تابع الفحص الميكروسكوبى

من طرف **المسلمانى** الجمعة أبريل 02, 2010 11:45 pm

Examining thin smears
Thin smears are useful for species identification of parasites already detected on thick smears, screening for parasites if adequate thick smears are not available, and a rapid screen while the thick smear is still drying.

Screen at low magnification (10× or 20× objective lens) if this has not been done on the thick smears.
Carefully examine the smear using the 100× oil immersion objective lens. NCCLS standards recommend examination of at least 300 fields using the 100× oil immersion objective.

Quantifying parasites
In some cases (especially malaria) quantification of parasites yields clinically useful information. If this information is needed by the physician, malaria parasites can be quantified against blood elements such as RBCs or WBCs.
To quantify malaria parasites against RBCs, count the parasitized RBCs among 500-2,000 RBCs on the thin smear and express the results as % parasitemia.
% parasitemia = (parasitized RBCs/total RBCs) × 100
If the parasitemia is high (e.g., > 10%) examine 500 RBCs; if it is low (e.g., <1%) examine 2,000 RBCs (or more); count asexual blood stage parasites and gametocytes separately. Only the former are clinically important and gametocytes of P. falciparum can persist after elimination of asexual stages by drug treatment.
To quantify malaria parasites against WBCs: on the thick smear, tally the parasites against WBCs, until you have counted 500 parasites or 1,000 WBCs, whichever comes first; express the results as parasites per microliter of blood, using the WBC count if known, or otherwise assuming 8,000 WBCs per microliter blood.
Parasites/microliter blood=(parasites/WBCs) × WBC count per microliter
Results in % parasitized RBCs and parasites per microliter blood can be interconverted if the WBC and RBC counts are known, or otherwise (less desirably) by assuming 8,000 WBCs and 4,000,000 RBCs per microliter blood