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Ova and Parasite (O&P) Examinations

Introduction | Macroscopic | Microscopic |

Macroscopic Examination

If the stool consistency can be determined it may give an indication of the types or stages of organisms present. Trophozoites (motile forms) of the intestinal protozoa are usually found in liquid specimens; both trophozoites and cysts might be found in a soft specimen. Cyst forms are generally found in formed specimens; however, there are always exceptions to these general statements. Coccidian oocysts and microsporidian spores can be found in any type of fecal specimen; in the case of coccidian oocysts, the more liquid the stool, the more oocysts that are found in the specimen. Helminth eggs may be found in any type of specimen, although in a liquid stool the chances of finding eggs are reduced by the dilution factor. Tapeworm proglottids may be found on or beneath the stool on the bottom of the collection container. Adult pinworms and Ascaris lumbricoides may occasionally be found on the surface or in the stool.

The presence of blood in or on the specimen may be clinically relevant and should be reported. Dark-colored stools may indicate bleeding high in the gastrointestinal tract, and fresh (bright red) blood most often is the result of bleeding at a lower level. In certain parasitic infections (amebiasis with the true pathogen, Entamoeba histolytica), blood and mucus may be present. These areas of blood and mucus should be carefully examined for the presence of trophic amebae. Occult blood in the stool may or may not be related to a parasitic infection and could result from a number of different conditions. Ingestion of various compounds may give a distinctive color to the stool (iron, black; barium, light tan to white).

Many laboratories prefer that stool specimens from both in- and outpatients be submitted in some type of preservative. Rapid fixation of the specimen immediately after passage (by the patient) provides an advantage in terms of recovery and identification of intestinal protozoa. This advantage (preservation of organisms before distortion or disintegration) is felt to outweigh the limited motility information that might be gained by examining fresh specimens. Each laboratory will have to decide for itself, often basing the decision on the types of procedures ordered by the physicians who use the laboratory service, the test methodologies selected (traditional methods or new immunoassay detection kits), and the lag time between specimen collection and submission to the laboratory. If specimens are received that do not meet specimen requirements for quality, they should be rejected and additional specimens requested.

Freshly passed specimens are necessary for the detection of trophic amebae, flagellates, and ciliates. Examination of liquid specimens must be carried out within 30 min of passage (not 30 min from the time the specimen reaches the laboratory). Soft specimens should be examined within an hour of passage. Immediate examination of a formed specimen is not as critical; however, if the stool cannot be examined on the day it is collected, portions of the specimen should be preserved. In a routine laboratory setting, these time frames are often neither practical nor possible.