Stool testing or stool examination is performed in laboratories for different diagnostic purposes.It is a specimen that is easily obtained, but it can be a reluctance on the part of the patient to provide the stool specimen because of it’s offensive nature and foul smell.In most cases, a clean container is used in stool specimen collection which contains neither detergent or disinfectant is sufficient for all types of stool tests, including stool culture.A comprehensive stool analysis is carried out in 3 different way in the laboratory
1. MACROSCOPIC EXAMINATION OF STOOL
During fecal test,macroscopic examination is to observe the following aspects of stool sample
The consistency and form
Normal stools are well formed. The stools in diarrhea and dysentery are semi- solid or watery in nature. In malabsorptive states, the stool is also semi- solid or watery depending on the severity of the disease. In cases of fat malabsorption (fat in stool), the stools are pale bulky and semi- solid.
In intestinal amoebiasis the stools tend to be voluminous. Whereas in bacillary dysentery the amount of stools is small due to Shigella.
Normal stools are light to dark brown in colour due to the presence of stercobilinogen, a product of bilirubin metabolism; in case of bleeding in the intestinal tract, the stools become dark and tarry. in nature due to the formation of acid hematin if the bleeding is in the small intestine.
In biliary obstruction, the stools may be clay- coloured due to the absence of stercobilinogen.The patient’s diet can also lead to a change in the colour of the stool.For example, if the patient previously had spinach, the stool may be green.For those who had a barium examination, the stools may be white.
Stool odour can become offensive under circumstances such as intestinal amoebiasis. In bacillary dysentery and cholera, the stools do not smell because there are no faeces.
If present, blood should be detected in stool as it indicates ulcerations or the presence of another pathology such as malignancy. It should also be considered whether the blood is bright red or changes color, as this may be an indication of the location of pathology in the intestinal tract.
Mucus is present under certain conditions such as amoebic and bacterial dysentery.
Stools may contain adult helminths. Nematodes like Ascaris are easy to recognize due to their size. Hookworms and proglottids of cestodes may also be present. These can be visible to the naked eye.
2. MICROSCOPIC EXAMINATION
The laboratory diagnosis of most parasitic infections is by demonstrating the ova of the parasite (stool ova and parasites test) in the infected person ‘s stools. Stool is collected in a clean container. The following techniques can be used to examine the stool.
- Iodine preparation
- Wet mount examination
- Buffered methylene blue stain: (Nuclear stain in E histolytica)
- Cellophane tape test:- NIH swab
- Concentration techniques
For stool ova and parasite test procedure and stool test results interpretation read : Stool Ova and Parasite wet mount
Stool iodine Preparation
The preparation of iodine leads to better visualization of the morphological details of ova and cysts as the glycogen stains.However, it has the disadvantage that the Entamoeba histolytica live trophozoites can not be seen as the iodine kills it.
One gram of iodine and two grams of potassium iodide are mixed in 100 ml distilled water. Potassium iodide is mixed in water and the iodine crystals are then added and vigorously shaken.The solution is filtered into a dark glass bottle and kept out of the light.
Stool saline wet mount examination
The stool is emulsified in normal saline solution and a large drop is placed on a microscope slide and then covered with a cover glass.This is then inspected with a light microscope. It is better to keep the condenser low and the light intensity low for the correct visualization of the ova and cysts. The film’s thickness should be such that the printed letters of the journal can be seen through it.
Buffered methylene blue stain
It is used for nuclear stain in Entamoeba histolytica
Cellophane tape test
The eggs of Enterobius vermicularis from the Perianal region are collected with the NIH swab( National Institute of Health).
Two types of concentration techniques are used for stool testing
A small amount of stools are emulsified in a wide mouth container in a saturated salt solution.When the stool becomes homogeneous, several drops of saturated saline are mixed and more saturated saline is added.A glass slide is placed over the container so that the slide is in contact with the saturated solution surface.If the fluid is not in touch with the slide then more saturated saline should be added till the fluid level touches the slide.The slide is left for fifteen minutes.After this, the slide is gently lifted off the container and turned upside down carefully to ensure that no fluid is spilled from the slide.A cover slip is placed over the fluid on the slide and the slide it is examined under a microscope.
Principle of floatation technique
The specific gravity of ova and cysts is lower and therefore floats to the top of the saturated salt solution, where it sticks to the glass slide underneath.
A. Sedimentation technique
It comprises the following
- Formol ether technique
- Formol ether SAF
- Formol ether PVA
B. Floatation technique
Comprises the following
- Zinc sulfate
- Saturated salt solution
3. CHEMICAL TESTING OR EXAMINATION OF STOOLS
Occult blood (fecal occult blood test or stool occult blood test)
Occult blood (occult stool test or blood in stool test) may be present in a variety of conditions, including malignancy of the gastrointestinal tract.The reagent used is benzidine powder.Add a pinch of benzidine powder to a test tube and acidify with 1- 2 drops of glacial acetic acid and mix well.To this is added 1 ml of hydrogen peroxide, which is mixed well again.Place a clean glass slide and put a small amount of stool on it.Add 1- 2 drops of the previously prepared benzidine mixture to the stool sample taken on the slide and observe the colour change. The development of a green to blue colour indicates the presence of occult blood in the stool sample.
The pH of stools is acidic in amebic dysentery and in bacillary dysentery is alkaline.
HOW TO PRESERVE STOOL SAMPLES (stool sample storage)
Stool samples containing eggs and parasite cysts may be conserved using one of the following methods
- 10% formol saline
- Buffered formol saline
- Merthiolate-iodine formalin
- Sodium acetate-acetic acid formalin (SAF)
- Polyvinyl alcohol (PVA)
- Schauddins preservative
Morphology of some common helminthic ova and cyst found in stool
The following are parasite testing labs or fecal parasite test(stool lab)
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