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.
