COLLECTION AND PRESERVATION OF BLOOD EVIDENCE

Blood is one of the most vital biological and physical evidence found in crime scene. It helps to establish a link between the victim, suspect and the crime.

· DNA profiling can identify the exact source of even tiny blood samples.

· Blood typing helps to narrow the suspect pool.

· Bloodstain Pattern Analysis (BPA) is the use of physics to help reconstruct how a crime occurred.

· Bloodstain patterns can reveal positions, movements, and the type of force used during the incident.

Forensic investigators treat wet and dry blood with different methods. Proper collection and preservation are required to maintain the quality of the blood and prevent the growth of bacteria that may cause degradation of the blood.

General Precautions to be Followed

1. Wear appropriate Personal Protective Equipment (PPE Kit) to prevent contamination and ensure self-protection. This includes the following:

• Disposable gloves (latex or nitrile) – usually officers wear two gloves. The top layer is removed regularly to prevent cross contamination.

• Face mask

• Hair cap

• Protective gown (made of Tyvek material)

• Safety Goggles

• Face shield

• Shoe nets

2. Use clean and sterile tools. The tools are regularly cleaned using alcohol and disinfectants.

3. Avoid touching the bloodstains with bare hands.

4. Make sure all the wet evidence are thoroughly air-dried.

5. Label and seal all evidence properly with relevant information.

6. Maintain chain of custody for each evidence collected.

Methods of Collection:

1. Wet Blood

· Liquid blood is highly perishable.

Small Amount of Blood: If only a small amount of blood is present, a sterile cotton swab is used to collect it.

Large Volumes: If the blood is present in the form of a large pool, it may be collected using a sterile syringe and stored in a glass vial coated with preservatives.

2. Dry Blood

Small Objects: If blood is present in a movable object like a knife or towels, then the item itself is packed.

Fixed Surfaces: If the blood is present on large objects that are immovable, like tables or walls then the following methods are used:

• Scrapping: Scraped using a sterile scalpel and packed in a paper bindle or druggist fold.

• Tape Lifting: Lifting the dry blood using a forensic tape and mounted on backing card.

• Rehydration: The blood is rehydrated using a sterile swab containing 0.9% NaCl (saline solution). This then is treated like wet blood evidence.

Methods of Preservation:

1. Physical Preservation

· Air dry: Wet blood evidence should be air-dried before it is packed.

· Paper envelopes: Never put the swabs in a plastic envelope and always use paper bags. Plastic traps moisture which acts a greenhouse for the growth of molds and bacteria that may destroy the DNA in the blood.

Once the sample is collected it must be refrigerated or frozen and brought to the laboratory as quickly as possible

2. Chemical Preservation

· When large amounts of liquid blood are collected, it is stored in glass viles that is coated with preservatives.

I. EDTA (Ethylenediaminetetraacetate)

This is an anticoagulant. It grabs onto the calcium ions in the blood that is responsible for the clotting of blood. This way the calcium is removed and prevents the blood from turning solid. This does not interfere with the genetic code of the DNA. It is the Purple Top vile and is used for DNA testing.

II. NaF (Sodium Fluoride)

This is an enzyme inhibitor. It stops the glycolysis process and stops the growth of bacteria. It is used in toxicology and to test blood alcohol levels. This is found in the Grey top vile.

III. (Potassium Oxalate)

This is also an anticoagulant that also removes calcium ions from the blood and prevent clotting. It is found in the Grey top vile and is used to test for blood glucose or other toxins.

By carefully following proper collection and preservation procedures, forensic investigators can ensure that blood evidence remains reliable and can be used effectively to help reconstruct events and support criminal investigations.

References

Schiro, G. (n.d.). Collection and preservation of blood evidence from crime scenes. Crime Scene Investigator Network. https://www.crime-scene-investigator.net/blood.html

Joseph, A. (2024, April 18). Blood evidence collection and preservation. Crime Museum. https://www.crimemuseum.org/crime-library/forensic-investigation/blood-evidence-collection-and-preservation/

Collection and preservation of blood evidence. (n.d.). ResearchGate. https://www.researchgate.net/publication/404718907_Collection_and_Preservation_of_Blood_Evidence

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