Examination of peripheral blood smears for malarial parasites may be requested in the patient with fever, especially if there is a history of overseas travel. Certain guidelines should be followed to ensure that an adequate evaluation has been done.
Parameter
Discussion
assess risk
determine if any risk factors exist; onset may be delayed months or years after exposure
history of prophylaxis
a history of prophylaxis does not exclude malaria due to compliance and resistance issues, but it may mean that the number of parasites may be low
number of specimens
a single examination does not exclude malaria, but may be sufficient for a low risk patient; a patient at risk should have several specimens examined (perhaps 4 over 48 hours)
timing of specimens
for patient at risk, one at presentation, then every 12 hours times 3 (at 12, 24 and 36 hours); specimens should be collected during a fever paroxysm, as parasites may be absent or scarce in interval specimens
adequate smears
thick smears are preferred for screening but may be problematic for the inexperienced; classic features of malarial parasites require proper staining at the correct pH
adequate examination
200 oil-immersion (1000x) fields for thick smears; 1,000 oil-immersion fields for thin smears
exclusion of artifacts
platelets and debris may mimic parasites
other parasites
Babesia may resemble the ring forms of P. falciparum
If a high level of clinical suspicion is held, then additional studies may be warranted:
(1) serology tests (may be negative in early disease; may be positive but unrelated to etiology for current fever)
(2) PCR (may not be available)
Thick smears require some skill to make:
(1) A capillary pipette can be used to apply the blood, going in concentric circles from the center of the slide outward.
(2) The smear should not be so thick that newsprint cannot be easily read.
(3) The smear should be well dried before staining.
(4) If large areas of the blood film come off during staining, then the smear was improperly prepared and should be done again.
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