Thick drop
As thicknesses drops (engl. Thick blood film ) is referred to a of Ronald Ross introduced 1903 microscopic method for the detection of pathogens in the blood. Because of its ease of implementation even under simple tropical conditions and its good diagnostic informative value, the "thick drop" quickly found widespread use and is still considered the standard examination for many diseases today.
It is mainly used in the diagnosis of parasitic infectious diseases such as malaria , Chagas disease , sleeping sickness , filariasis , and bacterial infections with spirochetes such as relapsing fever .
In contrast to the normal blood smear (sometimes referred to as "thin drop"), about 5–10 µl of native blood from veins or capillaries is dried on a microscope slide as a spot about one centimeter in diameter, causing the pathogens on the observed area to be about 10 - can be enriched up to 20 times. After allowing it to dry, the thick drop is treated unfixed with a coloring solution (e.g. Giemsa solution). Due to the lack of fixation, the staining solution leads to a lysis (dissolution) of the red blood cells, while the parasite structures contained therein are retained and become visible through the staining under the microscope.
To determine the species of malaria parasites, the normal blood smear is better than the thick drop, as typical findings such as Maurer's stain or Schüffner's stippling are hardly recognizable in the thick drop.
The thick drop should not be confused with the " hanging drop ". This remains liquid, while the thick drop has to dry up.