Reader technique as a source of variability in determining malaria parasite density by microscopy.
BACKGROUND
Accurate identification and quantification of malaria parasites are critical for measuring clinical trial outcomes. Positive and negative diagnosis is usually sufficient for the assessment of therapeutic outcome, but vaccine or prophylactic drug trials require measuring density of infection as a primary endpoint. Microscopy is the most established and widely-used technique for quantifying parasite densities in the blood.


METHODS
Results obtained by 24-27 expert malaria microscopists, who had independently read 895 slides from 35 donors, were analysed to understand how reader technique contributes to discrepancy in measurements of parasite density over a wide range of densities.


RESULTS
Among these 35 donations, standard deviations ranged from 30% to 250% of the mean parasite density and the percent discrepancy was inversely correlated with the mean parasite density. The number of white blood cells indexed and whether parasites were counted in the thick film or thin film were shown to significantly contribute to discrepancy amongst microscopists.


CONCLUSIONS
Errors in microscopy measurements are not widely appreciated or addressed but have serious consequences for efficacy trials, including possibly abandoning promising vaccine candidates.
O'Meara
Wendy Prudhomme
WP
Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA. prudhomw@mail.nih.gov
Barcus
Mazie
M
Wongsrichanalai
Chansuda
C
Muth
Sinuon
S
Maguire
Jason D
JD
Jordan
Robert G
RG
Prescott
William R
WR
McKenzie
F Ellis
FE
D000818:Animals; D004912:Erythrocytes / Q000469:parasitology; D006801:Humans; D007962:Leukocytes / Q000469:parasitology; D008288:Malaria / Q000097:blood / Q000469:parasitology*; D008859:Microscopy, Polarization / Q000379:methods* / Q000592:standards; D015588:Observer Variation; D010961:Plasmodium / Q000302:isolation & purification*