Stool wet mount (saline and iodine)

Principle

• Saline (0.85% NaCl) mount shows living, motile trophozoites and larvae, plus cysts, eggs, RBCs and WBCs in their natural state.
• Iodine mount (Lugol's or D'Antoni's, diluted) stains glycogen and nuclei of cysts, aiding identification, but kills trophozoites.

Specimen

• Fresh stool in a clean, leak-proof container, free of urine, water and barium.
• Examine liquid stool within 30 minutes, soft stool within 1 hour; formed stool can wait longer (same day).
• Three specimens collected on alternate days (within about 10 days) increase detection.

Reagents and equipment

• Physiological saline, iodine solution, glass slides, 22 × 22 mm coverslips, applicator sticks, microscope.

Procedure

• Place a drop of saline on the left and iodine on the right side of a slide.
• Emulsify a small portion of stool (from bloody or mucoid areas if present) in each drop.
• Apply coverslips; the preparation should be thin enough to read print through it.
• Scan the whole coverslip systematically with the 10× objective and reduced light; use 40× for detail.

Results

• Report helminth eggs, larvae, protozoan trophozoites and cysts to species where possible.
• Iodine: cyst nuclei and glycogen masses (e.g. in Entamoeba and Iodamoeba cysts) appear brown.
• Trophozoites of Entamoeba histolytica containing ingested RBCs indicate invasive amebiasis.
• Note Charcot–Leyden crystals, pus cells and RBCs.

Quality control and pitfalls

• Check reagents regularly; iodine should be the color of strong tea and be replaced when it fades.
• Calibrate the ocular micrometer, as size is key to identification.
• Artifacts (pollen, plant cells, yeasts, fat droplets) can mimic parasites.
• Too thick or dried preparations are unreadable; oil and barium make specimens unsuitable.
• Wet mounts are a screen only; follow with concentration and permanent stained smears.

Clinical use

• Diagnosis of intestinal parasites such as Giardia, Entamoeba, Ascaris, hookworm, Trichuris and Strongyloides larvae.