Forensic Characterization of Semen Testing for Seminal Stains

Forensic Characterization of Semen

Testing for Seminal Stains Many cases involve sexual offenses Need to examine for seminal stains Seminal stains are visible on fabric because they have a stiff, crusty appearance They must be tested to prove the identity

Acid Phosphatase Test This is an enzyme secreted by the prostate gland into seminal fluid Can be detected when mixed with an acidic solution of sodium alpha naphthylphosphate and Fast Blue dye

Procedure Moisten filter paper with water and rubbing it over the suspect area transfers any acid phos to the filter paper Place 1 -2 drops of test solution on the filter paper Purple indicates acid phosphatase is present (positive reaction) A negative reaction produces no color change and is interpreted as an absence of semen

False results Some vegetables (cauliflower) and fruit (watermelon, grapes), fungus, contraceptive creams, and vaginal secretions These all give a positive result to the acid phosphatase test But, none react with the speed of semen A reaction time of less than 30 seconds is considered a strong indication of semen Anything longer is not considered to be semen

MUP Another substance, MUP will fluoresce under UV light when it comes in contact with acid phosphate (positive for seminal fluid) MUP stands for 4 methyl umbelliferyl phosphate Test

Once we have a positive result Must examine the suspect semen for the presence of spermatozoa Criminalist locates them by immersing the stain in water Stirring rapidly transfers a small percent of spermatozoa into water Place a drop on a microscope slide, let it dry, then stain it for exam under microscope

Sperm Issues Male normally releases 250 -600 million sperm during ejaculation Chance of locating one is high, but not always true Sperm may bind tightly to cloth Stain must not be rubbed too hard, or it may disintegrate sperm Sex crimes may involve males with low sperm counts, oligospermia Or men that have no sperm at all in their semen, aspermia They can be infertile or have had a vasectomy

Prostate Specific Antigen (PSA) or p 30 Must examine stains suspected of semen that have a positive acid phos. test but have no detectable sperm How can the presence of semen be proven? Test for PSA (p 30) which is unique to seminal plasma Inject a rabbit to get antiserum Once proven to be semen, must tie it to an individual by DNA Can prove someone as being guilty and can exonerate anyone wrongfully accused

Collecting Rape Evidence No sperm doesn’t mean rape didn’t occur Injuries (bruises & bleeding) show a violent assault has occurred Contact may transfer physical evidence-blood, semen, hair, fiber Clothes must be removed/packaged separately in paper bags Place a bed sheet on floor with a paper sheet over it Have person stand on it while they disrobe to collect any material falling from clothing Object on which assault took place should be sent to lab If too large, cut out two pieces, one without stain and one with semen stain for DNA typing Must collect from victim: pubic combing, pubic hair, vaginal swab and smears, cervix swab, rectal swab and smear, oral swab and smear, head hair, blood, fingernail scraping, clothes, urine, saliva residue from skin If suspect is apprehended, collect: clothing, pubic hair combings, pulled head and pubic hair, penile swab within 24 hrs, blood sample or buccal swab

Determining time of sexual attack Living, motile sperm survive 4 -6 hrs in vaginal cavity of a living female Non-motile sperm may be found 3 -6 days after intercourse Intact sperm (with tails) not normally found 16 hrs after sex, but have been found 72 hrs after in rare instances Finding seminal acid phos. decreases with time after sex, with little chance of finding it after 48 hrs Investigators should determine when and if voluntary sex last occurred before sexual assault PSA, or p 30, will not be detected beyond 24 hrs after sex