Rapid diagnosis of malaria by use of fluorescent probes

Malaria diagnosis relies on observation of parasites in blood smears and the Giemsa-stained thick blood smear (G-TS) is the reference test. Diagnosis by G-TS in low-density infections requires long periods of observation and experienced microscopists. Examination of Giemsa-stained thin smears enables more reliable differentiation of species but may miss low-grade infections. Fluorescent stains may offer an alternative technique. We compared the Giemsa technique with 4,6-diamidine-2-phenilindolo-propidium iodide (DAPI-PI) stainings in order to evaluate the time required for diagnosis. A Plasmodium falciparum-infected blood specimen was diluted to obtain concentrations ranging from 6192 to 24 parasites/microliters (p/microliter), and thin and thick smears were stained with the two methods. The DAPI-PI proved useful: parasites were easily recognized and their morphology was preserved in thin and thick smears. The method allowed more rapid evaluation of thin smears as compared with Giemsa staining and enabled recognition of parasites in case of low-level parasitemias. The DAPI-PI staining technique may acquire an important role in malaria diagnosis, especially in nonendemic countries where technicians are not experienced with G-TS; in developing countries, it could be used in epidemiologic surveys of populations with low-density parasitemias, for which it enables a fast examination of smears and possibly the identification of parasite species.