Preparation of Blood Smears
Introduction of Preparation of Blood Smears
Preparation of Blood Smears are made by placing a drop of blood on a clean glass slide and spreading it with another glass slide at an angle. The slide is then stained and microscopically examined.
Thick Blood Smear
- is a large amount of blood spread to form a small circle on a glass slide and allowed to dry.
- It is made up of many layers of blood cells piled on top of each other.
- A large amount of blood can be examined quickly this way.
A thick blood smear is used to detect the following blood parasites:
- Plasmodium species
Thin Blood Smear
- A small amount of blood is spread over a large area of a glass slide so that the blood cells do not pile on top of each other.
- Individual blood cells can be examined and studied in this manner.
A thin blood smear is used for:
- Blood parasite identification.
- Differential white cell count and blood cell morphology.
Preparation of Blood Smears for Blood Parasites: Malaria, Trypanosomes, Borrelia, and Microfilariae
Thick and thin blood smears prepared on the same glass slide
- When collecting blood and preparing blood smears, it is critical to pay close attention to technique.
- A variety of viral, bacterial, and parasitological diseases can be transmitted through blood and blood products.
- A thin and thick smear can be made on the same slide for routine malaria microscopy.
- The thin film is used as a label, but it can also be used for species confirmation if properly prepared.
- For examination, the thick smear should be used.
Materials and Equipment used in Blood Smear Preparation
- Slide folders
- Wooden draining rack with grooves (to hold glass slides)
- Gauze pads
• Alcohol pads or swabs
- Sterile Lancets
- Pen or marker for labeling
- Glass slides
- Micropipette tips
- Staining dishes/racks
- •Towels (paper) or sponge
- Alcohol – 70% ethanol or isopropanol
- Blood slide template
Procedure of how to prepare a Thin and Thick Blood Smear on the Same Slide
- Blood smears are performed after the patient has been recorded in the appropriate form or register:
- Select the third finger with the patient’s left hand, palm upwards (the big toe can be used in infants) o The thumb should never be used for adults or children.
- Clean the finger with cotton wool lightly soaked in 70% alcohol, using smear strokes to remove dirt and grease from the ball of the finger.
- Dry the finger with a clean cotton towel or gauze, using firm strokes to stimulate blood circulation.
- Using a sterile lancet, quickly roll across the ball of the finger.
- Using gentle pressure on the finger, express the first drop of blood and wipe it away with dry cotton wool. Check that no cotton strains remain on the finger.
- Collect the blood as follows, working quickly and only by the edges of clean slides:
- Apply gentle pressure to the finger and collect a single small drop of blood in the center of the slide. This is for the thin film.
- Apply more pressure to express more blood and collect two or three larger drops about 1 cm from the drop intended for the thin smear. Wipe the remaining blood away from the finger with cotton wool.
Creating a thin blood smear
- Using a clean slide as a’spreader,’ and with the slide containing the blood drops resting on a flat, firm surface, touch the small drop with the spreader and allow the blood to run along its’ edge.
- Firmly push the spreader along the skid away from the largest drops, maintaining a 45 angle.
- Ensure that the spreader is in constant contact with the slide’s surface while spreading the blood.
- To avoid infection of the investigator, the blood smear should not extend beyond the edges of the slide.
figure 1. preparation of thin smear
Creating a thick blood smear
To make a thick smear, always handle slides by the edges or by a corner as follows:
- Using the spreader’s corner, quickly join the large drops of blood and spread them to form an even, thick smear.
- The blood should not be overly stirred, but it can be spread in a circular or rectangular pattern with three to six movements.
- Allow the thick smear to dry in a flat, level position away from flies, dust, and extreme heat (in a slide folder)
- Label the dry smear with a pen or marker pencil by writing the patient’s name or number and date at the thick portion of the thin smear (as shown below). Do not label the slide with a ballpoint pen.
- Wrap the dry slide in clean paper and send it to the laboratory as soon as possible with the patient’s record form.
figure 2: creating of thick smear and thin smear
image source: NMCP 2008
Advantages of Thick and Thin Blood Smears
Thick Blood Smear
- In the same amount of time, a thick smear can examine approximately 20 times more blood than a thin smear.
- A thick smear is best suited for detecting malaria parasites quickly. In areas where P. malariae is present. Because parasitaemia is normally low in this species, infection is likely to be missed unless a thick smear is examined.
- A thick smear allows for more blood to be examined. The red cells in a thick smear are not fixed during staining. The parasites are not destroyed, and after staining, they can be found among stained haemoglobin and red cell stroma (with careful focussing).
Thin Blood Smear
- If the Plasmodium species is not clear from the thick smear, a thin blood smear is required.
- A thin smear is fixed, allowing the parasites to be seen in the red cells. Depending on the species, parasitized red blood cells may become enlarged, oval in shape, and stippled.
- These characteristics, along with the parasite forms present, can greatly aid in confirming a mixed infection and identifying P. ovale and P. malariae, which are more difficult to distinguish in thick smears (but best detected in thick smears).
- Thin films can also be used to determine whether a patient with a severe falciparum infection is responding to treatment in areas where resistance is suspected by counting the percentage of red cells infected before and after treatment.