysis. The Hemastix test is another commonly used
presumptive test for blood. This test is performed
Presumptive tests are based on the peroxidase- with commercial Hemastix strips, plastic strips
like activity of hemoglobin contained in red with a reagent-treated filter paper at one end.
blood cells. Peroxidases are enzymes that Tetramethylbenzidine (TMB), the reactive reagent
quicken the oxidation of a number of classes of on Hemastix strips, is another well-known
organic compounds. There are two categories presumptive reagent. This, product designed as a
of presumptive tests: those that change color urine dipstick test, has been adapted to be used
and those that cause a glowing reaction. In in the field for the detection of blood. To perform
color change presumptive tests, a sterile swab this test a swab is moistened with distilled water
is moistened with distilled water and placed in and placed in contact with the stain. The swab is
contact with a small sample presumed to be blood. then placed onto the tip of the dipstick; if blood is
A drop of both a presumptive reagent and hydrogen present, the Hemastix strip will turn green.
peroxide is then added to the swab. An immediate
color change indicates the possible presence of In some cases bloodstains can not be seen under
blood. Alternatively, a presumptive test may be normal lighting and viewing, such as a stain that
performed by placing a thread or fragment of the has been cleaned up by the suspect. Therefore,
dried material on a spot plate and adding the above before performing any tests, a high intensity light
reagents as in the swab test. source is used to detect latent bloodstains. If a
stain cannot be visualized with a light source,
When performing the presumptive test, a substrate Luminol or Fluorescein tests may be used.
control test is required which will confirm that the
test result is not brought about by the material that Luminol is highly sensitive, and is known to detect
the stain was on. This is done by taking a swab of blood that has been diluted up to 1 in 10,000,000
the original, unstained surface (as close as possible times, unless a solvent such as bleach was used. It
to the stain) and adding all similar reagents as works similar to other color tests in that Luminol
the non-substrate control swab. Results for all and an oxidizer (hydrogen peroxide) are applied

14 THE MYSTERY OF LYLE AND LOUISE