DIAGNOSIS
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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