Module 2: Introduction to Common Fecal Diagnostic Procedures

Module 2.2: Direct Fecal Smear (Wet Mount)

Common fecal techniques used in diagnosing parasitism

In the next few pages, we will discuss common techniques used to recover parasite ova, or eggs, larvae, and motile forms (trophozoites) in feces. Many of these techniques will be practiced in our laboratory sessions over the next few weeks.

Direct fecal smear

There are two separate techniques used for direct fecal smear analysis. These are the dry mount fecal cytology (DM) or wet mount fecal test (WM). Both are performed by collecting a small amount of fecal material directly from the lumen of the rectum.  DM and WM are considered more sensitive than rectal scrapings for the detection of Giardia sp. and Trichomonads, as these protozoans by the time they reach the rectum are no longer associated with the mucosa.

Which one should I choose?

Dry mount fecal cytology

This test is commonly selected when we are looking for abnormal bacteria or yeast organisms in feces. These samples are stained using a Romanoswky stain, such as Diff Quik, and evaluated at 100x, 400x, and 1000x magnification. This is an uncommon method for evaluating feces for parasites.

Wet mount fecal test

Wet mount fecal test is the preferred method for the identification of motile protozoans, such as Giardia trophozoites, or in cases in which you have too little feces to use fecal flotation techniques. If you are looking for motile organisms, such as Giardia trophozoites, these samples must be reviewed within 30 minutes of collection and are not suitable as a send-out test. This test is not indicated for the detection of bacteria or yeast.

Wet mount fecal test procedure:

Video: This video demonstrates the wet mount fecal test. In this video, they refer to this test as a “direct fecal smear” this is another common name for this test that you will encounter.

Materials and equipment needed

  • Page 4 of your Parasitology Laboratory Techniques book
  • Glass slides and coverslips
  • Water (saline preferred)
  • Microscope and light source
  • Lugol’s iodine (helpful for identifying Giardia cysts)**

Technique

  1. Take a small amount of feces (i.e., the amount of feces adhering to a rectal thermometer) and emulsify with a few drops of water on a glass slide. The resultant mixture should not be opaque (place the slide on top of this page – if you can’t see the letters on this page under the mixture, the mixture is too thick→ add more water or use less feces).
  2. Add a coverslip and examine under 100x magnification (eyepiece 10x * objective 10x = 100x total magnification) and 400x magnification.  Be sure to reduce brightness (adjust light intensity) and increase contrast (adjust condenser to down position) so that eggs or trophozoites and their internal structures are easily seen.
  3. Examine entire smear systematically for parasite ova, worm larvae, or protozoan structures.

** Lugol’s iodine can be used to help identify Giardia & Hexamita cysts. Before placing the coverslip, add a drop to the mixture. Iodine will enhance the internal structures of the cyst but will kill Giardia and Hexamita trophozoites. We do not have Lugol’s iodine in our laboratory.

Coverslip placement on a glass slide during wet mount procedure
Coverslip placement for wet mount procedure

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