Parasitology
This domain tests specimen timing, stain selection, and organism sizing - the difference between a correct and an incorrect parasite report is often a few micrometers.
Specimens and stains
- Ova and parasite exam: three stools collected over 5–7 days, one preserved (formalin or zinc-PVA) and one unpreserved if antigen testing is planned.
- Trichrome or iron hematoxylin for permanent stains; iodine wet mounts for cysts; modified acid-fast (1% sulfuric acid) for Cryptosporidium, Cyclospora, and Cystoisospora.
- Blood parasites: Giemsa-stained thick and thin films on EDTA blood collected during fever; Knott concentration for microfilariae.
- Scotch tape preparation for Enterobius; Baermann or agar plate culture for Strongyloides; duodenal aspirate or string test for Giardia.
Worked example
A patient with AIDS has watery diarrhea. Modified acid-fast stain of concentrated stool shows spherical 8–10 µm oocysts that are variably acid-fast and autofluorescent under ultraviolet light. Reasoning: size separates the organisms - Cryptosporidium oocysts are 4–6 µm, Cyclospora 8–10 µm, and Cystoisospora 20–30 µm. Variability of acid-fast staining and autofluorescence fit Cyclospora cayetanensis. Report it and notify as required.
Traps
Examining a single stool; relying on trichrome alone and missing coccidia; using iodine on a wet mount intended for motility; measuring at the wrong magnification; and mistaking pollen, crystals, or white cells for parasites.