The most important role for malaria diagnostics is to help health care workers, whether they be in a village in Mali or in a sophisticated hospital in New York City, select the most appropriate treatment for patients whose illness may be due to infection with malaria parasites. Since the symptoms of malaria vary and can resemble those of other diseases, diagnosing malaria solely on the basis of clinical symptoms is unreliable. If evidence of malaria is found, particularly in nonimmune individuals, rapid and appropriate therapy is essential to prevent further progression of the disease. Alternatively, if parasites are absent, other explanations for the symptoms must be sought so that appropriate treatment can be started and the use of toxic antimalarial drugs can be avoided.
The simple microscopic diagnostic tests available today have two drawbacks, even when performed correctly. In patients who live in malarious areas and who are partially immune to the disease, malaria infections may be asymptomatic and of little clinical significance. The presence of parasites in such patients cannot be assumed to cause the symptoms of an illness, which may have other causes. There is currently no diagnostic to associate malaria infection with disease in such patients. In addition, repeated blood films may be necessary to detect malaria parasites in nonimmune patients, in whom symptoms can arise from very low parasitemias. Failure to detect parasites in a single blood film from such a patient (for instance, an American recently returned from a malarious area) cannot be used to exclude a diagnosis of malaria.
Diagnostic tests are also important for certain types of epidemiologic surveys. Used for this purpose, rapidity and pinpoint accuracy are not as critical as the need for the safe collection, preparation, and evaluation of a large number of samples. Because of the danger of transmitting other diseases, such as AIDS or hepatitis B, through the use of contaminated lancets, the collection of blood samples raises significant biosafety issues. In addition, the transportation and examination of blood slides for epidemiologic surveys can be both cumbersome and logistically difficult, particularly in remote areas. In many countries, there is a backlog of slides to be examined, and results may not be available for many months. The relevance of out-of-date results to the planning and evaluation of malaria field operations is questionable.
Currently, the “gold standard” for diagnosing malaria in individual patients and for epidemiologic surveys is the microscopic examination of blood smears. The presence of malaria parasites, identified by their characteristic morphology, is considered definitive proof of infection. A number of potential improvements to current microscopic methods and alternatives to microscopy for diagnosing malaria are discussed below.