Thick and Thin Blood Smear for Malaria Diagnosis

A blood film or Peripheral blood smear (PBS) must be prepared from capillary blood or a drop of blood from an EDTA (anticoagulant) blood sample. Blood examination under the microscope requires blood smears. The Differential Leukocyte Count (DLC) is typically determined by blood smears. The DLC entails counting 100 white blood cells (WBCs) and noting the number, type, and characteristics of all blood cells, which aids the physician in diagnosing viral infections, bacterial infections, parasitic infection, and certain blood disorders such as anemia, leukemia, bacterial or parasitic infection, etc.

For the diagnosis and prognosis of any abnormalities (if present), two varieties of blood smears are typically prepared. Thin blood smears are prepared for the study of blood cell morphology and the identification of microorganisms. Blood parasites such as Plasmodium spp. (malaria parasite), Wuchereria bancrofti & Brugia malayi (lymphatic filariasis), Leishmania donovani or other species of Leishmania (leishmaniasis), Babesia spp. (babesiosis) etc. are detected using thick blood smears.

Thick and Thin Blood Smear

• Thick and thin blood smears are laboratory techniques used to diagnose various blood-borne infections, including malaria and other parasitic diseases.
• A thick blood smear involves placing a drop of blood on a microscope slide and spreading it in a circular motion to create a thick film. The sample is then fixed with methanol, stained with a specific dye such as Giemsa, and examined under a microscope. This technique allows for the detection of higher concentrations of parasites in the blood.
• On the other hand, a thin blood smear involves placing a small drop of blood on a microscope slide and using a second slide to spread it out into a thin film. The sample is then fixed and stained in the same way as a thick smear. This technique allows for a more detailed examination of the blood cells and identification of specific parasites, but is less sensitive in detecting low concentrations of parasites in the blood.
• Both thick and thin blood smears are commonly used in the diagnosis of malaria, with thick smears being more sensitive in detecting the presence of parasites and thin smears allowing for the identification of the specific species of malaria parasite. In addition to malaria, these techniques can also be used to diagnose other parasitic infections such as leishmaniasis and trypanosomiasis.
• It is important to note that interpreting thick and thin blood smears requires specialized training and expertise, and false negative or false positive results can occur. Therefore, these techniques are typically performed by trained laboratory professionals and confirmed by further testing methods such as molecular diagnostics.

Thick Blood Smear

A thick layer of dehemoglobinized (lysed) red blood cells (RBCs) composes dense smears. The blood components (including parasites, if any) are approximately 30 times more concentrated than in a narrow smear of equal area. Thus, dense smears facilitate the detection of parasites more effectively (increased sensitivity). However, they do not allow for a thorough examination of parasite morphology. For instance, they are frequently insufficient for malaria parasite species identification: if the dense smear is positive for malaria parasites, the thin smear should be used for species identification.

Principle of Thick Blood Smear

A Thick blood film stain requires a greater volume of blood than Thin blood films, which allow for a more efficient detection of blood parasites. A thick blood smear is created by distributing a large blood drop over a small area of about 1 centimeter, which enables the detection of various parasitic forms against a more transparent background.

Requirement

• Syringe & needle
• EDTA vial
• Tourniquet
• 70% isopropyl alcohol
• Sterile Lancet
• Microscopic glass slides

Procedure of Thick Blood Smear

Method 1

1. Collect blood samples via capillary or venipuncture.
2. Using a capillary tube, transfer a large drop of blood from the tube onto the clean, grease-free microscope glass slide if the blood sample was obtained via venipuncture. If a finger or heel puncture is performed, discard the initial drop of blood and then place a drop of blood from the puncture site on the sterile microscope slide. The Blood drop should be placed in the center of the glass slide.
3. Place the glass slide of the specimen on a flat surface and grasp it with the index finger and forefinger of the left hand (if you are right-handed).
4. Now, uniformly distribute the blood in a smear or film of approximately 10 mm in width and a thickness that allows you to read the words when the slide is placed on printed paper.
5. Allow the blood stain to dry entirely in the air. To avoid distorting the smear, do not blast air on the slide from any source in an effort to expedite drying.
6. On the opaque end of the slide, using a lead pencil or glass marking pencil, write the Name, Identification Number, and Date. Do not use wax pencils, markers, or pens, as they dissolve and are removed during the staining process.

