Stool Concentration Techniques
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Direct answer
A single faeces sample is mostly debris with parasites scattered thinly through it, so concentration stacks the diagnostic odds. Two principles dominate: sedimentation and flotation. Formalin-ethyl acetate (formalin-ether) sedimentation — the Ridley-Allen technique in Indian laboratories — emulsifies stool in saline or formalin, strains it, and centrifuges with ether or ethyl acetate that dissolves fat and lifts debris, leaving ova, larvae and cysts packed into a small sediment button for saline and iodine mounting. Zinc sulfate flotation at specific gravity about 1.18 floats ova and cysts to the surface while dense matter sinks; lighter parasites are recovered from the surface film. Both kill trophozoites — so a direct saline wet mount within thirty minutes of passage remains mandatory in acute dysentery, where motile Entamoeba histolytica trophozoites are the target.
What you must remember
- Sedimentation technique steps: emulsify in 10 per cent formalin, filter through gauze, centrifuge, add ether or ethyl acetate, shake, recentrifuge; examine the sediment in saline and iodine — recovers almost all ova, larvae and cysts.
- Flotation technique: zinc sulfate solution at specific gravity 1.18-1.20 floats cysts and most ova for a cleaner preparation; misses dense and operculated ova — unfertilised Ascaris and schistosome eggs sink and are lost.
- Saline then iodine: saline preserves motility for trophozoites and larvae; Lugol's iodine stains glycogen and nuclei yellow-brown for cyst identification — but kills trophozoites, so saline comes first.
- Oocyst exception: Cryptosporidium, Cyclospora and Cystoisospora oocysts concentrate poorly by either method and need modified acid-fast staining or auramine to show up at all.
- Quantitative option: the Kato-Katz thick smear, using a standard 41.7 milligram template, counts eggs per gram of stool — the WHO field standard for soil-transmitted helminth and schistosome burden surveys.
- Site-directed extras: cellophane (Scotch) tape slide pressed over the perianal skin in the morning for Enterobius vermicularis; Baermann's larval migration or charcoal culture for Strongyloides; duodenal aspirate or string test for Giardia and Strongyloides when stool stays negative.
- Repeat sampling logic: three samples on alternate days raise yield because shedding is intermittent — Ascaris and hookworm ova output varies daily, Enterobius lays nightly, and Giardia excretes cyclically.
- Programme link: India's National Deworming Day (10 February, repeated 10 August) administers albendazole to school-age children — surveys behind such programmes are built on Kato-Katz and concentration methods.
Why the laboratory asks for three samples on alternate days
Explain the request to the parent of a child with recurrent abdominal pain and you explain parasitology. Egg output is not a steady stream: adult Ascaris and hookworms shed at varying daily rates, Enterobius females migrate nocturnally to deposit eggs that a morning stool may miss entirely, and Giardia cyst excretion cycles through peak and trough days. A single concentration study therefore misses a genuine infection a disconcerting fraction of the time; three samples on alternate days substantially raise cumulative yield, which is the evidence behind the request. Sequence matters too: if dysentery is the story, the first sample needs a direct motility search within half an hour of passage — concentration would kill the trophozoites and report "no cysts", a technically true and clinically useless answer.
Then match technique to target. A child with perianal itching gets tape, not stool; a farmer with larva currens rash needs Strongyloides agar culture or Baermann, since eggs rarely appear in stool; an immunocompromised adult with persistent watery diarrhoea gets modified acid-fast staining for coccidian oocysts that flotation barely concentrates. The laboratory that asks one question — which parasite am I hunting — picks the right technique every time.
Where students slip
The flotation-versus-sedimentation trade-off is the classic trap: flotation gives a cleaner field but loses operculated and heavy ova, so a negative flotation never excludes Ascaris or schistosomiasis — sedimentation is the safer default when one technique must be chosen. Second, iodine-first mounting kills trophozoites and dissolves their diagnostic motility; saline first, iodine second is the sequence to write. Third, students report "Entamoeba histolytica cysts" from a concentration without qualifying size and nuclear detail — the ten-micrometre four-nucleus cyst is E. histolytica/dispar/moshkovskii complex on morphology alone, and the honest report requires that hedge unless antigen or PCR settles it. A final viva point: concentration kills trophozoites but does not distinguish viable from dead cysts, and after deworming, dead eggs may still pass for days — cure is judged by follow-up clearance, not the immediate post-treatment sample.
Frequently asked questions
Why does formalin-ethyl acetate sedimentation recover more parasites than flotation?
It concentrates ova, larvae and cysts regardless of specific gravity, including operculated and dense eggs that sink and would be missed by zinc sulfate flotation.
Which parasites need modified acid-fast staining after concentration?
Cryptosporidium, Cyclospora cayetanensis and Cystoisospora belli oocysts, which are small, poorly concentrated and best seen as pink-red acid-fast oocysts against a blue background.
What is the Kato-Katz technique used for?
A measured 41.7 milligram stool template makes a cleared thick smear for counting eggs per gram — the WHO's quantitative field method for helminth burden estimates.
How is Enterobius vermicularis best diagnosed?
By cellophane tape pressed on the perianal skin early morning and mounted for microscopy, since eggs are deposited perianally and rarely appear in stool.
Why must a direct saline wet mount precede concentration in dysentery?
Motile trophozoites of Entamoeba histolytica and flagellates are killed and distorted by concentration and iodine; only an immediate saline mount shows the motility that identifies them.