Direct Saline/ Iodine Wet Mount for Diagnosis of Intestinal Parasites

Stool is the most common specimen submitted to the diagnostic laboratory; the most commonly performed procedure in parasitology is the ova and parasite (O &P) examination.

Egg of Ascaris in Wet Mount
Egg of Ascaris in Wet Mount

Gastro-intestinal infestations (infections) by parasites (Protozoan/Helminthes) are primarily diagnosed by detecting live motile trophozoites (for protozoans); cyst (inactive dormant stage of Protozoa) or eggs (in case of Helminthes) in stool.

Microscope slides made from faecal specimen of the patients can be examined under low and high power. These etiological agents are identified based on their morphological/staining (bile stained or not bile stained; in case of eggs of helminthes) characteristics.

Trophozoites and Cyst of Giardia lamblia
Trophozoite (left) and Cyst of Giardia lamblia (right)

Saline wet mount is made by mixing a small quantity (about 2 mg) of faeces in a drop of saline placed on a clean glass slide. The smear is then examined under microscope. Saline wet mount is used for the detection of trophozoites and cysts of protozoa, and eggs and larvae of helminthes.
It is particularly useful for detection of live motile trophozoites of E. histolytica, Giardia lamblia and Balantidium coli.

Uses of Direct Wet Mount

  1. To assess worm burden of patient
  2. To provide quick diagnosis of heavily infected specimen
  3. To check organism motility (primarily protozoan trophozoties)
  4. To diagnose organisms that might not been seen from permanent stain methods


  1. Normal Saline (0.85% NaCl);  Lugol’s Iodine
  2. Glass slides
  3. Cover slips
  4. Pipettes
  5. Gloves
  6. Microscopes


  1. Apply the patient’s sample to a small area on a clean microscope slide. Remove any gross fibers and particles.
  2. Immediately before the specimen dries, add 1 or 2 drops of saline with a pipette. Mix with pipette tip.
  3. Cover the specimen with a cover slip. (Note: Avoid air bubbles by drawing one edge of the coverslip slightly into the suspension and lowering it almost to the slide before letting it fall. The mount should be just thick enough that newspaper print can be read through the slide.)
  4. If desired the coverslip (s) can be sealed using petroleum jelly and Paraffin oil or other suitable sealing preparations. Sealing the coverslip keeps organisms from moving when using oil immersion objectives and prevents the preparation from drying out.
Suitable Wet Mount Preparation (Img Source: DPDx)
Suitable Wet Mount Preparation (Img Source: DPDx)


  1. Examine the specimen with the low power objective (10x) and low light. Begin at one corner of the smear and systematically examine successive adjacent swaths with the low power microscope. Low power examination includes entire area of 22 by 22 mm coverslip preparation (both saline and iodine).

    Scanning the stained smear (Image source: dpdx)
    Scanning the stained smear (Image source: dpdx)
  2. When a parasite like object comes into view, it should be more closely examined and identified under high power(40x) objective. High dry power examination should include at least one third of the coverslip area (both saline and iodine).


Results from the direct smear examination should often be considered presumptive; however, some organisms could be definitely identified (Giardia lamblia cysts and Entamoeba coli cysts, helminth eggs, and larvae, Isospora belli oocysts). These reports should be considered “preliminary”, while the final report would be available after the results of concentration and permanent stained smear were available.

Wet Mount Smear of Stool showing E. histolytica


  1. Once Iodine is added to the preparation, the organism will be killed and motility will be lost.
  2. Oil immersion examination is not recommended (organism morphology not that clear)


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