Method 1

1. Place a tiny drop of blood in the middle of the previously cleansed and labeled slide.
2. Using the corner of another slide or an applicator stick, distribute the drop until it is the size of a dime (1.5 cm2) in a circular pattern.
3. The proper density of a thick smear is one that, when applied (wet) over newspaper, makes it difficult to read the words.
4. Place the slides on a horizontal surface and permit the smears to dry completely (protect from dust and insects!). During staining, smears that are insufficiently dried or too dense can detach from the slides. Blood specimens made with anticoagulant-treated blood carry a greater degree of danger. At room temperature, drying can take several hours; 30 minutes is the bare minimum; when staining, manage the smudge with extreme care. You can speed up the drying process by using a fan or hair dryer (set to lukewarm). To prevent thick smears from becoming heat-fixed, shield them from heated environments.
5. Avoid using methanol or heat to repair thick smears. If there will be a delay in staining smears, briefly immerse the dense smear in water to lyse the red blood cells.

Scratch Method for Thick smears

The scratch method is an alternative technique for producing thick films that offers enhanced adhesion and a quicker turnover time. Instead of a stick, the edge of a glass microscope slide is used to spread the blood, while firm pressure is applied to produce small scratches on the underlying slide. The scratches enhance the blood film’s adhesion to the slide without altering the morphology of the smear. As soon as the smear is set, typically within 20 to 30 minutes of preparation, it can be stained.

Limitation of Thick Smear

• Subjectivity and operator dependence: The interpretation of thick blood smears is subjective and operator-dependent, requiring trained and experienced personnel to obtain accurate results. The quality of the sample and the staining procedure can also influence the interpretation of the test.
• False negatives: The sensitivity of the thick blood smear technique may be reduced in cases of low parasitemia or in infections with species of parasites that are difficult to detect. False negative results may occur if the parasite load is too low or if there is insufficient blood in the smear.
• False positives: The thick blood smear technique is not specific for detecting a particular parasite species and may identify other non-malarial parasites as malaria parasites, leading to false positive results.
• Limited information: Thick blood smears only provide information on the presence and density of parasites in the blood, and do not provide information on the stage of infection or the response to treatment.
• Time-consuming: The preparation, staining, and examination of thick blood smears can be time-consuming and requires specialized equipment and trained personnel, which can be a limitation in resource-limited settings.
• Variability: Thick blood smear results can vary between different laboratories, technicians, and even between different parts of the same slide, which can lead to inconsistencies in the diagnosis and treatment of infections.
• Malaria parasite species identification may be impossible, even for experienced technicians.
• Always scrutinize a thin film when a definitive identification based on morphology is required.
• Anticoagulated blood must be used to produce smears within one hour of venipuncture. Due to the direct action of the anticoagulant, the parasitic forms and erythrocytes acquire an atypical morphology after this time.

Therefore, while thick smears are an important tool in the diagnosis of malaria and other parasitic infections, they should be used in conjunction with other diagnostic techniques to improve accuracy and ensure the best possible patient outcomes.

Advanatges of Thick Blood Smear

• Sensitivity: Thick blood smears are more sensitive than other diagnostic methods, such as rapid diagnostic tests (RDTs), in detecting low levels of parasites in the blood. This makes it a valuable tool for early detection of infections.
• Cost-effective: The thick blood smear technique is a low-cost diagnostic method, making it an attractive option for resource-limited settings where other diagnostic methods may not be available.
• Wide availability: Thick blood smears can be performed in most laboratories with basic microscopy equipment, making it widely available in many settings.
• Quantitative: Thick blood smears allow for the quantification of parasites in the blood, which can be useful in monitoring the progression of infections and evaluating treatment outcomes.
• Information on parasite morphology: The thick blood smear technique provides information on the morphology of the parasites, which can be useful in identifying specific species of parasites and guiding treatment decisions.
• Easy to store and transport: Thick blood smears can be stored for a long time and transported easily, making it possible to perform retrospective analysis of samples and compare results over time.
• Identification of co-infections: Thick blood smears can be used to identify co-infections with multiple parasitic diseases, which is important for guiding appropriate treatment.

