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Ova and Parasite (O&P) Examinations

Introduction | Macroscopic | Microscopic |

Microscopic Examination - Ova and Parasite Examination

Introduction | Direct Wet Smear | Concentration | Permanent Stained Smear

Direct Wet Smear

Introduction | Formulas | Quality Control | Procedures | Results and Reporting | Procedure Notes | Procedure Limitations | Procedure Review


Examination of the fecal material as a direct smear may or may not reveal organisms; in light infections, the number of organisms present may be very low. The direct wet smear is prepared by mixing a small amount of stool (about 2 mg) with a drop of 0.85% NaCl; this mixture will provide a uniform suspension under a 22- by 22-mm coverslip. A 2-mg sample of stool forms a low cone on the end of a wooden applicator stick. If a fresh stool specimen is received and if blood and mucus are present, the specimen should be examined as a direct mount making sure to sample the bloody areas. The entire 22- by 22-mm coverslip should be systematically examined with the low power objective (10 x) and low light intensity; any suspicious objects may then be examined with the high dry objective (40 x). Use of an oil immersion objective (100 x) on wet mounts is not routinely recommended. Many workers think that the use of the oil immersion objective on this type of preparation is impractical, especially since morphological detail is more readily seen by oil immersion examination of the permanent stained smear. This is particularly true in a busy clinical laboratory situation.

The direct wet mount is used primarily to detect motile protozoan trophozoites. These organisms are very pale and transparent, two characteristics that require the use of low light intensity. Protozoan organisms in a saline preparation will usually appear as refractile objects. If suspicious objects are seen on high dry power, allow at least 15 s to detect motility of slowly moving protozoa. Heat applied by placing a hot penny on the edge of a slide may enhance the motility of trophic protozoa. Tapping on the coverslip can also stimulate the fluid to move; objects will roll over, thus providing a better view of the parasite or artifact. After the wet preparation has been thoroughly checked for trophic amebae, a drop of iodine can be placed at the edge of the coverslip or a new wet mount can be prepared with iodine alone. A weak iodine solution is recommended; too strong a solution may obscure the organisms. The color should resemble "strong tea." Several types of iodine are available; D'Antoni's will be discussed here. Gram's iodine used in bacterial work is not recommended for staining parasitic organisms.

If preserved specimens are submitted to the laboratory, it will be more cost-effective and clinically relevant to delete the direct smear and begin the stool examination with the concentration procedure, particularly since motile protozoa will not be viable because of the prior addition of preservative. With few exceptions, intestinal protozoa should never be identified on the basis of a wet mount alone; permanent stained smears should be examined to confirm the specific identification of suspected organisms.