Microscopic Examination الفحص الميكروسكوبى

من طرف **المسلمانى** الجمعة أبريل 02, 2010 11:38 pm

Microscopic Examination

Examining thick smears
Since the erythrocytes (RBCs) have been lysed and the parasites are more concentrated, the thick smear is useful for screening for parasites and for detecting mixed infections.

First screen the entire smear at a low magnification (10× or 20× objective lens), to detect large parasites such as microfilaria.
Then examine the smear using the 100× oil immersion objective lens. Select an area that is well-stained, free of stain precipitate, and well-populated with white blood cells (WBCs) (10-20 WBCs/field).
If you see parasites, make a tentative species determination on the thick smear and then examine the thin smear to determine the species present. Most often, the thin smear is the appropriate sample for species identification.
Determination of "No Parasites Found" (NPF): For malaria diagnosis, WHO recommends that at least 100 fields, each containing approximately 20 WBCs, be screened before calling a thick smear negative. Assuming an average WBC count of 8,000 per microliter of blood, this gives a threshold of sensitivity of 4 parasites per microliter of blood. In nonimmune patients, symptomatic malaria can occur at lower parasite densities, and screening more fields (e.g., 200, 300, or even the whole smear) might be warranted, depending on the clinical context and the availability of laboratory personnel and time. NCCLS standards recommend examination of at least 300 fields using the 100× oil immersion objective.