BY Preeti Shah | April 27, 2022
Various evidence is found at the crime scene that helps forensic investigators find the suspects and carry out further investigations. And one of the most common types of biological evidence is blood. You can find bloodstains at the majority of the crime scene, like robbery, homicide, hit and run, burglary, etc. Bloodstains also assist in DNA fingerprinting.
Blood traces are considered a powerful forensic tool that helps analyze the situation under which the crime might have been committed. It also assists in carrying forward the criminal investigation in the right direction and eventually helps decide a suitable punishment for the offender.
Let us now look at various blood detection methods forensic scientists use.
Before performing any forensic test on the suspected blood sample, the difference between presumptive and confirmatory tests is vital to understand. Also, investigators must know the proper sequence to perform the tests to avoid spoilage of the evidence.
Presumptive tests are also known as screening, preliminary, or field tests, and as the name suggests, they are not final tests. Still, they are done to determine the likelihood of the presence of a particular body fluid. It helps determine what further tests need to be performed to establish the presence of that fluid in combination with confirmatory tests.
• Easy to perform
• Narrow down possibility
• Can cover larger areas
• Sometimes overly sensitive
• Risk of false positives
As the name suggests, these tests help forensic investigators conclude whether a specific body fluid is present in the sample or not. A single test or a combination of different procedures is followed to identify the evidence.
• Lesser risk of false positives
• Delivers convincing results
• Can be expensive
• Requires professional equipment
• Take more time
It means establishing whether the stain is blood or not. Fresh stains are easily identifiable, but old stains might be difficult to identify.
This examination allows:
• Determining the size and number of stains
• Quantity of bloodshed
• Direction from which the blood has fallen. It is determined from the tip of the elongated stain.
• Height from which the blood has fallen. The shape of the stain determines it; Round with sharp edges (bloodstains fallen from a height of approx. 50 centimeters), Small with spiked projections at the edges (bloodstains fallen from a height of approx. 50 to 150 centimeters), and Stains with corrugated edges (bloodstains fallen from a height of over 150 centimeters).
• The victims' and culprits' movements can be analyzed from the stains’ positions.
• The age of the stains is determined by the degree of dryness and color changes.
• Other foreign material like the presence of flesh, hair, bones, etc., helps identify where the injury occurred.
Once the visual examination of blood stains is over, color tests are the first to be performed. A positive color reaction in any two-color test is required to ensure that the stain is a bloodstain. If color reactions do not occur, then there is a probability that it is not a bloodstain.
Benzidine reaction: The stain is sprayed with a solution of benzidine and sodium perborate dissolved in glacial acetic acid. If the stains turn dark blue, it indicates blood.
Leucomalachite green reaction: The reagent is prepared by mixing leucomalachite green and sodium perborate in glacial acetic acid and applied to the stain. If intense green color appears, it indicates blood.
Phenolphthalein reaction: After reducing phenolphthalein, it is dissolved in acetic acid, and sodium perborate is mixed into this solution, then applied to the stain. If pink color appears, it indicates blood.
Luminol test: Luminol, when sprayed on stains present on the material in a dark room, a reaction with the blood takes place that gives strong luminescence, and stains become visible. It is prepared by dissolving sodium perborate in water and adding 3-arninophthalhydrazide and sodium carbonate to the solution.
Crystal test: Certain reagents react with haemoglobin in blood to form crystals, and Pyridine is the most widely used reagent and forms pink crystals upon reaction. The test is performed by adding the reagent to the stain on a microscopic slide, and it is then observed under a microscope.
Though not widely used nowadays, this technique can yield positive results on blood stains as old as 20 years. Also, if the outcome is not positive, it does not eliminate the presence of blood but might be due to mishandling while implementing the technique.
Catalyst test: It is based on the fact that the haem group of haemoglobin catalyzes hydrogen peroxide breakdown. Upon breakdown, various substrates react with the oxidizing species formed, resulting in color change. The most common substrates used are benzidine, leucomalachite green, leucocrystal violet, and phenolphthalein. Also, it is essential to note that a color change does not guarantee the presence that a stain is a blood. Several enzymes and metals also react and lead to positive results.
Instrumental method (High-performance liquid chromatography): It is a suitable method when stains are faint or hard to see. It enhances the stains so that a visible print can be taken and matched with the suspect.
Also, while performing chemical tests, you must ensure that these tests do not close the way for the conduction of other tests to identify to which the blood belongs.
Microscopic studies and micro measurements can assist in determining the species of origin of the fresh bloodstain.
With this technique, you can identify the body part from which the blood came out.
The presence of diseases like leukemia or syphilis can be detected.
You can also identify menstrual blood.
The presence of a puss helps in identifying blood from an infected place.
This technique is used when you need to study body proteins, and various enzyme systems present in the blood can be separated using this technique. It is a powerful method to distinguish among various blood samples. When subjected to an electric field, enzymes start moving towards polarity opposite to theirs. Depending upon the enzyme structure, weight, and electric charge, the rate of movement varies.
Often, the bloodstains on materials like furniture, clothes, floor, doors, etc., get washed away or are too light in appearance to analyze. Ultraviolet or infrared rays can detect these stains and reveal even minute blood traces. These rays are also helpful to identify bloodstains present on colored fabric and painted surfaces.
Spectrophotometry means measuring light intensity in the spectrum upon transmission from a specific substance. Haemoglobin and its derivatives are identified based on their absorption spectra. They exhibit characteristic absorption bands at specific wavelengths that a spectrophotometer can observe.
Forensic biology is not limited to blood detection methods alone, and there are several more aspects that need to be mastered to become a forensic biologist.
SIFS India is a pioneer in delivering forensic education and is backed by a team of highly-qualified forensic experts. An in-depth study material focusing on practical hands-on training has been developed under Dr. Ranjeet Singh, CEO of SIFS India.
You can contact their support team and learn about forensic courses best suited for you.