Pediatric Malaria Workup

Demonstration of the parasite in a smear of the blood definitely establishes the presence of malaria. A negative finding on examination does not rule out malaria. Only 50% of children with malaria have positive smear findings, even on repeated examination.

Asymptomatic parasitemia is common in children from an endemic area who have partial immunity to the disease. A positive smear for malaria parasite does not always confirm a diagnosis of malaria in these children. More importantly, a positive smear should not stop the diagnostic effort in a febrile illness.

Lumbar puncture is indicated to rule out meningitis in cerebral malaria and febrile seizures with malaria. Severe P falciparum malaria is often associated with hypoglycemia. Lower blood glucose levels are associated with higher mortality rates.

Serologic tests provide confirmation of past malaria in patients and are valuable for epidemiologic studies. These tests are also useful for screening donated blood and diagnosing hyperactive malarial splenomegaly. Among the tests used are:

• Indirect fluorescent antibody test (IFAT)

• Indirect hemagglutination antibody (IHA) test

• Enzyme-linked immunosorbent assay (ELISA)

• Immuno-chromatographic test (ICT) for filariasis

All of these tests produce positive results several days after malarial parasites appear in the blood and so do not help in the diagnosis of the acute infection for treatment purposes.

Dipstick tests based on the detection of P falciparum histidine-rich protein-2 (PfHRP-2) antigen are specific for P falciparum infections and do not detect the other 3 species. Dipstick tests based on the detection of parasite lactate dehydrogenase are now available; these tests can detect P falciparum and P vivax. These tests have high sensitivity and specificity, require no special equipment or training, and produce results rapidly. They offer an advantage where the disease is uncommon and blood smear examination expertise is not readily available. However, they remain positive for a week or more after the treatment and cure and, in this situation, can yield false-positive results.

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) probes and polymerase chain reaction (PCR) assays have good sensitivity and specificity but require sophisticated expensive equipment.

Both thick and thin films are essential. If the parasitemia is light, a thin film examination may miss the diagnosis. Thick films save time in diagnosis of scanty infections but make species identification of the parasite difficult. At least 100-200 fields of a thick film should be scrutinized before a slide is reported as negative for malaria. In doubtful cases, the examination can be repeated after 4 hours.

Obtaining thick and thin blood films at the bedside is important. These films may have to be taken repeatedly for diagnosis. Earlobe or finger prick is used for older children; the great toe is used in infants.

Take a large drop of blood on one end of a clean slide, spread uniformly, and air-dry. A smaller drop should be spread thinly so that the end of the smear does not reach the end of the slide. Giemsa staining is suitable for identifying malarial parasites in these films.

Various techniques to enhance the diagnostic value of the peripheral blood smear examination are in use. Fluorescent staining and microscopy, centrifugation, selective magnetic separation techniques, and other techniques have been used but have only a moderate effect.

Histologic findings

The appearance of the parasite varies in the thick and thin films. The thick unfixed film shows only leucocytes and parasites; erythrocytes are destroyed in the staining process. The parasites themselves are also altered.

Young trophozoites appear as incomplete rings or spots of blue cytoplasm with discrete red nuclei. In mature trophozoites, the cytoplasm may be fragmented, and the various characteristics of the different species are often indistinct.

Gametocytes and schizonts usually retain their characteristic appearances. Thin film examination is essential for the accurate identification of plasmodial species, which has an important bearing on treatment.