Thick and Thin Smears for Microscopy detection of Malarial Parasites
Background Information of Thick and Thin Smears
Traditionally the "GOLD-STANDARD" for laboratory diagnosis of malaria has been based on the microscopic examination of a well stained blood film. Both thick and thin peripheral blood films are used for detection and species identification.
[A] Thin Films
Species identification is more apparent on the thin film because red cell morphology is better preserved than in the thick film. This enables the observer to confirm the diagnosis and identify the species of parasite.
Making Thin Films
- Transfer a small drop of blood about 1.0 cm from the end of a clean glass slide.
- Place the slide on a flat surface and hold the long edges between your thumb and forefinger.
- Using your other hand, place the narrow end of a second (spreader) slide on the specimen slide, just in front of the drop of blood. Hold the spreader at a 300 angle and draw it back until it just touches the drop of blood. Allow the blood to run almost to the edges of the spreader.
- Push the spreader forward with one light, smooth motion. A thin film of blood in the shape of a bullet or tongue will be formed.
[B] Thick Films
Thick films are extremely useful when parasites are scanty but identification of species may be more difficult than in thin films, especially for P. ovale which is indistinguishable from P. vivax. Thick films should give 10 to 20 times the concentration of thin blood films. Mixed infections may be missed using thick films as during the process of staining the red cells are completely lysed and parasites may be distorted thus making identification of the species much more difficult. Thick films, if well stained, may also be used to perform the differential white cell count in cases with severe leucopenia.
Making Thick Films
- Place a drop of blood in the center of a microscope slide and, using the corner of another slide, spread it in a circular manner to cover an area approximately two centimeters in diameter. The correct thickness has been achieved if, when placed over a wristwatch, the figures on the dial are just visible.
- Allow thick films to air-dry for at least one hour at room temperature or thirty minutes at 370C before staining. If films are not completely dry, or are too thick, they may be lost during the staining process.
Routine Staining procedures using Giemsa Stain