infectious diseases
Malaria
Diagnosis of Malaria
Light microscopy of blood (thin and thick smears) Rapid diagnostic tests that detectPlasmodiumantigens or enzymes in blood Fever and chills in an immigrant or traveler returning from an endemic region should prompt immediate assessment for malaria. Symptoms usually appear in the first 6 months after infection, but onset may take up to 2 years or, rarely, longer. Malaria can be diagnosed by finding parasites on microscopic examination of thick or thin blood smears. The infecting species (which determines therapy and prognosis) is identified by characteristic features on smears (see table). If the initial blood smear is negative, additional smears should be repeated at 12- to 24-hour intervals until 3 smears are negative. Thin blood smears stained with Wright-Giemsa stain allow assessment of parasite morphology within red blood cells (RBCs), often speciation, and determination of percentage parasitemia (parasite density), evaluated using oil immersion magnification of portions of the smear where RBCs are more or less touching, which should show about 400 RBCs per field. Thick smears are more sensitive but more difficult to prepare and interpret as the RBCs are lysed before staining. Sensitivity and accuracy of the results depend on the examiner's experience. Commercial rapid diagnostic tests for malaria are based on the presence of certain plasmodium antigens or enzymatic activities. Assays may involve detection of a histidine-rich protein 2 (HRP-2) associated with malaria parasites (especiallyP. falciparum) and detection of plasmodium-associated lactate dehydrogenase (pLDH). The rapid diagnostic tests are generally comparable in sensitivity to microscopy in detecting low levels of parasitemia, but they do not differentiate single infection from concurrent infection with more than onePlasmodiumspecies or allow speciation except forP. falciparum. Light microscopy and rapid diagnostic tests are complementary, and both should be done when available. They have similar sensitivity. Negative results even in both does not exclude malaria in a patient with low parasitemia. Polymerase chain reaction and species-specific DNA probes can be used but are not widely available at the point of care. They can help identify the infectingPlasmodiumspecies after malaria is diagnosed. Because serologic tests may reflect prior exposure, they are not useful in the diagnosis of acute malaria. Severe malariais defined by the presence of one of more of the following clinical and laboratory features. Severe malaria tends to result fromP. falciparum. Clinical criteria for severe malaria: Acute respiratory distress syndrome/pulmonary edema Bleeding Coma or impaired consciousness Jaundice Seizures (recurrent) Shock Laboratory criteria for severe malaria: Anemia (severe: < 7 g/dL [70 g/L]) Disseminated Intravascular Coagulation (DIC) (DIC) Hemoglobinuria Metabolic acidosis Parasite density > 5% Renal failure 