The standard tool for diagnosis of malaria is a smear examination. Rapid diagnostic testing should be used if microscopy is not available. The preparation of smears consists of applying a drop of blood to a glass microscopy slide, followed by drying and staining. There are two types of blood smears: thin and thick smears. Smears are generally stained with the Wright/Giemsa stain.
Thin smears maintain the integrity and morphology of red blood cells so that the parasites are visible within the cells. They allow for the identification of the infecting parasite species and can be used to measure parasite density. Thin smears are made using the same technique as performed in the hematology lab for Wright stained smears.
Thick smear preparation involves mechanical lysis of RBCs so that malarial parasites can be visualized independently of cell structures. It is typically used as a screen for the presence or absence of parasites.
Parasites are best seen under 1000X magnification using an oil immersion objective lens. It is necessary to screen at least 200 to 500 fields or examination for 20 to 30 minutes. If malaria is suspected but the initial smear is negative, then additional smears should be prepared and examined over 48 to 72 hours. CDC recommendations are to repeat a thick and thin smear every 12 to 24 hours for a total of three sets before ruling out malaria.
