Overview
Stool examination is a routine diagnostic laboratory investigation, though patients may be reluctant to provide samples due to their offensive nature. A clean container free of detergent or disinfectant is generally sufficient for all types of stool examination, including culture.
This lesson covers the physical, microscopic and chemical examination of stool, along with concentration and preservation techniques essential for accurate detection of ova, cysts and other pathological findings.
Learning Objectives
After this lesson you will be able to…- Explain the physical examination of stool (quantity, consistency, colour, odour, blood, mucus, parasites)
- Describe the microscopic examination techniques for stool
- Explain the chemical examination of stool including pH and occult blood testing
- Describe the sedimentation and floatation concentration techniques
- Describe methods used for preservation of stool specimens
Clinical Story
Why This MattersA 6-year-old child presents with chronic diarrhoea, abdominal pain and poor weight gain. The physician requests a stool examination. The lab technician performs a formol-ether concentration technique before microscopy, since a direct wet mount alone had failed to detect any ova in a heavily diluted, watery sample.
Core Concepts
Laboratory Principle
Stool examination principles rest on physically observing gross abnormalities, then using differential specific gravity (floatation) or differential solubility/sedimentation (formol-ether) to concentrate low-density parasitic elements (ova, cysts, larvae) away from bulky faecal debris, followed by microscopic identification based on characteristic size, shape and internal structures.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Normal Saline | 0.85% | Wet mount preparation for motility and general morphology | Room temperature |
| Iodine Solution (Lugol's-type) | 1g iodine + 2g KI in 100ml distilled water | Stains glycogen, improves visualisation of ova/cyst internal structures | Dark glass bottle, away from light |
| Formol-Ether Reagents | 10% formalin + diethyl ether | Sedimentation concentration technique | Room temperature |
| Benzidine Powder + Glacial Acetic Acid + H2O2 | Standard reagent grade | Chemical test for occult blood in stool | Room temperature, prepare fresh |
Step-by-Step Procedure
Record quantity, consistency, colour, odour, presence of blood, mucus, or visible parasites.
Emulsify a small stool sample in normal saline on a slide; examine for motile trophozoites, ova and cysts.
Prepare a parallel iodine mount to enhance visualisation of internal ova/cyst structures.
Perform formol-ether sedimentation or saturated salt floatation if direct mount is negative but clinical suspicion remains.
Perform pH testing and benzidine-based occult blood testing when clinically requested.
Flow Diagram
Quality Control
Maintain fresh iodine reagent stored away from light, since it degrades and loses staining ability over time. Verify saturated salt solution specific gravity regularly using a hydrometer to ensure effective floatation.
Participate in external quality assurance panels distributing coded stool concentrates to verify inter-technician consistency in ova/cyst identification.
Reference Values
Normal Ranges⚠️ Values summarised from standard parasitology/microbiology references. Always confirm with your laboratory's SOP.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Voluminous, foul-smelling stool with mucus and blood | Suggestive of amoebic dysentery | Wet mount for E. histolytica trophozoites/cysts |
| Scanty stool, not foul-smelling, with blood/mucus | Suggestive of bacillary (Shigella) dysentery | Stool culture recommended |
| Rice-water stool | Suggestive of cholera | Urgent stool culture on TCBS agar; rehydration therapy |
| Clay-coloured (pale) stool | Suggestive of biliary tract obstruction (absent stercobilinogen) | Liver function tests, imaging of biliary tract |
Common Errors & How to Avoid Them
Cause: Adding iodine to a wet mount intended for motility assessment
Prevention: Always examine a fresh saline mount first for motile trophozoites before adding iodine.
Cause: Direct wet mount only performed on a dilute or low-parasite-load sample without concentration
Prevention: Always perform a concentration technique (formol-ether or floatation) when clinical suspicion is high despite a negative direct mount.
Cause: Container washed with detergent or disinfectant residue killing motile trophozoites or altering pH
Prevention: Use only clean, dry containers free of any chemical residue for stool collection.
Laboratory Tips from the Bench
Examine the wet mount at a thickness where you can just about read newspaper print through it — too thick obscures ova, too thin reduces yield.
Always examine at least three stool samples on separate days when parasitic infection is suspected but an initial sample is negative.
'NIH swab for pinworm' — the cellophane tape test targets Enterobius vermicularis, which lays eggs at the peri-anal skin, not inside the stool itself.
Important Notes
Detergent or disinfectant residue in the collection container can kill trophozoites or alter cyst morphology, leading to false-negative results.
Because E. vermicularis females migrate to the peri-anal skin to lay eggs at night, the cellophane tape (NIH swab) test — not routine stool examination — is the preferred diagnostic method.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeChronic watery diarrhoea for three weeks with poor weight gain and intermittent abdominal cramping. No visible blood or mucus reported by caregiver.
Despite a negative direct wet mount, the formol-ether concentration technique successfully revealed Giardia lamblia cysts, explaining the chronic diarrhoea and malabsorption picture.
- →Direct wet mount alone can miss low-burden parasitic infections.
- →Concentration techniques significantly improve diagnostic yield.
- →Chronic diarrhoea in children warrants a thorough parasitological work-up.
Frequently Asked Questions
In cholera, massive fluid secretion into the intestinal lumen occurs with minimal faecal matter, producing a watery stool containing flecks of mucus and epithelial cells that resemble water in which rice has been washed.
Iodine kills live trophozoites, destroying the motility that is essential for their identification; a fresh saline wet mount must be examined first for motile forms.
Stool pH can offer a clue to the type of intestinal infection — acidic in amoebic dysentery and alkaline in bacillary dysentery — supporting, but not replacing, microscopic and culture confirmation.
Quick Revision
10-Minute ReviewKey Takeaways
- Stool examination includes physical, microscopic and chemical components.
- Physical characteristics (colour, consistency, odour) offer valuable diagnostic clues.
- Direct wet mounts (saline and iodine) are complementary, not interchangeable.
- Concentration techniques (sedimentation and floatation) improve detection of low-burden infections.
- The cellophane tape (NIH swab) test, not routine stool exam, is used for Enterobius vermicularis.
- Proper collection container hygiene and prompt processing (or correct preservation) are essential for accurate results.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 50: Stool Examination.
- Cheesbrough M. District Laboratory Practice in Tropical Countries, Part 1. 2nd ed.
- Garcia LS. Diagnostic Medical Parasitology. 6th ed. ASM Press.