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Diagnosis of malaria involves identification of malaria parasite
or its antigens/products in the blood of the patient. Although this seems simple, the
efficacy of the diagnosis is subject to many factors. The different forms of the four
malaria species; the different stages of erythrocytic schizogony; the endemicity of
different species; the population movements; the inter-relation between the levels of
transmission, immunity, parasitemia, and the symptoms; the problems of recurrent malaria,
drug resistance, persisting viable or non-viable parasitemia, and sequestration of the
parasites in the deeper tissues; and the use of chemoprophylaxis or even presumptive
treatment on the basis of clinical diagnosis can all have a bearing on the identification
and interpretation of malaria parasitemia on a diagnostic test.
The diagnosis of malaria is confirmed by blood
tests and can be divided into microscopic and non-microscopic tests.
Microscopic Tests
For nearly a hundred years, the
direct microscopic visualization of the parasite on the thick and/or thin blood smears has
been the accepted method for the diagnosis of malaria in most settings, from the clinical
laboratory to the field surveys. The careful examination of a well-prepared and
well-stained blood film currently remains the "gold standard" for malaria
diagnosis.
The microscopic tests involve staining
and direct visualization of the parasite under the microscope.
1. Peripheral smear study
2. Quantitative Buffy Coat (QBC) test
Non-Microscopic Tests
Several attempts have been made to
take the malaria diagnosis out of the realm of the microscope and the microscopist. These
tests involve identification of the parasitic antigen or the antiplasmodial antibodies or
the parasitic metabolic products. Nucleic acid probes and immunofluorescence for the
detection of Plasmodia within the erythrocytes; gel diffusion,
counter-immunoelectrophoresis, radio immunoassay, and enzyme immunoassay for malaria
antigens in the body fluids; and hemagglutination test, indirect immunofluorescence,
enzyme immunoassay, immunochromatography, and Western blotting for anti-plasmodial
antibodies in the serum have all been developed. These tests have found some limited
applications in research, retrograde confirmation of malaria, investigation of cryptic
malaria, transfusion blood screening, and investigation of transfusion acquired
infections.
Rapid
Diagnostic Tests (RDTs)
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Para Sight F
test
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OptiMal Assay
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The immuno
chromatographic test (ICT Malaria P. f. test)
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Polymerase
Chain Reaction
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Detection
of antibodies by Radio immuno assay, immunofluorescence or enzyme immuno assay
The simplest and surest test is the
time-honoured peripheral smear study for malarial parasites. None of the other newer tests
have surpassed the 'gold standard' peripheral smear study.
Remember this:
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Ask for MP test in all cases of fever and
related symptoms and also whenever there is high level of suspicion.
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MP test can be done at
any time. Do not wait for typical symptoms and signs or for chills.
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A negative test DOES
NOT rule out malaria. Repeated tests may have to be done in all doubtful cases. Duration
of the illness, level of parasitemia, expertise of the technician and the method of
examination may all have a bearing on the result of the M.P. test.
Peripheral
smear study for malarial parasites - The MP test
Peripheral smear study for malarial
parasites is the gold standard in diagnosing malarial infection. It involves collection of
a blood smear, its staining with Romanowsky stains and examination of the Red Blood Cells
for intracellular malarial parasites.
The smear can be prepared
from blood collected by venipuncture, finger prick and ear lobe stab. In obstetric
practice, cord blood and placental impression smears can be used. In fatal cases,
post-mortem smears of cerebral grey matter obtained by needle necropsy through the foramen
magnum, superior orbital fissure, ethmoid sinus via the nose or through fontanelle in
young children can be used.
Sometimes no parasites
can be found in peripheral blood smears from patients with malaria, even in severe
infections. This may be explained by partial antimalarial treatment or by sequestration of
parasitised cells in deep vascular beds. In these cases, parasites, or malarial pigment
may be found in the bone marrow aspirates. Presence of malarial pigment in circulating
neutrophils and monocytes may also suggest the possibility of malaria.
Thick and thin smears are usually
prepared. Thick smears are used to identify the parasites and thin smears for identifying
the species.
Staining methods: 1. Giemsa 2.
Lieshman's 3. Jaswanth Singh Battacharya
Staining
methods - See details
An experienced technician
can detect as few as 5 parasites/µl in a thick film and 200/µl in a thin film.
Thick Film Examination |
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P.
vivax - trophozoites and gametocytes |
P.
falciparum - trophozoites and gametocytes |
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Thin Film Examination |
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P. vivax - trophozoites |
P. vivax - schizont |
P. vivax - gametocyte |
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P. falciparum - trophozoite |
P. falciparum - gametocyte |
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Other
Tests:
1. Quantitative
Buffy Coat (QBC) test
2. RDT - Para Sight F test
3. RDT - OptiMal Assay
4. RDT - The immuno chromatographic test (ICT Malaria P. f. test)
5. Polymerase Chain Reaction
6. Detection of antibodies by Radio immuno assay,
immunofluorescence or enzyme immuno assay
Missed parasites -
SEE Misreport
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