Lecture 5 - Serology (continued) Flashcards
Between a false + and a false -, which is preferred?
False - can be accepted, but labs try to avoid false +. False + can implicate an innocent person in a crime.
Historically, Presumptive/Confirmatory Tests would always occur before DNA profiling. The serological test was used to direct what samples got subjected to DNA profiling.
Now, some labs go for “direct to DNA”. They put the sample directly into DNA profiling because serology tests aren’t as effective and DNA profiling has gotten so good that they want to skip serology. If DNA is found and matches a suspect, then serology is done to see if what bodily fluid it is.
Sexual assaults, especially in the states, have been the basis or the primary casework that has created all the backlogs at both federal levels and also state and local levels. And because of that, it’s been the target of many serological research projects.
Sperm-Vaginal mixtures
vaginal swab of victim taken post-coital
Semen
hugely important for sexual assaults. Both presumptive
and confirmatory tests exist
Semen Components
-Cellular material: sperm and epithelial cells
sloughed from linings of vas deferens,
ejaculatory duct, urethra, bladder -Other proteins and chemicals: water, choline,
inorganic ions, fructose, flavins, acid
phosphatase, P-30 protein, urea
Sperm head has nucleus with DNA
Presumptive Test 1 for Semen
ALS - when at crime scene or laboratory and searching large areas, surfaces. Allows CSI to search large areas quickly
Confirmatory Test 1 for Semen
Microscopic Identification
one of the most common methods still to confirm the presence of semen is microscopic examination, because sperm cells look different.
the reason why this is not universally done, or why it’s still difficult, still can be ambiguous, is that it’s really hard in most forensic samples to spot sperm cells. First, sperm cells are super small.
Identifying sperm cells within vaginal tissue is also difficult due to the size difference.
Sample may be complex mixture containing other human cells, bacteria, particulates, etc.
As sample ages/degrades tails fall off, only the head is visible. Even in a collected sample, the head falls off within one to two days. Sperm heads can also stick together. Hard to make out.
Presumptive Test 2 for Semen
Acid phosphatase test - This is done at the forensic lab and not the crime scene. Acid phosphatase is an enzyme with high abundance in semen (but can be found in other
tissues, materials).
Use filter paper to press where you think the stain might be on a fabric or material. The substrate is alpha-naphthyl phosphate. This is combined with the FastBlue Dye to form the reagent which is then applied to the filter paper/swab with the stain. In the presence of acid phosphatase, a color change will occur from colorless to pink.
Most common Presumptive test for semen.
Confirmatory Test 1 for Semen
Christmas Tree Stain
It has two different dyes that stain different parts of the sperm cell structure. So basically it dyes the middle part and the tail blue, and then the head stains red.
Azoospermia
semen is present but no sperm. can result from disease, age, and vasectomy. May represent a significant proportion of perpetrators.
Prostate Specific Antigen (PSA, p30) test
Confirmatory. This is an antibody based reaction where the antibody binds to the prostate specific antigen and the PSA protein is secreted in seminal fluid regardless of whether or not sperm cells are present.
Vaginal Tissue
There are almost no serological tests to confirm the presence of vaginal tissue.
There are microchemical tests based on glycogen content of cells (e.g.,
periodic acid-Schiff reagent). However, there’s so much variability in glycogen content, obviously general chemistry between different individuals, vaginal tissue either due to biological variability, differences in menstrual cycle. You can also have glycogen content that’s higher in other types of tissue that would give you a false positive.
Why is there such little research about tests for vaginal tissue?
One of the reasons why there’s been so little research and development in vaginal tissue is because the United States has been so consumed with the backlog of vaginal swabs and post-coital detecting semen that the research and development, all of the money for literally 30 years has just gone into sperm vaginal mixtures.
because that’s what made most of the backlog.
Recent Developments of Serological Tests.
So many labs have pretty much stopped doing serology. They don’t get enough information on serology. So the confirmatory tests for serology mostly focused on semen. Those are good. P30, even acid phosphatase, microscopic examination. But for example, saliva tests, don’t yield anything useful. You also need high amounts of body fluid for these serological tests. DNA profiling has also gotten more sensitive and reliable, amplifying the weaknesses of serological tests.
