Forensic experts come across different types of evidence ranging from physical to biological. Body fluids among them are some major ones. According to Locard’s principle of exchange, every contact leaves a trace. Thus a contract between two individuals especially when it comes to violent crimes could deposit sufficient amounts of bodily evidence between two of them, body fluids such as blood, semen, saliva or sweat could hence help in relating a person to the crime or victim or suspect.
Many body fluids are invisible to naked eyes or might not be easily noticeable due to their presence in very small quantities or mixtures. Hence some tests that relied historically on the use of chemical or enzymatic assays were often presumptive and generally limited in specificity or sensitivity. Similarly, there are confirmatory tests that rely on microscopic and immunological tests that confirm the presence of certain body fluid or its components.
Body fluids and tissues of interest in forensic science include blood, saliva, menstrual blood, semen, vaginal materials, and skin. Any test required for their localization or identification should be sensitive and easy to be conducted in the laboratory or at the crime scene, specific both to body fluid and the species and preferably non-destructive.
Non-destructive, as technologies had developed quite subsequently to extract DNA samples from the body fluids which helps in an easier method of identification. Thus post the identification process, samples should be left without being destroyed for further processes such as DNA typing.
Common Body Fluids Encountered At Crime Scene
The most common body fluid that could be encountered at the crime scene is blood irrespective of the type of crime as long as it involves violence. Blood evidence such as peripheral blood is present throughout a person’s body that circulates through the heart, arteries, veins, and capillaries. This is the reason why experts encounter it in almost all cases.
Other bodily fluids are transcellular fluids. These fluids are considered to be a sub-compartment of the extracellular fluid that is contained within epithelial-lined extracellular spaces. Blood plasma and other various bodily fluids usually contain small amounts of nucleic acids (DNA or RNA) known as extracellular nucleic acids.
The nucleic acids circulating in the plasma are referred to as circulating nucleic acids. Ones found in other body fluids, such as saliva and urine, are referred to as cell-free nucleic acids. Extracellular nucleic acids can remain soluble or form complexes with proteins and lipids.
1. Mesntrual Blood & Vaginal Secretions
Vaginal secretions are usually encountered in sexual assaults. There is a high probability of internal vaginal bleeding due to the hymen tear or wounds created in most rape cases. Sometimes in a woman who is menstruating these secretions could be mixed up with menstrual blood. Hence learning the characteristics and analysis of menstrual blood is important.
The most important factor about menstrual blood is the presence of traces of endometrial cells which are viewed through microscopes. Immunochromatography tests for D-dimer, a soluble fibrin degradation product detected clinically for the diagnosis of thrombosis, is recognized as a possible test for menstrual blood.
An approach using ELISA could also help in targeting estrogen receptor alpha and fibrinogen to differentiate between peripheral and menstrual blood. Lugol’s staining for the squamous epithelial cells of the vaginal wall and detection of lactate dehydrogenase isoenzymes are useful in the detection of vaginal cells.
Saliva could be common evidence in suicides near the victim’s mouth or neck and in genital areas during sexual assaults. Saliva tests mainly look for the presence of buccal cells, salivary amylase, or starch. The Phadebas test is the common presumptive test used to determine the presence of saliva and is based on the detection of α-amylase.
The test is not confirmatory for the presence of human saliva as small amounts of the α-amylase enzyme are known to be present, as salivary and pancreatic forms, in other body fluids including breast milk, sweat, semen, vaginal fluid, feces, and in other mammals.
An alternative colorimetric test for saliva, which has also proved to be a useful localization reagent more sensitive than the Phadebas test, is the SALIgAE saliva test. The RSID saliva test is another test based on anti-human salivary anti-amylase antibodies.
Usually, when a victim is put in a position of a matter of life and death, urination is obvious due to fear. Other than these suicidal deaths or even when a person is hanged to death urine can be found as one of the major body fluids present as evidence.
Localization of urine stains is difficult as they are typically diffuse, pale, and spread over large areas. Presumptive tests are typically based on the detection of urea, urease, or uric acid. These tests are not specific, as sweat and other substances containing high amounts of urea also react positively.
Tests for creatinine have also been used to detect urine. The detection of Tamm–Horsfall glycoprotein protein (TMP), which is also present in the urine of animals, has been reported previously and incorporated into the RSID Urine test. TMP appears to be suitable as a specific test for urine although the presence of vaginal fluid can inhibit the result of the test and the presence of blood in the sample can make the text difficult to read.
To date, there is no practical screening test for identifying sweat. Although DNA is frequently recovered and profiled from areas of clothing likely to contain sweat, little research has been undertaken. ELISA-based assays have been developed for the detection of sweat-specific protein G-81 and dermcidin but have not been widely adopted.
Blood is the fluid connective tissue of our body and hence controls the whole system. Bloodstains from crime scenes could be dried stains or wet. It helps in species identification, DNA typing and eventually links the suspect to the crime. Preliminary tests such as color tests or colorimetric assays are conducted to prove if the suspected stain would be blood or not. They include the benzidine color test, Kastle Meyers test, and phenolphthalein test.
Confirmatory tests on the other hand confirm the presence of blood through crystal tests forming colored rhomboidal or needle crystals which can be visualized under a microscope. These are Teichmann and Takayama also known as Hematin or Hemochromogen assays. These reagents react with the iron element of hemoglobin forming protoporphyrin crystals. Hence they confirm the presence of hemoglobin and hence the blood.
In semen analysis, an alternative light source like a laser is used to find stains on bed sheets or clothing. Once a stain is found it is sent to be tested to determine the type of biological fluid it is. If the biological fluid is not blood the other options are saliva, urine, or semen. DNA profiling starts from the identification of a stain as seminal fluid.
Once this is determined the DNA analyst needs to determine if sperm is present in the fluid. It’s important to determine if sperm is present because it contains a great deal of DNA, which could help to identify a suspect. Once it has been determined that sperm is present, the analyst must determine which item of evidence or swab sample has the most sperm so that getting a DNA profile is more likely.
Other than these tests such as spectroscopic and UV analysis could yield more detailed information on the body fluid as important evidence in the scene of a crime.