Blood smear

What is it and why is it done?

The blood smear is a laboratory test that allows to obtain a sort of photograph, a "snapshot of the cell population present in a drop of blood. Through the smear on the slide it is in fact possible to evaluate the number - but also the morphology, the stage of maturation and the numerical percentages - of red blood cells, leukocytes and platelets.

In addition to the number, one can therefore also evaluate the shape of these cells; thus, for example, the smear of a subject affected by sickle cell anemia shows the typical sickle conformation of red blood cells, which are instead too small in the case of microcytic anemia and too large in the presence of macrocytic anemia. Furthermore, through the smear it is possible to evaluate the hemoglobin content of the erythrocytes, establishing based on the color scheme whether this is normal or insufficient.

As shown in the figure, thanks to the blood smear it is also possible to highlight the presence of parasites, such as those responsible for malaria, heartworm and sleeping sickness. We are therefore talking about a precious diagnostic aid, a second level test which however requires a certain dexterity and experience on the part of the operator. Generally, the execution of the smear follows a first-line examination, such as the blood count, considered suspicious The complete blood count is an automated test that determines the number of blood cells; the smear, on the other hand, is a manual examination, which involves the observation and direct counting of these cells.

When taking the smear

Your doctor may prescribe a blood smear when:

CBC and differential leukocyte counts indicate a suspicion of abnormality or immature cells;

in the presence of a suspicion of a deficiency, a disease or a disorder in the production of blood cells;

you want to monitor the efficacy or side effects of a therapy (eg chemotherapy) or the evolution of a haematological disease (eg leukemia).

How is a blood smear done?

To make a smear

1. place a drop of the patient's blood near the end of a special slide which will act as a base, suitably washed, degreased in alcohol and carefully dried.
2. near the end of the drop of blood which is furthest away from the corresponding side of the basal slide, place the shorter side of a second ground slide slightly smaller than the previous one at an angle of about 40 °
3. pull the ground slide towards the drop, so that the latter spreads by capillarity along the contact line of the two slides.
4. push the ground slide towards the opposite end, with constant, rapid and light movement, in order to obtain the smear;
5. dry the smear, then proceed to the specific fixing and staining step
6. a specialist (laboratory doctor) examines the blood smear thus obtained under a microscope, making a general assessment of the cell populations present, such as number, size, shape and general appearance. As shown in the image at the beginning of the article, the streaked drop contains thousands of erythrocytes, hundreds of leukocytes and numerous platelets (cell fragments involved in clotting). Red blood cells are the predominant cells by number.

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