Rationale:

• Evaluation of Giemsa-stained thick and thin smears is the accepted standard for malaria diagnosis.
• P. falciparum parasites, the cause of the most deadly human malaria, sequester in the deep venous system and may not be apparent on initial blood smear.
• Parasite load is an important indicator of disease severity and must be monitored periodically to ensure adequate response to therapy.
• Detectable parasitemia often lags behind aches, fevers, and chills, sometimes for many days.
• Malaria is a reportable disease in the U.S.

Evidence:

• Microscopy remains the method of choice for parasitologic diagnosis of malaria worldwide. However, its accuracy varies depending on the expertise of the microscopist, parasite density, microscope quality, quality of reagents, and preparation of blood films (61; 62; 63; 64).
• Diverse technical problems, such as inappropriate staining solution pH, the presence of stain debris, and suboptimal application of blood to slides for thick smears, are some of the factors that may reduce accuracy of diagnosis (65).

Comments:

• Thick blood smears concentrate erythrocyte layers 30- to 50-fold and are used to screen a relatively large amount of blood for the presence of parasites. Because erythrocytes are lysed in the process of staining in the thick smear technique, parasites are visualized outside of erythrocytes. Parasitemias can be determined from thick smears by counting the number of parasites until 200 leukocytes have also been counted. This count, when multiplied by 40, gives an estimation of the number of parasites per microliter of blood (assuming an average leukocyte count of 8,000/µL). Before reading a thick blood smear as negative, at least 200 to 500 fields should be examined (some experts recommend examining a thick smear for 20 minutes). Blood smears should be examined at 1,000x with an oil immersion objective.
• Thin smears are prepared from a much smaller amount of blood and are used to determine the Plasmodium species that have distinct morphologies and other characteristics when visualized inside erythrocytes. Speciation of malaria parasites has important implications for treatment. Infections with P. falciparum are suggested by: thin delicate rings, absence of mature forms (e.g., trophozoites and schizonts), multiply infected erythrocytes, positioning of rings against the inner surface of the erythrocyte membrane (so-called accolade forms), and banana-shaped gametocytes. Plasmodium vivax can be distinguished by the presence of mature forms in the peripheral blood and spherically shaped gametocytes. By counting the number of infected cells in 10,000 total erythrocytes, an estimation of parasitemia on the slide can be determined. Bench aids that are useful in the diagnosis of malaria are available at the CDC's Laboratory Identification of Parasites of Public Health Concern.
• Although the level of parasitemia is frequently associated with severity of disease, this is not always the case. Persons with antimalarial immunity in endemic areas may have detectable parasitemias yet may be asymptomatic. Persons with no antimalarial immunity may have severe malaria in the absence of detectable parasites on blood smear.
• Babesia sp. produce intraerythrocytic forms that may be confused with Plasmodium sp., but experienced personnel are able to distinguish between them.
• The current list of Nationally Notifiable Infectious Diseases for the U.S. can be found at http://www.cdc.gov/epo/dphsi/phs/infdis2005.htm.