Examining stool for the parasites themselves is the traditional method of diagnosis, and remains the primary method for many labs. There are multiple ways that we can approach the microscopic exam; however, the following will help us determine which method to use:
1. As you will see, many protozoa (but not all) have both an active trophozoite or troph phase, and a resistant cyst phase. Typically, the trophozoites are found in runny or loose stools and the cysts are found in more formed stools. If you have a loose stool specimen, direct examination should occur while the specimen is very fresh (less than a few hours old), or the trophs may die and no longer be visible.
2. If you have a loose stool and don't have time for a direct exam, it is best to put it into a fixative (preservative).
Table 1 provides some guidelines on specimen procurement for microscopic exam. The test is usually called Stool for O&P (ova and parasites)
Table 1. Specimen Procurement for Microscopic Exam for Ova and Parasites.| Specimen options | Collection & Transport | Processing | Comments |
Fresh stool
| Waxed container; delivery: 30 min if liquid, 60 min if semi-formed, 24 h if formed | Direct wet prep (not on formed specimens); concentration for cysts; permanent stained smear | Unacceptable if patient received: barium, mineral oil, anti-diarrhea medications |
| Preserved stool - fixatives | 5/10% formalin; MIF*; SAF*; Schaudinn's; PVA*; single vial systems; universal fixative | Concentration; permanent stained smear | Same as above |
* MIF=Merthiolate iodine formalin; SAF=sodium acetate-acetic acid-formalin; PVA=polyvinyl alcohol
