Parasitology
HIGH-YIELD EXAM REVIEWER
MOST TESTABLE FACTS (memorize these first)
Fact | Detail |
|---|---|
Iodine wet mount | Best for protozoa; poor for helminths |
Saline wet mount | Detects motile trophozoites & larvae |
Watery/loose stool | → Trophozoites likely |
Formed/soft stool | → Cysts predominate |
Unevenly distributed eggs (exception to "uniform distribution") | Schistosome, pinworm, Taenia eggs |
Trophozoites most numerous | Last portion of stool / in mucus |
Concentration procedures | ↑ cysts & eggs/larvae but bad for trophozoites |
Kato-Katz NOT good for | Small trematode eggs; protozoa (little/no value) |
Kato-Katz egg that disappears | Hookworm — collapses 30–60 min after clearing; read ASAP |
Kato-Katz eggs that last months | Ascaris & Trichuris |
Schistosome eggs (Kato-Katz) read within | 24 hours (endemic areas) |
Direct smear ideal feces amount | ~2 mg (size of a match head) |
Finger for blood smear | Ring finger, left hand; infants = big toe; NEVER thumb or heel |
Pencil type for labeling blood film | HB lead pencil only — never ballpoint/gel (ink spreads on fixing) |
Where to fix with methanol | Thin film ONLY — never let methanol touch thick film |
Thick film stir rule | Do NOT stir the blood when spreading |
Thick film size | ~1 cm diameter (size of 5-centavo coin) |
Giemsa stock solution | Never add water; never shake; never return unused stain to bottle |
EPG MULTIPLIERS (Kato-Katz) — easy to test as math problem
Template weight | Multiply by |
|---|---|
50 mg | × 20 |
20 mg | × 50 |
41.7 mg | × 24 |
High counts → Stoll quantitative dilution (0.1 mol/L NaOH)
RAPID vs SLOW GIEMSA METHOD (classic comparison Q)
Rapid (10%) | Slow (3%) | |
|---|---|---|
Slides at once | 1–15 | 20+ |
Used for | Quick malaria dx | Surveys/research/teaching |
Dilution | 3 drops stock : 1 mL buffer | 3 mL stock : 97 mL buffer (pH 7.2) |
Stain time | 8–10 min | 45–60 min |
Rinse method | Drops of water added gently | Water poured into thin-film end of trough |
Both: fix thin film w/ methanol → dry film-side-down → avoid scum touching film.
MICROSCOPIC EXAM METHOD MATRIX (DFS Exercise)
Method | Best Detects | Weak For |
|---|---|---|
Direct wet mount (fresh) | Motile trophozoites, larvae | — |
Direct mount (preserved) | Parasites that don't concentrate well | — |
Concentration (centrifuge/floatation/formalin-ethyl acetate) | Cysts, eggs, larvae | Trophozoites |
Permanent stain | Trophozoite & cyst morphology | — |
Watery stool → simple centrifugation, not floatation
Preserved specimen → direct wet mount may be omitted
Formed stool → minimum = concentration procedure
"NEVER / ALWAYS" RULES (common trick-question targets)
Never use thumb or heel for blood collection
Never let methanol touch the thick film
Never stir blood when making thick film
Never add water to Giemsa stock
Never shake Giemsa stain bottle before use
Never pour unused stain back into stock bottle
Never use ballpoint/gel pen to label slides
Always wipe away the first drop of blood before collecting sample
Always dry thick films completely before staining (avoid overheating/heat-fixing)
Always read Kato-Katz hookworm preps immediately after clearing
BLOOD COLLECTION QUICK SEQUENCE
Glove → select ring finger
Clean w/ alcohol → dry w/ cotton cloth (stimulates circulation)
Lancet puncture (rolling action) → wipe away first drop
Small drop → thin film | 2–3 larger drops (~1cm away) → thick film
Thin film: spreader at 45°, smooth push
Thick film: join drops w/ spreader corner, 3–6 quick strokes, no stirring
ORGANISMS DETECTED IN BLOOD (Exercise 7)
Leishmania donovani, Trypanosoma spp., Plasmodium, Babesia, microfilariae
Trypanosoma & microfilariae → seen motile in wet prep; definitive ID needs permanent stain