Serology Flashcards
(33 cards)
Why is it used?
Used more as a screening method to see if there is a need to take a sampling of evidence for DNA
What is the definition of serology?
Defined as the examination and analysis of body fluids
what are the types of body fluid testing?
Blood, saliva, semen, and urine
Presumptive testing
Analyze a sample and establish one of the following:
1. The sample is definitely not a certain substance
2. The sample probably is the substance
A positive reaction indicates the possibility of X
- Important to remember indicates not specific for
- Ex. bleach would test positive
- It tests for what you’re looking for, but also could be positive for other things as well. It’s probably blood, but because it can react with other things, we cannot say it is definite
“Presumptively positive for whatever you are testing for. It is indicating it, but does not mean specifically that it is there.”
- It could show positive for blood, but could be bovine or canine
Confirmatory testing
- Required to confirm the analysis
- Confirmatory tests cost more than simpler presumptive tests so presumptive tests are often done to see if confirmatory tests are necessary
- Usually takes more time as well
4 S’s of Choosing a Reagent
Sensitivity
Specificity
Safety
Simplicity
Sensitivity
Neat: body fluid in its natural state (not diluted in any way)
Ex. Vaginal fluid and saliva with dilute semen
specificity
You want a test that tests for what you are testing for. You don’t want one that tests for 30 things, but rather 3.
safety
Can be performed on a crime scene or at a lab
simplicity
At a crime scene you want to go with less is more
Target stain must be _____ for genetic testing
viable
What are other important considerations for choosing a reagent
Substrate and environmental conditions may be important considerations (what works well in the lab may be cumbersome in the field)
Benzidines and Derivatives
Used extensively as blood detection reagents because of their good sensitivity
Active dye in Hemastix
What is the active dye in hemastix?
Benzidines and Derivatives
How do hemastix work?
Constructed by embedding active materials at end of a plastic strip contributes to the safety and ease of use for testing
Hemastix is a one step test since both parts of the test are contained in the stick
Presumptive
Always required to do a positive and negative control before using at a scene
What is a substrate control?
A small unstained portion of whatever you are testing to show that the substance itself is what is making it turn and not a dye coming off the carrier
(do not do it on an area not in question)
what is the most common presumptive test used in blood identification?
Triarylmethanes
(similar to Phenolphthalein tests)
What are the Triarylmethanes
Phenolphthalein (Kastle-Meyer Reagent), Leucomalachit Green, Leucrystal Violet and Fluorescein
Structural variations give rise to their different color reactions and cause the reaction to fluoresce (fluorescein)
PhenolphthaLIN (colorless and reduced form of phenolphthalein) + H2O2
Reacts with hemoglobin
Always use a (+) control
read results immediately
very high sensitivity
false positives
Phenolphthalein AKA (Kastle-Meyer reagent)
Take phenoLIN to hydrogen peroxide and it turns pink, it’s a positive
But if it turns pink before adding the hydrogen peroxide, it is a compromised test and not valid.
Always need to do a control (around once a day is usually good)
Wait to make sure there is no reaction
Steps to conduct a Kastle-Meyer test
- Moisten swab with distilled H2O
- Swab area you want to test
- Add drop of ethanol (no color change) - Increases sensitivity of the test
- Add a drop of phenolphthalein (should be no color)
- Add a drop of h2o2
Chemiluminescence (and types)
process by which light is emitted as a product of a chemical reaction; no additional light is required for the reaction to take place (Luminol)
First made in 1853
First used in the application FS in 1937
In 1951, Grodsky combined luminol, sodium perborate, sodium carbonate, and distilled water which is the common mixture used today
Good because it can be used on really old stains
Bluestar is also a type
Fluorescence (and types)
occurs when a chemical substance is exposed to a particular wavelength of light and light energy is emitted at longer wavelengths
Fluorescein
- catalytic reaction between the hemoglobin, blood, and oxygen
- very good on latent bloodstaines
- longer half-life
- does not react with bleach
Luminol test
Luminol + Oxidizer → chemiluminescence
Reacts to the hemoglobin in the RBC
More luminol must be added for further chemilumi to occur
Light blue glow
- Bleach tends to appear white
As you add more luminol, you are diluting the stain though so you have to be careful
Bluestar test
Competeter of luminol
Fairly new forensic tool (2000)
Derivative of luminol
Application by atomizing/ light spraying
Stringer and longer lasting luminescence than luminol
Do not need complete darkness
Lasts for a couple days once mixed
