Seminal fluid (semen) is a complex mixture of secretions from at least four male urogenital glands.
Seminal vesicle 60%
Prostate gland 30%
Epididymis & Bulbourethral gland 10%
An average male ejaculate measures around 3.5 millimeters containing 10-50 million sperm cells per millimeter. This number varies with the age, medical conditions, genetic background, and habits such as smoking and use of drugs.
The identification of semen is of great value in medicolegal practice like alleged rape cases or sexual assaults. The primary aim of semen forensic analysis is to sample and examine smears or other biological material taken from the assailant, the victim or stains found on clothes and/or other evidence at the crime scene. In cases of sexual assault, two steps are required: locate the stain and prove its identity. Semen analysis can also help in successfully linking semen to a suspect through DNA typing.
Presumptive Tests for Semen
1. Visual and Alternate Light source (ALS) – Semen can be detected with the naked eye in clothing and undergarments or through the use of ALS such as ultraviolet light.
2. Acid Phosphatase (AP) test – The male prostate gland produces and secretes high amounts of acid phosphatase (AP), an enzyme, in the semen. Using standard chemical reactions, laboratories can analyse a given stain for the presence of this enzyme. In the presence of certain stains, AP will produce a dark purple color in less than a minute.
3. Prostate Specific Antigen (PSA) – Prostate specific antigen also known as PSA or P30 can be detected in semen. PSA is produced in high amounts by the male prostate gland but can also be found in very small amounts in fecal matter and breast milk.
Confirmatory Tests for Semen:
1. The Christmas Tree Stain
The Chridtmas Tree Stain is a positive identification of sperm cells. Two reagents are used consecutively to produce this distinctive stain: Picroindigocarmine stains and Nuclear Fast Red. The first stain stains the neck and tail of the sperm cell in green and blue whereas the latter one stains the sperm head and acrosomal cap with red and pink, respectively.
2. RSID-Semen Strip Test
This test provides sensitivity and specificity to human semen. Similar to pregnancy test strip, the RSID-semen test identifies the presence of the seminal vesicle-specific antigen. This antigen is unique to semen and thus, there is no cross reactivity with other body fluids.