FORS-2106EL Study Guide - Final Guide: Chromogenic, Communicate!, Abo Blood Group System
27 Jun 2018
School
Department
Course
Professor
Page ! of !113
Forensic Biology Final Quiz Review
Day 12
Forensic Serology: Blood Identification
-blood leading event: general term for bleeding
-when there’s a lot, it is easy to tell if it is blood or not
-when there was not a lot, you must test to check if it blood or not
-a presumptive test will test for negative or positive of a sample, vs a controlled
sample
-these could give false positive, that will come with the test to ensure you are aware
of the possibility of this
-there is no more dangerous substance that you can encounter, then blood due to the
diseases blood can carry (bacteria, prion (no cure) , viral)
-purpose: to identify liquid and dried substances as being blood or not
Serology
-They are big on blood type (class physical evidence)
-Blood analyzed to ascertain (know) its source
-Forensic contexts deals with fresh, dried, and degraded samples
-You will not get there fast enough to get liquid blood, most of the time it will be dried
or sucked into a source
-degraded occurs when its so dry that it can be pealed in flakes
Degradation Factors
-Heat
-Humidity
-Careless handling of dried stains
Identification Protocol
-Visual examination for stains
-position, condition, does it make sense, how is it deposited, was it transferred
through touch, did someone smear it (a blood wipe), transferred through blood
source
-Presumptive test of the stain
-will give a positive or negative result, using a controlled sample, another area of the
material where you see no blood, then test the stain itself to see if positive or
negative
-Specific test to confirm blood
-can test for blood types
find more resources at oneclass.com
find more resources at oneclass.com
Page ! of !213
-h is an antigen we all have, but we still test for it to tell us if the blood is degraded,
because if this comes back negative then you know the blood is not intact enough
to test
-Then it will be tested for A and B, as well as Rh + or -
-Determine origin
-blood type will give us class evidence for finding the origin
-Characterization genetically
Presumptive Tests
-Positive or negative
-Positive: qualified conclusion
-there is a caveat (warning) on this conclusion, so it is positive but it is not 100%
certain something else may not have reacted with it
-Negative: elimination of sample
-if reagents are 100% effective, this test will exclude the possibility of blood
Catalytic Color Tests
-The chemical oxidation of a chromogenic substance by an oxidizing agent catalyzed
by the presence of blood, or more specifically, the hemoglobin present in red blood
cells
-chromogenic meaning it produces a colour
-hemoglobin
General Procedures
-Colour tests
-First add a solution of chromogen to the sample
-Followed by an oxidizing agent
-Usually hydrogen peroxide (3% solution).
-Catalyst is the heme group that is chemically peroxidase-like
-peroxidase like, like an enzyme
-A colour from the chromogen results.
False Positives
-Tests that appear to have a positive result, can react with a substance other than
blood.
-Mitigated by (avoid by):
-Experience.
-Careful observation.
-ex. age of reagent
-Confirmatory tests
find more resources at oneclass.com
find more resources at oneclass.com
Page ! of !313
Test Methodology
-Use a clean and moistened cotton swab
-Rub the swab onto the stain to effect a transfer
-Add a drop of colour reagent
-Follow with a drop of hydrogen peroxide
-A color develops
-A spot plate can also be use
-when flaky blood is present
Benzidine (Alder Test)
-Formerly extensively used to identify blood
-Reaction in an ethanol/acetic acid solution
-Results in a blue to dark blue colour
-It is a carcinogen: therefore, not used today.
Phenolphthalein (Kastle-Meyer Test)
-Uses phenolphthalein as the acid-base indicator
-Produces a bright pink colour
-No colour or other colours indicate a negative
result.
o-Tolidine
-ortho-tolidine
-3,3’ dimethyl derivative of benzidine.
-Test is done under acidic conditions
-Produces a blue colour reaction
-Hemastix®Test to detect blood in urine
-Replaced by tetramethylbenzidine (TMB).
Leucomalachite Green (LMG)
-Oxidation is catalyzed by heme to produce a green
colour.
-Carried out in an acid (acetic acid) medium with
hydrogen peroxide as the oxidizer.
Tetramethylbenzidine (TMB)
-The 3,3’,5,5’ tetramethyl derivative of benzidine
-Used in an acid medium (acetic acid) when in solution
-Positive colour is green to blue-green
find more resources at oneclass.com
find more resources at oneclass.com
