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New method for early malaria recognition

In cooperation with the Charité Clinic for Infectiology (Berlin), a method based on laser flow cytometry has been developed to support malaria diagnostics. Owing to the detection of rare white blood cells (monocytes) modified by the infection, it has for the first time been possible to recognize malaria in non-immune patients with high specificity and sensitivity. The method is suitable for screening to detect malaria.

(Fig. 1): Scatter plot (about 250000 white blood cells) of a blood sample of a malaria patient. Each dot represents at least one white blood cell (Gn = neutrophilic, Ge = eosinophilic granulocytes). In this scatter plot the pigment carrying monocytes occur in the region denoted as PCM. Their identification was realized by sorting and subsequent microscopic analysis.

With about 2,5 millioncasese of death per year, malaria is one of the most dangerous infectious diseases worldwide. In the industrialized countries, too, there are fatalities which might be prevented if the infection was recognized as early as possible. The standard method of malaria diagnostics is microscopy which allows the four different kinds of parasites to be distinguished and the concentration of the parasitized red blood cells to be determined. Such examinations are, however, carried out only if an infection is clinically suspected, i. e. establishing of a diagnosis for persons returning from a journey to endemic regions might be delayed.

To check the suitability of flow cytometry as a screening tool for malaria recognition, the concentration of monocytes containing malaria pigment (PCM) was measured. After incorporation of the birefringent malaria pigment haemozoin, a degradation productof the parasites, these cells are distinguished by their high intensity of depolarized orthogonal light.

The flow cytometric method investigated by the PTB together with their medical partner allows malaria to be recognized with a specificity of 100 % a sensitivity of 86 % (100 %) for non-immune (semi-immune) patients was achieved.

(Fig. 2): For three cohorts (patients, volunteers) the normalized cumulative frequency is plotted as a function of the relative concentration of the cells in the PCM region. The open symbols stand for malaria negative persons and closed circles (triangles) for non-immune (semi-immune) malaria patients. The threshold of 3,3 x 10-5 which corresponding to the vertical line represents the biological background, i. e. events in the PCM region are recorded even for malaria negative persons. Generally, non-immune patients are persons having returned from a journey, while semi-immune patients live in epidemic regions and have become partially immune because they are in permanent contact with the parasites. In this group of patients, the characteristic symptoms of a malaria infection can be observed at a higher concentration of the PCM.

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