title: Sources of variability in determining malaria parasite density by microscopy.
abstract: Enumeration of parasites by microscopic examination of blood smears is the only method available for quantifying parasitemia in infected blood. However, the sources and scale of error inherent in this technique have not been systematically investigated. Here we use data collected in outpatient clinics in Peru and Thailand to elucidate important sources of variation in parasite density measurements. We show that discrepancies between readings from two independent microscopists and multiple readings from a single microscopist are inversely related to the density of the infection. We present an example of how differences in reader technique, specifically the number of white blood cells counted, can contribute to the differences between readings. We discuss the implications of this analysis for field studies and clinical trials.
authors.affiliation: Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA. prudhomw@mail.nih.gov
authors.forename: Wendy Prudhomme
authors.initials: WP
authors.lastname: O'Meara
authors.forename: F Ellis
authors.initials: FE
authors.lastname: McKenzie
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Magill
authors.forename: J Russ
authors.initials: JR
authors.lastname: Forney
authors.forename: Barnyen
authors.initials: B
authors.lastname: Permpanich
authors.forename: Carmen
authors.initials: C
authors.lastname: Lucas
authors.forename: Robert A
authors.initials: RA
authors.lastname: Gasser
authors.forename: Chansuda
authors.initials: C
authors.lastname: Wongsrichanalai
mesh_terms: D000818:Animals; D006801:Humans; D007962:Leukocytes / Q000469:parasitology; D016778:Malaria, Falciparum / Q000175:diagnosis* / Q000453:epidemiology*; D016780:Malaria, Vivax / Q000175:diagnosis* / Q000453:epidemiology*; D008853:Microscopy / Q000379:methods*; D015588:Observer Variation; D010568:Peru; D013785:Thailand