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SOURCES OF VARIABILITY IN DETERMINING MALARIA PARASITE DENSITY BY MICROSCOPY

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.

Received February 25, 2005. Accepted for publication April 18, 2005.

Acknowledgments: The authors thank Dr. J. Sirichaisinthop for his contributions to this work. We also thank the Armed Forces Research Institute of Medical Sciences and the Naval Medical Research Center Detachment field study teams for their dedication and technical assistance.

Financial support: This work was supported by the United States Army Medical Research and Materiel Command.

Disclaimer: The views presented are those of the authors, and do not necessarily reflect the official policy or position of the U.S. Departments of the Navy or Army, the U.S. Department of Defense, the U.S. Government, or any other organization listed.

^* Address correspondence to Wendy Prudhomme O’Meara, Fogarty International Center, National Institutes of Health, Bethesda, MD 20892. E-mail: prudhomw{at}mail.nih.gov

Authors’ addresses: Wendy Prudhomme O’Meara and F. Ellis Mc-Kenzie, Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, E-mails: prudhomw{at}mail.nih.gov and mckenzel{at}mail.nih.gov. Alan J. Magill, J. Russ Forney, and Robert A. Gasser, Jr., Walter Reed Army Institute of Research, Silver Spring, MD 20910, E-mails: Alan.magill{at}na.amedd.army.mil, John.Forney{at}usma.edu, and rgasser{at}erols.com Barnyen Permpanich, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand. Carmen Lucas, United States Naval Medical Center Detachment, Lima, Peru, E-mail: clucas{at}nmrcd.med.naqvy.mil. Chansuda Wongsrichanalai, United States Naval Medical Research Unit No. 2, Jakarta, Indonesia, E-mail: chansuda{at}namru2.org.

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