Blood Smear

A blood smear allows the evaluation of white blood cells (WBCs, leukocytes), red blood cells (RBCs, erythrocytes), and platelets (thrombocytes). These cell populations are produced and mature in the bone marrow and are eventually released into the bloodstream as needed. WBC’s main function is to fight infection, while RBCs carry oxygen to the tissues. Platelets appear as small cell fragments and, when activated, form a plug as one of the first steps in blood clotting. The number and type of each cell present in the blood is dynamic but generally maintained by the body within specific ranges. Values can fluctuate at times of illness or stress; intense exercise or smoking can also affect cell counts.

A peripheral blood smear is a snapshot of the cells that are present in the blood at the time that the sample is obtained. To create a blood smear, a single drop of blood is spread in a thin layer across a glass slide, dried, and then stained with a special dye. Once the stain has dried, the slide is evaluated under a microscope by a laboratorian.

The drop of blood on the slide contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. Under the microscope, the stained WBCs can be easily seen and counted to estimate the number of each type of cell present. In addition, one can compare their size, shape, and general appearance to the established appearance of “normal” cells. It is possible to distinguish between the five different types of WBCs and to determine their relative percentages by counting 100 consecutive cells. During this examination, one can also evaluate the size, shape, and color (indicators of hemoglobin content) of the RBCs and also estimate the number of platelets present.

A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a finger, ear or, in the case of an infant, a heel.

No test preparation is needed.