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Identification of Blood & Bodily Fluids


Blood and body fluid identification is a fundamental aspect of forensic biology and has been conducted in laboratories and crime scenes for many decades. Investigators consider blood and body fluids as the greatest kind of evidence at a crime scene because they contain the DNA evidence that can easily identify the suspect or vindicate an innocent person. This essay explores the techniques used to identify blood and body fluids such as semen, blood, sweat, urine, and saliva. It focuses more on modern techniques developed in the recent past. Current tests, presumptive tests, and the emerging tests carried out are discussed for each type of fluid.

Identification of Blood & Bodily Fluids

Determining whether or not there are blood and body fluids and subsequently extracting it from a crime scene for purposes of carrying further laboratory tests is a critical step in forensic biology. According to Virkler & Lednev (2009), the task is not as obvious as it may seem since the body fluids are not only invisible to a naked eye but also similar to other substances. Saliva, blood, and semen are the most popular fluids, but urine, sweat, and vaginal fluids are also crucial in providing investigators with DNA evidence. A major problem with the majority of tests is the destruction of the sample. For this reason, it is necessary to examine the small quantities effectively and efficiently through non-destructive tests. Another drawback of the current methods is that they only detect whether or not a particular body fluid is present. This means that the investigator decides which test to carry out depending on the fluid present. This paper analyzes the traditional and current methods of identifying blood and body fluids. It also focuses on the latest developments in forensic biology in the recent decades. The paper also analyzes the advantages and disadvantages of these tests and discusses the new methods under development. With further testing, it is believed that the new methods under development will deliver better results is an easy manner.

The Composition of Blood and Body Fluids

Each fluid comprises a unique composition and so the presence of a certain component in a particular fluid relative to the other forms the basis of identification. James, Nordby & Bell (2014) note that different body fluids may contain common components, but the difference in the ratio of the components matters and informs the effectiveness of the tests. The saliva, for instance, contains a high ratio of amylase relative to other body fluids like vaginal fluids and semen. The ratio of lactate and citrate differs significantly in vagina fluids compared to semen. Blood is rich in hemoglobin, albumin, erythrocytes, fibrinogen, and glucose. Semen, on the other hand, contains spermatozoa, chlorine, lactic acid, fructose, urea, citric acid, ascorbic acid, and acid phosphatase. Saliva is rich in amylase, bicarbonate, potassium, glucose, phosphorous, and lysozyme. The vaginal fluids contain urea, lactic, acid, citric acid, acetic acid, peptidase, and pyridine. Sweat contains urea, sodium, potassium, lactic acid, and chloride while urine is rich in uric acid, chlorine, urea, and creatinine.


The majority of crime scenes have blood as the primary body fluid. Several presumptive and confirmatory tests are available to identify blood in crime scenes. The alternate light source is the simplest presumptive test that laboratory technicians use to identify blood, especially where the bloodstains are not clear or are found in a dark place. Luminol test is another presumptive blood test that investigators use in crime scenes. In this method, the technicians spray the crime scene with a Luminol solution (Elkins, 2013). The blood components are oxidized in the Luminol solution if blood is present. Luminol is considered the most sensitive presumptive test because of its intensity, sensitivity, and the duration of time it can be applied even in an area that has already been cleaned. Other presumptive blood tests include the catalytic and non-catalytic tests. Several confirmatory tests exist for identifying an unknown stain on the blood once the presumptive test has positively identified blood. The most popular confirmatory blood tests include the crystal test, chromatographic test, immunological and spectroscopic tests. Most emerging tests under investigation for blood identification involve RNA.


