A blood smear, also known as a peripheral blood film, is a fundamental procedure in the laboratory that involves spreading a thin layer of blood onto a glass microscope slide for subsequent analysis. This technique provides a direct, visual look at the cellular components of the blood, including red blood cells, white blood cells, and platelets. Preparing a high-quality smear is necessary for accurate diagnosis, as subtle differences in cell appearance can indicate significant health issues. The blood smear remains an important diagnostic tool, complementing automated cell counters by offering a manual, morphological assessment that machines cannot fully replicate.
Why Blood Smears are Performed
The primary application of a blood smear is to allow for the manual differential count of white blood cells (WBCs). This count determines the proportions of the five main types of WBCs—neutrophils, lymphocytes, monocytes, eosinophils, and basophils—which helps health professionals differentiate between various conditions like bacterial or viral infections and inflammation. The manual differential offers a confirmation and a deeper look at the cell maturity and morphology beyond what automated analyzers provide.
Beyond white cell analysis, the blood smear is essential for evaluating the morphology of red blood cells (RBCs) and platelets. Technicians examine the size, shape, and color of RBCs, looking for abnormalities like sickle shapes, target cells, or fragmented cells, which can be indicators of anemias or other blood disorders. The smear also allows for an estimate of platelet numbers and a check for platelet clumping. Furthermore, the slide can be used to detect blood-borne parasites, such as those causing malaria.
Essential Materials and Sample Preparation
The preparation of a blood smear begins with gathering the correct, clean materials to prevent artifacts that could interfere with analysis. Requirements include two high-quality, grease-free glass slides: one stationary base slide and a second spreader slide used to create the film. The blood sample is typically venous blood collected in a tube containing the anticoagulant Ethylenediaminetetraacetic acid (EDTA). It is important to prepare the smear within two hours of collection, as prolonged exposure to EDTA can cause white blood cells to show morphological changes.
Prior to smearing, the EDTA sample must be gently mixed by inversion to ensure the blood cells are evenly distributed. A small, isolated drop of blood, around 2 to 3 millimeters in diameter, is placed onto the base slide, approximately 1 to 2 centimeters from one end. The size of this drop is critical; too large a drop yields a smear that is too thick and long, while a drop that is too small results in a film that is too short for proper examination.
Mastering the Smear Technique
Safety protocols must be observed throughout the procedure, including wearing gloves and disposing of all contaminated materials in appropriate biohazard and sharps containers. The wedge technique is the most common method for creating a peripheral blood smear. The spreader slide is positioned at an angle, typically between 30 and 45 degrees, against the base slide, just in front of the blood drop.
The spreader is drawn backward until its edge makes contact with the blood drop, allowing the blood to rapidly spread by capillary action along the entire width of the contact line. This full, even spread is crucial before the forward motion begins to ensure the final smear is uniform and has straight lateral edges. Once the blood has spread, the spreader slide is pushed forward in a single, smooth, and steady motion across the base slide. A rapid movement generally produces a thinner, longer smear, while a slower movement results in a thicker, shorter one.
Maintaining an even pressure and speed is necessary to prevent streaks or an irregular distribution of cells. The goal is to create a film that covers approximately two-thirds of the slide’s length and ends in a thin, translucent area known as the feathered edge. This feathered edge should have a rounded shape and a rainbow sheen when held up to the light, indicating the ideal thinning of the blood film. The angle of the spreader may need slight adjustment based on the patient’s hematocrit; a lower angle is often necessary for thicker blood to prevent a short smear.
Assessing Smear Quality and Next Steps
Once the blood smear is complete, its quality must be visually assessed. A well-made smear will be smooth, without any noticeable ridges, streaks, or holes, and will taper gradually from the thick application end to the feathered edge. The feathered edge is the most important area for microscopic analysis, often called the monolayer, where the red blood cells should be separated and not overlapping.
Common errors include making the smear too thick (due to too large a blood drop or too steep an angle) or creating a smear that is too short because the forward motion was too slow. If the smear is too thick, the cells will be distorted and difficult to evaluate. Another frequent issue is the uneven distribution of white blood cells, where larger cells are pushed disproportionately to the edges, which compromises the accuracy of the differential count.
To correct these faults, the operator must adjust the drop size, the angle of the spreader slide, or the speed of the forward push when preparing a new slide. The acceptable smear must then completely air-dry at room temperature to fix the cells to the glass. The slide is then ready for staining, typically with a Romanowsky-type stain such as Wright or Giemsa, which colors the cellular components and makes them visible for microscopic examination.