Abstract
Malaria is a global health problem responsible for nearly one million deaths every year around 85% of which concern children younger than five years old in Sub-Saharan Africa. In addition, around million clinical cases are declared every year. The level of infection, expressed as parasite density, is classically defined as the number of asexual parasites relative to a microliter of blood. Microscopy of Giemsa-stained thick blood films is the gold standard for parasite enumeration. Parasite density estimation methods usually involve threshold values; either the number of white blood cells counted or the number of high power fields read. However, the statistical properties of parasite density estimators generated by these methods have largely been overlooked. Here, we studied the statistical properties (mean error, coefficient of variation, false negative rates) of parasite density estimators of commonly used threshold-based counting techniques depending on variable threshold values. We also assessed the influence of the thresholds on the cost-effectiveness of parasite density estimation methods. In addition, we gave more insights on the behavior of measurement errors according to varying threshold values, and on what should be the optimal threshold values that minimize this variability.
Highlights
Malaria is caused by one or a combination of four species of Plasmodia: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale
Parasitemia was categorized as either low (PDƒ100 parasites/ml), intermediate (100vPDv10,000 parasites/ml) or high (PD§10,000 parasites/ml)
To the best of our knowledge, this is the first study of thresholdbased counting technique performance using the theoretical properties of parasite density (PD) estimators
Summary
Malaria is caused by one or a combination of four species of Plasmodia: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale. P. falciparum is the Plasmodium species responsible for 85% of the malaria cases and causes the most severe form of the disease. The thin smear is airdried for 10 minutes After drying, it is fixed in methanol and stained with Giemsa. The thick smear is dried for 30 minutes, but not fixed with methanol It is dehemoglobinized in water, and stained with Giemsa [7]. Thick blood smears are most useful for detecting the presence of parasites because they allow examining a larger sample of blood [8,9]. Thick films are more sensitive than thin films in case of low levels of infection and are used to detect infection, and to estimate parasite density (PD) [1,10,11]. The level of infection, expressed as parasite density is classically defined as the number of asexual forms of parasite relative to a blood volume (e.g. microliter) or a percentage of white blood cells (WBCs)
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