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Overview :
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.
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Parameter |
Discussion |
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assess risk |
determine if any risk factors exist; onset may be delayed
months or years after exposure |
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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 |
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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) |
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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 |
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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 |
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adequate examination |
200 oil-immersion (1000x) fields for thick smears; 1,000
oil-immersion fields for thin smears |
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exclusion of artifacts |
platelets and debris may mimic parasites |
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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:
• serology
tests (may be negative in early disease; may be positive but unrelated to
etiology for current fever)
• PCR (not
widely available)
Thick smears
require some skill to make:
• A capillary
pipette can be used to apply the blood, going in concentric circles from the
center of the slide outward.
• The smear
should not be so thick that newsprint cannot be easily read.
• The smear
should be well dried before staining.
• 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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