Blood Smear Test: Procedure, Staining & Interpretation

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  • 0:04 The Blood Smear Test
  • 0:42 Procedure
  • 2:55 Tips for Perfect Tests
  • 3:54 Staining
  • 4:22 Interpretation
  • 5:40 Lesson Summary
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Lesson Transcript
Instructor: Shelly Watkins

Shelly has instructed college biology courses, as well as graduate students in health profession programs. She has a doctorate in physical therapy.

In this lesson, readers will learn about the procedure, staining methods, and interpretation of the blood smear test. This lesson also includes some general descriptions of conditions indicated by blood smear test results.

The Blood Smear Test

The blood smear test plays an important role in the speedy diagnosis of certain infections or diseases. This test uses a drop of blood spread onto a glass microscope slide that is then treated with a colored stain and examined using a microscope. The blood smear test shows a sample of blood components including platelets, leukocytes (white blood cells), and erythrocytes (red blood cells) that are present in plasma, the fluid part of blood. A blood smear test is typically used as a follow-up test after abnormal results were shown in a complete blood count test (CBC). The blood smear is a vital diagnostic aid.


A blood smear test is performed by first obtaining a 5 mL blood sample from the patient. The patient should be educated about the procedure before taking a sample from their vein, or, less often, from a capillary. The blood sample is stored in a bottle containing an anticoagulant called ethylene diamine tetra-acetic acid (EDTA). Anticoagulants prevent the blood from clotting. This sample is sent to the laboratory for testing within two hours of collection.

Preparation of the microscope slide is performed by trained personnel such as a pathologist, medical laboratory technologist, hematologist, or laboratory assistant. This personnel uses a base slide, a blood spreader slide, and a pipette or capillary tube. The wedge method is the most common way to prepare the slide for testing. Using this method, a mixed drop of blood 1 to 2 mm in diameter is placed in the center line about 1/4 inch from the edge of the microscope slide using a pipette or capillary tube. The slide containing the blood drop is called the base slide.

Another microscope slide called a spreader slide is used. This slide should have chipped edges, along with another smooth end. The side of the spreader slide with chipped edges is placed on the original slide (base slide) in front of the blood and moved backwards to touch the blood. This makes the blood spread along the base of the slide.

The smear is made with the spreader inclined at an angle of approximately 30° to the blood. The smear should cover two-thirds of the base slide and should have a feathered end. The smear should then be air dried. The frosted end of the slide should be labeled with the patient's name, identification number, and date. The dried smear is then fixed with methanol or ethyl alcohol and stained.

The smear is covered with stain for approximately ten minutes, then diluted with water and allowed an additional ten minutes for the cells to properly stain. Following the stain application, the slide is rinsed under running water. The slide should be wiped underneath with cotton to remove excess stain. Finally, the slide is placed on a rack to dry.

Tips for Perfect Tests

It is best to follow instructions for the procedure in obtaining, creating, and staining a blood smear. It is also better to create a new smear if the procedure is compromised than it is to interpret an inadequate smear. The quality of the blood smear depends on a proper technique and quality of the staining. Some additional guidelines should be followed to create the best blood smear.

At least two slides should be made during testing. A smear will be too thin if the spreader slide is moved too quickly or if the angle of the spreader is less than 30°. Conversely, the smear will be too thick if the spreader is moved slowly or if the angle is greater than 30°. Large blood drops may extend the smear over too much of the base slide, while a small drop can be insufficient for the smear. The stain needs adequate time with the sample to avoid over-staining or under-staining. If a sample is over-stained, debris might show up in the sample. This can also happen if the stain is not washed enough with running water.

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