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.

  • Formalin fixes and preserves parasitic structures.

  • Ether extracts fats and clears the sample.

  • Upon centrifugation, heavier parasitic elements settle at the bottom (sediment) and can be examined microscopically.

3. Materials

  • Fresh stool sample

  • 10% formalin solution

  • Ether (diethyl ether or ethyl acetate)

  • Glass centrifuge tubes (15 mL)

  • Gauze or strainer

  • Applicator sticks

  • Dropper or pipette

  • Centrifuge (capable of ~500 x g)

  • Glass slides and coverslips

  • Microscope (10x and 40x objectives)

  • PPE (gloves, lab coat, mask)

  • Waste container for biohazard and ether

4. Procedure

  • 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.

  • Add 3–4 mL of ether to the tube and cap tightly.

  • Shake vigorously for 30 seconds to mix.

  • Centrifuge at ~500 x g (typically 5–10 minutes).

  • After centrifugation, four layers will form:

    • Top layer: Ether

    • Middle layer: Debris plug

    • Third layer: Formalin

    • Bottom: Sediment (contains parasites)

  • Carefully discard top three layers, keeping only the sediment.

  • Place a drop of sediment on a glass slide, add a coverslip, and examine microscopically.

5. Result

  • Positive: Detection of helminth eggs, protozoan cysts, or larvae in sediment.

  • Negative: No parasitic elements detected.

  • Typically, parasites are easier to find due to concentration and reduced background debris.

6. Uses

  • Improves detection of low-intensity infections.

  • Ideal for routine stool examination in suspected parasitic infections.

  • Useful in research, surveillance, and diagnostic labs.

  • 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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