Stool Concentration Technique (Formalin-Ether Method)
1. Objective
To concentrate parasitic elements (ova, cysts, and larvae) in a stool sample using the formalin-ether sedimentation technique, enhancing their detection under the microscope.
2. Principle
The formalin-ether concentration method uses centrifugation to separate parasites from fecal debris.
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Formalin fixes and preserves parasitic structures.
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Ether extracts fats and clears the sample.
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Upon centrifugation, heavier parasitic elements settle at the bottom (sediment) and can be examined microscopically.
3. Materials
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Fresh stool sample
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10% formalin solution
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Ether (diethyl ether or ethyl acetate)
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Glass centrifuge tubes (15 mL)
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Gauze or strainer
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Applicator sticks
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Dropper or pipette
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Centrifuge (capable of ~500 x g)
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Glass slides and coverslips
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Microscope (10x and 40x objectives)
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PPE (gloves, lab coat, mask)
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Waste container for biohazard and ether
4. Procedure
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Mix about 1 g of stool with ~7 mL of 10% formalin in a beaker or tube.
Strain the mixture through gauze into a clean centrifuge tube.
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Add 3–4 mL of ether to the tube and cap tightly.
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Shake vigorously for 30 seconds to mix.
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Centrifuge at ~500 x g (typically 5–10 minutes).
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After centrifugation, four layers will form:
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Top layer: Ether
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Middle layer: Debris plug
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Third layer: Formalin
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Bottom: Sediment (contains parasites)
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Carefully discard top three layers, keeping only the sediment.
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Place a drop of sediment on a glass slide, add a coverslip, and examine microscopically.
5. Result
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Positive: Detection of helminth eggs, protozoan cysts, or larvae in sediment.
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Negative: No parasitic elements detected.
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Typically, parasites are easier to find due to concentration and reduced background debris.
6. Uses
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Improves detection of low-intensity infections.
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Ideal for routine stool examination in suspected parasitic infections.
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Useful in research, surveillance, and diagnostic labs.
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Often done alongside wet mount microscopy for confirmation.
7. Conclusion
The Formalin-Ether Concentration Technique is a sensitive and effective method to enhance the visibility of parasitic elements in stool. It is especially valuable when infections are light and not easily visible in direct wet mount microscopy.
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