Malarial parasites can be demonstrated in stained smears of the peripheral blood in virtu-ally all symptomatic patients. Typically, capillary or venous blood is used to prepare both thin and thick smears, which are stained with Wright or Giemsa stain and examined for the presence of erythrocytic parasites. Thick smears, in which erythrocytes are lysed with water before staining, concentrate the parasites and allow detection of very mild para-sitemia. Nonetheless, it may be necessary to obtain several specimens before parasites are seen. Artifacts are numerous in thick smears, and correct interpretation requires experi-ence. The morphologic differences among the four species of plasmodia allow their speci-ation on the stained smear by the skilled observer.
A number of attempts have been made to improve on the standard thin and thick smear. One such procedure involves acridine orange staining of centrifuged parasites in quantitative buffy coat (QBC) tubes. Although it is expensive, requires a fluorescence mi-croscope, and permits less reliable parasite speciation, its rapidity and ease of use make it attractive to laboratories that are only occasionally called on to identify patients with malaria. Simple, specific card antigen detection procedures are now available. The most widely used test, ParaSight F, detects a protein (HRP2) excreted by P. falciparum within minutes. The test can be performed under field conditions and has a sensitivity more than 95%. A second rapid test, OptiMAL, detects parasite lactate dehydrogenase, and, unlike ParaSight F, can distinguish between P. falciparum and P. vivax. Serologic tests for malaria are offered at a few large reference laboratories but are used primarily for epi-demiologic purposes. They are occasionally helpful in speciation and detection of other-wise occult infections. The recently completed sequencing of the malaria genome will lead to newer diagnostic methods.
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