Thick and Thin Blood Smear

The gold standard of malaria diagnosis is a parasitic diagnosis based on finding diagnostic morphologic forms by microscopic examination of thick and thin blood smears prepared with a patient’s blood. Presence of parasitized RBCs in the circulation is referred to as parasitemia; the percentage of RBCs involved is used to gauge the severity of the malaria infection and is expressed as percent parasitemia.

The morphology of the ring trophozoites, mature trophozoites, schizonts, and gametocytes is used to differentiate the various malaria species when blood smears stained with Giemsa are examined microscopically, and up to three blood samples taken 6 to 8 hours apart should be examined by an experienced microscopist (Figure 63-4). The gametocytes of P falciparum have a distinctive banana-shaped appearance on blood smears and may be contained within RBCs or lie outside of the cells.

On blood smears from infected humans, early ring trophozoite forms of the recently recognized P knowlesi may resemble those of P. falciparum, whereas mature forms of P knowlesi trophozoites, schizonts, and gametocytes can be mistaken for those of P malariae. An atypical appearance of the Plasmodium species seen in the blood smears and a higher than expected level of parasitemia than is usual for P malariae should raise suspicions of P knowlesi if the patient has traveled in Asia. A definitive diagnosis may not be possible from light microscopy, and samples of the patient’s blood may have to be sent to a reference laboratory for species confirmation by molecular testing, such as polymerase chain reaction (PCR) amplification and microsatellite analysis.

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