Applications of Thick Blood Smear

• Diagnosis of malaria: Thick blood smears are widely used in the diagnosis of malaria, particularly in resource-limited settings where other diagnostic methods may not be available. The technique allows for the detection and quantification of malaria parasites in the blood, which is essential for guiding appropriate treatment.
• Monitoring of treatment: Thick blood smears can be used to monitor the effectiveness of malaria treatment by evaluating changes in parasite density over time. This helps to ensure that treatment is effective and can identify cases of treatment failure.
• Identification of co-infections: Thick blood smears can be used to identify co-infections with multiple parasitic diseases, which is important for guiding appropriate treatment.
• Surveillance: Thick blood smears can be used in surveillance programs to monitor the prevalence of malaria and other parasitic diseases in a given population. This can help to identify areas of high transmission and guide targeted interventions.
• Research: Thick blood smears are widely used in research studies to evaluate the effectiveness of new diagnostic tools or to study the biology of the malaria parasite.
• Evaluation of control measures: Thick blood smears can be used to evaluate the effectiveness of malaria control measures, such as bed nets, insecticide-treated clothing, and indoor residual spraying.

Thin Blood Smear

Thin smears consist of a layer of blood with progressively decreasing thickness toward the feathered border. In the feathered border, the cells should be arranged in a monolayer without touching.

Principle of Thin Blood Smear

The Thin Peripheral Blood smear is created by placing a drop of well-mixed blood between 1 and 2 mm in diameter and 1/4 inch from the margin of a clean microscope glass slide. The drop should be positioned in the center of the glass slide. These margins are left on the glass slide in order to obtain a region where the cells can be enumerated and differentiated. Using a second slide as a spreader, the blood is striped into a thin film in the shape of a tongue, and then allowed to dry. For microscopic examination, the blood smear is then fixed and stained with Romanowski stain.

Requirement

• Syringe & needle
• EDTA vial
• Tourniquet
• 70% isopropyl alcohol
• Sterile Lancet
• Microscopic glass slides

Procedure of Thin Blood Smear

1. Collect blood samples via capillary or venipuncture.
2. Using a capillary tube, transfer a drop of blood from the tube onto the clean, lubricant-free microscope glass slide if the blood sample was obtained via venipuncture. If a finger or heel puncture is performed, discard the initial drop of blood and then place a drop of blood from the puncture site on the sterile microscope slide. Place the blood droplet in the center of one side of the glass slide, leaving approximately 1 cm of margins around it.
3. Place the glass slide of the specimen on a flat surface and grasp it with the index finger and forefinger of the left hand (if you are right-handed). Now, at an angle between 30° and 45°, place a flat, clean edge of the spreader slide on the specimen slide.
4. Move the dispersal slide toward the blood drop until it makes contact with the blood drop at the desired angle. Then, swiftly and smoothly move the spreader slide over the specimen slide, drawing the blood behind it into a thin, tongue-shaped film.
5. Allow the blood stain to dry entirely in the air. To avoid distorting the smear, do not blast air on the slide from any source in an effort to expedite drying.
6. On the opaque end of the slide, using a lead pencil or glass marking pencil, write the Name, Identification Number, and Date. Do not use wax pencils, markers, or pens, as they dissolve and are removed during the staining process.

Variations in Angle of Spreader

To acquire an optimal smear, the angle of the spreader varies with the thickness or viscosity of the blood. Blood viscosity increases due to conditions such as Polycythemia (increased red blood cell count), Dehydration, or any condition that increases the packed cell volume or hematocrit value. Reduces the angle between the Spreader and glass slide to obtain an optimal blood film in this state. Conversely, any condition that decreases the packed cell volume or hematocrit value, such as anemia, depression of the bone marrow, etc., is associated with a decrease in the viscosity of blood. In this state, the angle between the spreader and the specimen glass slide is increased to create the ideal blood film.

Advantages of a thin blood smear

• Specificity: Thin blood smears are more specific than other diagnostic methods, such as rapid diagnostic tests (RDTs), in identifying specific species of parasites in the blood. This is because thin blood smears provide a clearer picture of the parasite’s morphology, which is important for accurate identification.
• Differentiation of species: Thin blood smears allow for the differentiation of different species of parasites in the blood, which is important for guiding appropriate treatment. For example, different species of malaria parasites require different treatments, and accurate species identification is essential for effective treatment.
• Identification of other blood disorders: Thin blood smears can also be used to identify other blood disorders, such as leukemia and anemia, by examining the morphology of the blood cells.
• Quantitative: Thin blood smears allow for the quantification of parasites in the blood, which can be useful in monitoring the progression of infections and evaluating treatment outcomes.
• Better visualization: Thin blood smears provide a clearer and more detailed picture of the parasites in the blood, which can make it easier to identify and quantify parasites accurately.
• Minimal interference: Thin blood smears are less likely to be affected by interfering substances, such as white blood cells or debris, which can interfere with other diagnostic methods.
• Ease of storage: Thin blood smears can be easily stored for later analysis, making it possible to perform retrospective analysis of samples and compare results over time.