Semen is another common body fluid in crime scenes. Several confirmatory, presumptive, and emerging tests are available for identifying semen. Like blood, the alternate light source test can also be used to identify semen. According to Elkins (2013), the reason why this method is effective is due to its non-destructive and easy-to-use nature. Semen acid phosphatase test is also a universally accepted presumptive test for the presence of semen. Test for chlorine is also the widely used presumptive method used to test for the presence of semen. It is commonly known as Florence test and involves testing the extract on a microscope after treating it with potassium iodide. If semen is present, then brown crystals which are needle-like often form. Microscopic identification of the sperm cells remains the widely used confirmatory test for identifying semen. Other confirmatory tests include the testing of the extract for a prostate-specific antigen. The majority of emerging tests for the presence of semen rely on immunological markets and are capable of concurrently identifying species of an unknown semen stain.


Other than semen and blood, saliva is also a common body fluid in major crimes scenes. However, there are only a few known presumptive tests for identifying saliva. Additionally, there are no confirmatory tests specifically designed for identifying saliva. As Elkins (2013) points out, the alternate light source is a popular presumptive test for identifying saliva. When viewed through ultraviolet light, the saliva appears bluish-white. The disadvantage with the ultraviolet light is that it does not distinguish saliva stain from other types of body fluids. Due to the absence of solid particles in a saliva stain, it is difficult to identify saliva compared to semen. Test for the presence of amylase is another presumptive test used to identify saliva. The emerging tests under investigation to identify saliva use RNA markers and could be very helpful in the future.

Vaginal fluid

Unlike semen, blood, and saliva, vaginal fluids are not common in crime scenes but are crucial especially in cases involving sexual abuse. The absence of many tests to identify vaginal fluids is because it is not defined. According to Elkins (2013), the components of vaginal fluids may vary depending on the menstrual cycle of a woman. For this reason, it is difficult to test for a particular component for all cases. One of the methods used involves the test for an enzyme called vaginal peptidase which is present in vaginal fluids. Moreover, the concentration of citric acid and lactic acid in the vaginal fluid is used through comparison with the concentration in semen. While the concentration of lactic acid is high in vaginal fluids than in semen, the concentration of citric acid is high in semen compared to the vaginal fluids (Elkins, 2013). The emerging tests being investigated, like in other body fluids, involve the use of RNA markers and could become more effective in the future.


Detecting urine is not as easy as identifying other fluids like blood or semen. It’s easily diluted and spared on the surface. However, it is useful especially in determining cases involving sexual abuse or defilement. There are a few presumptive tests used to identify urine, but the alternate light source, like in the body fluids, is the most common method. Test for urea which is broken down into carbon dioxide and ammonia is another presumptive test used to detect urine (Elkins, 2013). Another method tests the concentration of uric acid and compares it to urea nitrogen in urine. There has been no research on better methods to detect urine stains in the recent past.


Compared to other body fluids, sweat is the least fluid in most crime scenes. However, like other fluids, sweat contains DNA evidence which is essential for conducting investigations. It is often extracted from hats, shirts, and pants. As Elkins (2013) was able to prove, the components of sweat are similar to those of urine, but the concentration of creatinine and urea are lower in sweat compared to urine. The only presumptive test for identifying sweat is EDX and SEM tests which detect the concentration of sulfur, sodium, potassium, calcium, and phosphorous metal traces. The emerging tests revolve around RNA markers and could prove to be more effective when approved.


In conclusion, blood and body fluid identification is a critical aspect of forensic biology. With technological advancements, the improved technique for detecting and identifying fluids in crime scenes are emerging. Traditionally, investigators and laboratory technicians have relied on presumptive and confirmatory tests to identify blood, sweat, semen, urine, saliva, and vaginal fluids. With the development of emerging methods such as RNA markers, identification of blood and body fluids will be more effective in the future.


Elkins, K. M. (2013). Forensic DNA biology: A laboratory manual. Oxford: Academic Press.

James, S. H., Nordby, J. J., & Bell, S. (2014). Forensic Science: An Introduction to Scientific and Investigative Techniques, Fourth Edition. Hoboken: CRC Press.

Virkler, K., & Lednev, I. K. (January 01, 2009). Analysis of body fluids for forensic purposes: from laboratory testing to non-destructive rapid confirmatory identification at a crime scene. Forensic Science International, 188, 1-3.


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