Approach Considerations
In returning travelers from endemic areas, malaria is suggested by the triad of thrombocytopenia, elevated lactate dehydrogenase (LDH) levels, and atypical lymphocytes. These findings should prompt obtaining malarial smears.
In general, blood cultures should be drawn in a febrile patient. Patients from tropical areas may have more than 1 infection; maintaining a high suspicion for additional infections should be considered when patients do not respond to antimalarials.
Assess hemoglobin (decreased in 25% of patients, often profoundly in young children), platelet counts (thrombocytopenia in 50-68% of patients), and liver function (results abnormal in 50% of patients). Also monitor renal function, electrolytes (especially sodium), and parameters suggestive of hemolysis (haptoglobin, LDH, reticulocyte count). Rapid HIV testing may also be indicated in select cases. Importantly, fewer than 5% of patients with malaria have an elevated white blood cell (WBC) count. If leukocytosis is present, the examiner should entertain a broader list of differential diagnoses. The British Committee for Standards in Haematology has guidelines on the laboratory diagnosis of malaria. [10]
If the patient is to be treated with primaquine, a G-6-PD level should be obtained because primaquine can result in severe hemolysis in these patients.
If the patient has cerebral malaria, obtain a blood glucose level to rule out hypoglycemia as a cause of mental-status changes. Note that intravenous (IV) quinine can induce hypoglycemia; therefore, blood glucose should be monitored when IV quinine is used.
The British Committee for Standards in Haematology revised its Guidelines for the Laboratory Diagnosis of Malaria, intended for use in the United Kingdom but also potentially applicable to other nonendemic areas. [11] Recommendations include the following:
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Thick and thin films should be routinely used for malaria diagnosis; thick films should be stained with Giemsa or Field stain, thin films with Giemsa or Leishman stain
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Two observers should examine thick films, with each viewing a minimum of 200 high-power fields; if the films are positive, the species should be determined through examination of a thin film
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In cases of P falciparum or Plasmodium knowlesi infection, the percentage of parasitized cells or the number of parasites per microliter should be estimated
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Rapid diagnostic tests for malarial antigen can be used on a supplementary basis when diagnosis is performed by inexperienced staff
Imaging studies
Chest radiography may be helpful if respiratory symptoms are present. If CNS symptoms are present, a computed tomography (CT) scan of the head may be obtained to evaluate evidence of cerebral edema or hemorrhage.
Microhematocrit centrifugation
Using this method with the CBC tube is a more sensitive method of detection of malaria infection. However, microhematocrit centrifugation does not allow the identification of the species of Plasmodium. To determine species, a peripheral blood smear must be examined.
Fluorescent dyes/ultraviolet indicator tests
Several different dyes allow laboratory results to be obtained more quickly. These methods require the use of a fluorescent microscope. Fluorescent /ultraviolet tests may not yield speciation information.
Polymerase chain reaction assay
PCR assay testing is a very specific and sensitive means of determining if species of Plasmodium are present in the blood of an infected individual. PCR assay tests are not available in most clinical situations. However, they are very effective at detecting the Plasmodium species in patients with parasitemias as low as 10 parasites/mL of blood.
Lumbar puncture
If the patient exhibits mental-status changes, and even if the peripheral smear demonstrates P falciparum, a lumbar puncture should be performed to rule out bacterial meningitis.
Blood Smears
A diagnosis of malaria should be supported by the identification of the parasites on a thin or thick blood smear. In rare occasions, P falciparum infection can present without detectable parasitemia. If no alternative diagnosis is found in an at-risk patient with possible cerebral malaria (ie, unrevealing lumbar puncture findings), initiate therapy for presumptive malaria and continue to obtain additional blood smears to confirm the diagnosis.
When reading a smear, 200-300 oil-immersion fields should be examined (more if the patient recently has taken prophylactic medication, because this temporarily may decrease parasitemia). One negative smear does not exclude malaria as a diagnosis; several more smears should be examined over a 36-hour period.
Thick smears
Three thick and thin smears 12-24 hours apart should be obtained. The highest yield of peripheral parasites occurs during or soon after a fever spike; however, smears should not be delayed while awaiting fever spikes.
Thick smears are 20 times more sensitive than thin smears, but speciation may be more difficult. The parasitemia can be calculated based on the number of infected RBCs. This is a quantitative test.
Thin smears
Thin smears are less sensitive than thick smears, but they allow identification of the different species. This should be considered a qualitative test.
Alternatives to Blood Smear Testing
Alternative diagnostic methods typically are used if the laboratory does not have sufficient expertise in detecting parasites in blood smears.