Limitations of a thin blood smear

• Subjectivity and operator dependence: The interpretation of thin blood smears is subjective and operator-dependent, requiring trained and experienced personnel to obtain accurate results. The quality of the sample and the staining procedure can also influence the interpretation of the test.
• False negatives: The sensitivity of the thin blood smear technique may be reduced in cases of low parasitemia or in infections with species of parasites that are difficult to detect. False negative results may occur if the parasite load is too low or if there is insufficient blood in the smear.
• False positives: The thin blood smear technique is not specific for detecting a particular parasite species and may identify other non-malarial parasites as malaria parasites, leading to false positive results.
• Time-consuming: The preparation, staining, and examination of thin blood smears can be time-consuming and requires specialized equipment and trained personnel, which can be a limitation in resource-limited settings.
• Limited information: Thin blood smears only provide information on the morphology and stage of parasites in the blood, and do not provide information on the parasite density or response to treatment.
• Variability: Thin blood smear results can vary between different laboratories, technicians, and even between different parts of the same slide, which can lead to inconsistencies in the diagnosis and treatment of infections.
• In light infections, parasites may be undetected. In these situations, a dense film must be examined.
• Anticoagulated blood must be used to produce smears within one hour of venipuncture. Due to the direct action of the anticoagulant, the parasitic forms and RBC acquire an atypical morphology after this time.

Applications of Thin blood smears

• Diagnosis of malaria: Thin blood smears are the gold standard for the diagnosis of malaria, particularly in resource-limited settings where other diagnostic methods may not be available. Thin blood smears allow for the identification and quantification of malaria parasites in the blood, which is essential for guiding appropriate treatment.
• Identification of other blood-borne infections: Thin blood smears can also be used to identify other blood-borne infections, such as filariasis, trypanosomiasis, and babesiosis, by examining the morphology of the parasites in the blood.
• Monitoring of treatment: Thin blood smears can be used to monitor the effectiveness of malaria treatment by evaluating changes in parasite density over time. This helps to ensure that treatment is effective and can identify cases of treatment failure.
• Differentiation of species: Thin blood smears allow for the differentiation of different species of parasites in the blood, which is important for guiding appropriate treatment. For example, different species of malaria parasites require different treatments, and accurate species identification is essential for effective treatment.
• Research: Thin blood smears are widely used in research studies to evaluate the effectiveness of new diagnostic tools or to study the biology of the malaria parasite.
• Surveillance: Thin blood smears can be used in surveillance programs to monitor the prevalence of malaria and other parasitic diseases in a given population. This can help to identify areas of high transmission and guide targeted interventions.
• Evaluation of control measures: Thin blood smears can be used to evaluate the effectiveness of malaria control measures, such as bed nets, insecticide-treated clothing, and indoor residual spraying.

Giemsa Staining of Thick and Think Blood Smear

Giemsa stain is the most trustworthy approach for staining both thick and thin blood films. Eosin and methylene blue (azure) constitute Giemsa solution. The cytoplasm is stained blue with methylene blue, while the nucleus is stained red with eosin.

Determines the quantity of transparencies to be stained. Staining each slide requires approximately 3 mL. If you need to stain 16 microscope slides, you can create 50 mL of Giemsa working solution. Two typical effective solutions are:

• 10% Giemsa stain working solution: The 10% Giemsa stain working solution is utilized in hospitals and diagnostic labs for rapid diagnosis. It is marginally more expensive as more stain is utilized.
• 3% working solution: This method is primarily used to stain microscope slides for educational or epidemiological objectives.

Staining the Thin blood smear with Giemsa stain

1. Prepare and air-dry a thin blood deposit on a clean and dry microscope glass slide.
2. Initially, fix the air-dried thin blood smear in absolute methanol by rapidly dipping it twice in a Coplin jar containing absolute methanol.
3. Remove the slide from the Coplin jar and allow it to dry naturally.
4. Giemsa Stain for Thin Blood swab dilution: In 1:20, the Giemsa stain is used to color the thin blood smear. Add 2 ml of Giemsa stain stock solution to 40 ml of phosphate buffer solution in a clean Coplin jar to create a 1:20 dilution. You can also substitute distilled water for the buffer, though the results may vary.
5. Stain the Methanol-fixed Blood smear for 20 minutes with diluted Giemsa stain (1:20, v/v). Place the slide in the Coplin jar containing the diluted Giemsa stain, or place it smear-side up on a staining rack or any flat surface and pour the stain over the smear so that it evenly covers the smear.
6. Now Wash the stained slides by rapidly dunking them in and out of a Coplin jar containing buffered water or distilled water once or twice.
7. Do not overexpose the stain to phosphate buffer solution (buffered water). Washing excessively may decolorize blood trace.
8. Allow the stain to cure thoroughly in the air.

