blood analysis

Blood smear

What is it and why is it performed?

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 a 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, the shape of these cells can also be evaluated; so, for example, the smear of a subject suffering from sickle-cell anemia shows the typical sickle conformation of the red blood cells, which are instead too small in the case of microcytic anemia and too large in the presence of macrocytic anemia. Furthermore, by means of the smear it is possible to evaluate the hemoglobin content of the erythrocytes, establishing on the basis of color if 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, filaria and sleeping sickness. We are therefore talking about a valuable diagnostic aid, a second level test that requires however a certain manual skill and experience on the part of the operator. In general, the execution of the smear follows a first-line examination, such as the blood count, considered suspect. Blood counts is an automated test that determines the number of blood cells; the smear is instead a manual examination, which involves the observation and direct counting of these cells.

When making the smear

Your doctor may prescribe a blood smear when:

the blood count and the differential leukocyte count indicate a suspicion of abnormalities or immature cells;

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

you want to monitor the effectiveness or side effects of a therapy (for example chemotherapy) or the evolution of a hematological pathology (eg leukemia).

How do you make a blood smear?

To make a smear

  1. place a drop of patient's blood near the end of a special glass that will act as a base, properly washed, degreased in alcohol and carefully dried.
  2. near the end of the farthest drop of blood to the corresponding side of the basal slide, place, with an inclination of about 40 °, the shorter side of a second cut glass slightly smaller than the previous one
  3. pull the ground glass towards the drop, so that the latter spreads by capillary action along the contact line of the two glass slides.
  4. push the ground glass towards the opposite end, with constant, rapid and light movement, so as to obtain the smear;
  5. dry the smear, then proceed with the fixing and specific coloring step
  6. a specialist (laboratory doctor) examines the blood smear thus obtained under a microscope, carrying out a general evaluation on 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 coagulation). Red blood cells are, by number, the predominant cells.