Rapid diagnostic tests (RDT)
Immunochromatographic tests based on antibody to histidine-rich protein-2 (PfHRP2), parasite LDH (pLDH), or Plasmodium aldolase appear to be very sensitive and specific. [12, 13] Some RDTs may be able to detect P falciparum in parasitemias that are below the threshold of reliable microscopic species identification. Only one RDT (BinaxNOW) has been approved to date for the diagnosis of malaria in the United State. [14]
In one study, RDTs were found to perform better than microscopy under routine conditions. RDTs performed by the health facility staff were 91.7% sensitive and 96.7% specific. Microscopy was 52.5% sensitive and 77% specific. [15] A recent sudy using loop-mediated amplification technique (LAMP)also suggests that RDTs have accuracy similar to that of nested PCR, with a greatly reduced time to result, and was superior to expert microscopy. [16]
In a study from Tanzania, d'Acremont et al reported that antimalarials could be safely withheld from febrile children (< 5 y) who had negative results from an RDT based on PfHRP2. [17]
RDTs are less effective when parasite levels are below 100 parasites/mL of blood, and, in rare instances, an RDT test is negative in patients with high parasitemias. For these reasons, confirm RDT test results with a second type of screening test when possible. A false-positive result from an RDT may occur up to 2 weeks or more after treatment due to persistence of circulating antigens.
Other tests
In addition to the RDT listed above, new molecular techniques, such as PCR assay testing and nucleic acid sequence-based amplification (NASBA), are also available for diagnosis. They are more sensitive than thick smears but are expensive and unavailable in most developing countries. [18]
The quantitative buffy coat (QBC) is a technique that is as sensitive as thick smears. Because results cannot be used to speciate Plasmodium, a thin smear must be examined.
Malaria is a reportable disease. Identification of parasites by any of the above techniques should prompt notification to the local or state health department.
Histologic Findings
The table below compares histologic findings for P falciparum, P vivax, P ovale, and P malariae.
Table 1. Histologic Variations Among Plasmodium Species (Open Table in a new window)
Findings |
P falciparum |
P vivax |
P ovale |
P malariae |
Only early forms present in peripheral blood |
Yes |
No |
No |
No |
Multiply-infected RBCs |
Often |
Occasionally |
Rare |
Rare |
Age of infected RBCs |
RBCs of all ages |
Young RBCs |
Young RBCs |
Old RBCs |
Schüffner dots |
No |
Yes |
Yes |
No |
Other features |
Cells have thin cytoplasm, 1 or 2 chromatin dots, and applique forms. |
Late trophozoites develop pleomorphic cytoplasm. |
Infected RBCs become oval, with tufted edges. |
Bandlike trophozoites are distinctive. |
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Malarial merozoites in the peripheral blood. Note that several of the merozoites have penetrated the erythrocyte membrane and entered the cell.
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This micrograph illustrates the trophozoite form, or immature-ring form, of the malarial parasite within peripheral erythrocytes. Red blood cells infected with trophozoites do not produce sequestrins and, therefore, are able to pass through the spleen.
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An erythrocyte filled with merozoites, which soon will rupture the cell and attempt to infect other red blood cells. Notice the darkened central portion of the cell; this is hemozoin, or malaria pigment, which is a paracrystalline precipitate formed when heme polymerase reacts with the potentially toxic heme stored within the erythrocyte. When treated with chloroquine, the enzyme heme polymerase is inhibited, leading to the heme-induced demise of non–chloroquine-resistant merozoites.
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A mature schizont within an erythrocyte. These red blood cells (RBCs) are sequestered in the spleen when malaria proteins, called sequestrins, on the RBC surface bind to endothelial cells within that organ. Sequestrins are only on the surfaces of erythrocytes that contain the schizont form of the parasite.
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Schema of the life cycle of malaria. Image courtesy of the Centers for Disease Control and Prevention.
Tables
Findings |
P falciparum |
P vivax |
P ovale |
P malariae |
Only early forms present in peripheral blood |
Yes |
No |
No |
No |
Multiply-infected RBCs |
Often |
Occasionally |
Rare |
Rare |
Age of infected RBCs |
RBCs of all ages |
Young RBCs |
Young RBCs |
Old RBCs |
Schüffner dots |
No |
Yes |
Yes |
No |
Other features |
Cells have thin cytoplasm, 1 or 2 chromatin dots, and applique forms. |
Late trophozoites develop pleomorphic cytoplasm. |
Infected RBCs become oval, with tufted edges. |
Bandlike trophozoites are distinctive. |