Staining the Thick blood smear with Giemsa stain

1. Prepare and air-dry a viscous blood smear on a clean and dry microscope glass slide.
2. If you don’t know how to prepare it, refer to the Thick blood Smear preparation.
3. Do not dry the thick blood smear in an incubator or with heat, as this will attach the blood smear to the slide and impede the lysis of red blood cells (RBCs).
4. If a rapid diagnosis of the malaria parasite is necessary, thick films can be made slightly thinner than usual, permitted to air-dry at room temperature for one hour, and then stained with Giemsa stain.
5. Giemsa Stain Dilution for Thick Blood Smear: For staining the thick blood smear, Giemsa stain is diluted 1:20. In a clean Coplin jar, combine 1 ml of Giemsa stain stock solution with 49 ml of phosphate buffer solution to create a 1:50 dilution. You can also substitute distilled water for the buffer, though the results may vary.
6. Stain the Air-Dried Blood smear for 50 minutes with diluted Giemsa stain (1:50, v/v). Place the slide in the Coplin jar containing the diluted Giemsa stain, or place it on a staining rack or any flat surface with the smear facing up and distribute the stain evenly over the smear.
7. Blood smears are cleaned by soaking them in buffered or distilled water for 3 to 5 minutes.
8. Allow the stain to cure thoroughly in the air.

Staining the Thin and thick blood smear on the same slide with Giemsa stain

1. Prepare a thin and thick smear of the specimen on the same slide by dividing the slide in half and making the thin smear on one side and the thick smear on the other. Permit the stain to air-dry.
2. Now, carefully fix the air-dried thin blood smear in absolute methanol by immersing the thin smear side in a Coplin jar containing absolute methanol (2 to 3 dips). Ensure that the Methanol or its vapors reach the dense film by tilting the slide slightly.
3. Note: The introduction of even a small amount of methanol into dense blood smears will prevent the RBCs from lysing during the staining process.
4. Remove the slide from the Coplin jar and allow the dense smear to air-dry. Before staining, ensure that the slide is thoroughly dried on both the thin and heavy smear sides.
5. Giemsa stain dilution for thin and dense blood smears on the same slide: For staining thick and thin blood smears on the same slide, a 1:50 solution of Giemsa stain is used. In a clean Coplin jar, combine 1 ml of Giemsa stain stock solution with 49 ml of phosphate buffer solution to create a 1:50 dilution. You can also substitute distilled water for the buffer, though the results may vary.
6. Stain the Air-Dried Blood smear for 50 minutes with diluted Giemsa stain (1:50, v/v). Place the slide in the Coplin jar containing the diluted Giemsa stain, or place it on a staining rack or any flat surface with the smear facing up and distribute the stain evenly over the smear.
7. Place the slide in the Coplin jar with the thick smear facing downward to prevent RBC debris from settling onto the thin blood smear.
8. Now is the crucial time to rinse the slide…… Rinse the thin smear side by rapidly submerging the smear in and out of a Coplin jar of phosphate buffer (one or two dips) and then transferring the smear to a paper towel. Wash the dense smear side with phosphate buffer or distilled water for three to five minutes. Ensure that the dense smear is completely submerged, but do not allow any water to cover the thin smear. If not, the fine smear will become decolored.
9. Allow the stain to cure thoroughly in the air.

