Parasitological diagnosis of malaria is based on the detection of asexual and sexual forms of the pathogen in a microscopic study of blood, which is possible only during its development in the red blood cell. To detect plasmodia and determine their type, blood preparations prepared by the method of "thin smear" and "thick drop", painted according to Romanovsky-Giemsa, are used. Both methods, which have their advantages and disadvantages, are complementary.
Detection of any stages of plasmodia (even 1 parasite) developing in erythrocytes (trophozoites - young and adults, schizonts - immature and mature, as well as sexual forms of gametocytes - male and female) in the blood smear or thick drop is the only indisputable proof of the presence of malaria. It should be borne in mind that the volume of blood in a thick drop is 20-40 times higher than in a thin smear, so a positive response can be given even after smear examination, and negative - only after the study of a thick drop with an immersion objective for at least 5 min , with viewing at least 100 fields of vision (WHO standard).
The sensitivity of the "thick drop" method is such that when viewing 100-150 fields of vision, approximately 8 parasites per 1 μl of blood can be detected. Care must be taken to detect a single formation resembling ring-shaped trophozoite in a thick drop, since the appearance of this stage of the parasite can be simulated by various artifacts. If it is not possible to detect plasmodia in the presence of a suspected malaria in a single study, it is sometimes necessary to conduct multiple studies (in tropical malaria, blood smears should be taken every 6 hours throughout the attack).