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Overview :
Greenwood
and Armstrong listed several methods for determining the density of malaria
parasites in the peripheral blood. The authors are from the Medical Research
Council (MRC) Laboratories in Banjul,
The Gambia.
Methods:
(1) based on
erythrocyte count: This is tedious but considered the gold standard method.
(2) based on
estimate from thick blood smears: This is quick but less accurate.
Erythrocyte method:
(1) Count the
patient's erythrocyte count in an automated analyzer.
(2) Prepare thin
smears of the peripheral blood and examine under oil.
(3) The number of
intra-erythrocytic parasites are counted in 100,000 RBCs (this would be
approximately 360-450 oil immersion microscopic fields, which might take up to
an hour to complete).
malaria parasite density in number per µL =
= (number of parasites per 100,000 RBCs) / (100,000) *
(erythrocyte count per µL)
Thick smear method:
(1) Prepare thick
smears of the peripheral blood.
(2) Examine the
thick smears under oil (10x ocular, 100x objective).
(3) The average
number of parasites per oil immersion microscopic field is calculated.
(3a) If the number
of parasites is low (< 1 per field), then 100 fields are counted.
(3b) If the number
of parasites is >= 1 per field, then 10-50 fields are counted (10 fields is
sufficient if the parasite density is very high).
(4) The authors
estimate that 5-8 µL of blood is used to make a thick smear, and one oil
immersion field represents 0.002 µL of blood if a Leitz Laborlux microscope is
used.
malaria parasite density in number per µL =
= (average number of parasites per oil immersion microscopic
field) * 500
Limitations:
• The erythrocyte
count method is very tedious.
• The thick smear
estimate would be affected by the microscopic field area, which varies between
microscopes.
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