Microscopic examination

Examining the thick film

• Blood film stained with Giemsa should be placed on the microscope stage with the label to the left. Align the dense film with the tenfold objective lens.
• Activate the microscope, optimize the light source, and locate the focus by peering through the ocular and 10x objective.
• Examine the blood film for parasites and other blood components. Select a portion of the film that is uniformly stained with white blood cells.
• Apply a drop of immersion oil to the dense film. To prevent cross-contamination, guarantee that the oil applicator never contacts the slide. Permit the 40x objective to avoid contact with the lubricant.
• Transfer the 100x oil immersion objective to the chosen portion of the dense film. Utilize the adjustment for precise focus to see the image. Raise the mechanical stage to prevent slide damage.
• Using the fine adjustment, concentrate on the cell elements and confirm that the film is suitable for routine examination; 15-20 white blood cells per thick film field will yield an acceptable film thickness. Films that contain fewer white blood cells per field will necessitate a more thorough examination.
• Examine the slide in a methodical manner. Start at the upper left of the film (indicated by a green arrow pointing vertically) and work your way horizontally to the right, field by field.
• When you reach the opposite end of the film, move the slide gently downwards, then to the left, field by field, and so on. Throughout the examination of each field, incessantly focus and refocus with the fine adjustment for efficient examination.
• Examine the dense film under the oil immersion objective, horizontally or vertically, field by field. Focus by using the precise adjustment.
• A minimum of one hundred high-power fields must be investigated before a thick film can be deemed “parasite-free.” The entire dense film should be scanned if possible.
• If parasites are discovered, 100 additional fields should be scanned to enhance the likelihood of identifying mixed infections.
• Identify and record all observed species and stages.

Examining Thin Films

After analyzing the thick blood film, the thin blood film must always be examined to identify parasite species or mixed infections. Thin film, unlike thick film, permits the visualization of parasite and red cell morphology. Examine the extremity or edge of the thin film where it is feathery.

• Place a drop of immersion oil on the thin film’s feathered edge.
• Change from the 10x oil immersion lens to the 100x oil immersion lens.
• Examine the feathery end of the thin film’s edge, where there is minimal overlap between adjacent red cells. Follow the movement pattern depicted in Figure 2. Move along the edge of the film, then move the slide one field outwards, then one field inwards, and so on.
• Examine the thin film until the presence and species of malaria parasites are confirmed. Identify and record all observed species and stages in the malaria blood microscopy register.

Differences Between Thick Blood Smear and Thin Blood Smear

Feature	Thick Blood Smear	Thin Blood Smear
Sample preparation	A thick layer of blood is spread on a slide using a dropper and then allowed to air dry	A thin layer of blood is spread on a slide using a spreader and then allowed to air dry
Useful	Thick blood smears are most useful for detecting the presence of parasites.	Thin blood smears helps to discover which species of parasite is causing the infection.
Staining	A thick blood smear is a drop of blood on a glass slide.	A thin blood smear is a drop of blood that is spread across a large area of the slide.
Purpose	The blood films must be laked before or during staining to rupture all the RBC so that only WBC, platelets and parasites are visualized.	The purpose is to allow malarial parasites to be seen within the RBC and to assess the size of the infected RBCs compared to uninfected RBCs
Fixed	It is not fixed in methanol.	It is fixed in methanol.
Parasite detection	Useful for detecting high parasite density, as the thick smear concentrates the parasites in a smaller area	Useful for detecting low parasite density, as the thin smear spreads the parasites out over a larger area
Sensitivity	Lower sensitivity due to lower parasite density in a larger field	Higher sensitivity due to higher parasite density in a smaller field
Interpretation	The morphology and stage of the parasites can be difficult to distinguish due to the high concentration of parasites	The morphology and stage of the parasites can be more easily distinguished due to the lower concentration of parasites
Time required	Faster preparation, staining, and examination process	Slower preparation, staining, and examination process
Diagnostic value	Useful for initial screening and rapid diagnosis in resource-limited settings	Useful for accurate species identification and monitoring of treatment response
Operator-dependent	Interpretation can be subjective and operator-dependent	Interpretation can be subjective and operator-dependent
False negatives	Parasites may be missed if the concentration is too low	Parasites may be missed if the concentration is too low
False positives	Other non-malarial parasites may be identified as malaria parasites	Other non-malarial parasites may be identified as malaria parasites

FAQ

References

• https://www.vetlexicon.com/treat/felis/technique/blood-smear
• https://www.cdc.gov/dpdx/diagnosticprocedures/blood/specimenproc.html
• https://paramedicsworld.com/hematology-practicals/preparation-peripheral-blood-smear/medical-paramedical-studynotes
• https://microbeonline.com/thick-and-thin-blood-smear/
• https://www.nyp.org/healthlibrary/tests-detail/thick-and-thin-blood-smears-for-malaria
• https://microbiologyinfo.com/differences-between-thick-blood-smear-and-thin-blood-smear/
• https://www.differencebetween.com/what-is-the-difference-between-thin-and-thick-smear/