Identification tests for Faeces
The identification of faeces usually involves a combination of physical and chemical examinations. Physical examination may include identifying the presence of faecal stains based on colour, odour, and appearance. Chemical examination may include the detection of metabolic by-products such as urobilin or stercobilin. Positive results from multiple tests may be taken into consideration during forensic investigations to confirm the presence of faeces.
PHYSICAL EXAMINATION
Colour and Appearance
The colour of faecal stains can vary depending on several factors, including the individual's diet, digestion, and underlying health conditions. Typically, faecal stains are brown in colour and have a soft, formed consistency.
The two main pigments responsible for the brown colour of faeces are urobilinogen and stercobilin. Intestinal bacteria convert bilirubin to urobilinogen, which is then oxidised to form stercobilin, giving faeces their distinctive brown colour. The shade of brown can vary depending on the amount of bile produced by the liver and the length of time it takes for the faeces to travel through the digestive system. Fresh faecal stains may appear darker or lighter in colour, depending on the amount of water present in the stool. The colour of faecal stains can also be affected by other factors, such as the type of surface the stain is on and the amount of time the stain has been exposed to air. As the stains age and dry out, they may turn darker in colour and eventually become black or dark brown.
Odour
The characteristic odour of faeces is caused by indole, skatole, and hydrogen sulfide, which are the metabolic by-products of intestinal bacterial flora. While odour can be a strong indicator of the presence of faeces, it is not always reliable and can be influenced by many factors such as the age of the stain, the environment in which the sample was collected, and the sensitivity of the individual performing the analysis.
Microscopic Examination
Microscopic examination can be used as both a preliminary test and a confirmatory test for the identification of human faecal stains in forensic investigations, depending on the context and purpose of the examination.
As a preliminary test, it can be used to provide initial information about the characteristics of the faecal matter, such as its texture, colour, and the presence of visible structures such as undigested food particles or parasite eggs. This information can then be used to guide further testing or analysis.
As a confirmatory test, it can be used to identify specific structures or features in the faecal matter that are characteristic of human faeces, such as the presence of human red blood cells, leukocytes, or intestinal epithelial cells. These structures can provide definitive evidence of the presence of human faecal matter and can be used to exclude other sources.
Figure: Microscopic examination of Faeces
✏️ Preparation of Reagents
Lugol’s Iodine Solution: Dissolve 1g of iodine and 2g of potassium iodide (KI) in 200ml of water. Mix well and store for further use.
✏️ Procedure:
Soften the suspected faecal stain with distilled water for 30 mins.
Take a small scraping from the softened stain and place the scraping on a slide using a clean stick or spatula.
Add 1-2 drops of Lugol's iodine solution to the slide and cover the slide using a cover slip by gently pressing it down to spread the sample.
Observe the slide under a microscope at 10x or 40x magnification.
Observations: The presence of undigested food particles, muscle fibres, epithelial cells, RBCs, WBCs, microorganisms, other parasites and their eggs, indicates the presence of faeces in the sample.
CHEMICAL EXAMINATION
Urobilinogen test
The urobilinogen test relies on the principle that urobilinogen can undergo oxidation to form urobilin in the presence of a strong oxidizing agent, such as mercuric chloride. Subsequently, urobilin reacts with alcoholic zinc chloride to form a green fluorescent zinc-urobilin complex that can be observed using UV light.
Figure: Urobilinogen test for the detection of Faeces
✏️ Preparation of Reagents:
a) 40% alcoholic mercury solution: Add 4g of mercuric chloride (HgCl2) to 10ml of methanol. Mix well and store in a bottle.
b) 40% alcoholic zinc solution: Add 4g of zinc chloride (ZnCl2) to 10ml of methanol. Mix well and store in a bottle.
✏️ Procedure:
1. Take 2-3 drops of liquid extract of the suspected faecal stain in a microcentrifuge tube.
2. Add 1-2 drops of a 40% alcoholic mercuric chloride solution to the tube, followed by 1-2 drops of amyl alcohol (C5H12O). Gently shake the tube to ensure the contents are well mixed.
3. Centrifuge the tubes and collect the clear supernatant.
4. Examine the tube under UV light; no fluorescence should be visible at this stage.
5. Add 1-2 drops of 40% alcoholic zinc chloride to each tube and shake well.
6. Incubate the tube at room temperature for 30-60 mins.
7. Re-examine the tube under UV light. A green fluorescence indicates the presence of urobilinogen, suggesting the presence of faecal matter in the sample.