Faecal Direct Smear

This is usually used to detect and identify protozoan trophozoites, cysts and oocysts in fresh or appropriately fixed faeces.


Purpose

This is usually used to detect and identify protozoan trophozoites, cysts and oocysts in fresh or appropriately fixed faeces. Occasionally, helminth eggs or nematode larvae may also be detected.

Principle

Mixing a small amount of fresh faeces with a drop of saline or dilute iodine on a microscope slide presents the cysts and oocysts reasonably clearly, provided that the faeces is fresh or appropriately fixed. In fresh faeces and using saline, the trophozoites may be observed moving.

Equipment

Slides and coverslips

Saline or Lugol’s iodine

Mounted needle

Microscope

Procedure

  1. Place a very small amount of faeces on a slide and add one drop of saline (for live protozoa) or Lugol’s iodine (which stain but kill the protozoa).
  2. Mix well with a mounted needle.
  3. Smear thinly enough to read through, apply a coverslip (it is possible to push aside larger faecal debris using the coverslip) and examine under the microscope.
  • This technique may demonstrate nematode, trematode or cestode eggs, but generally it is of limited usefulness for this.

Variations

This is a basic technique. Sometimes, smears may be fixed and specially stained, for example acid-fast staining for Cryptosporidium oocysts.

Problems

It is possible that if only small numbers of trophozoites, cysts or oocysts are present in the sample, they may be missed. In addition, some non-parasite structures can be misidentified as parasites.
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