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Blood Smear

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Also known as: Peripheral Smear; Blood Film; Manual Differential; Differential Slide; Red Blood Cell Morphology; Erythrocyte Morphology; Leukocyte Differential
Formal name: Peripheral Blood Smear

The Test Sample

What is being tested?

A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and examined under a microscope by a trained laboratorian. It is a snapshot of the cells that are present in the fluid portion of the blood (plasma) at the time the sample is obtained. The blood smear allows for the evaluation of these cells:

  • White blood cells (WBCs, leukocytes) — help fight infections
  • Red blood cells (RBCs, erythrocytes) — carry oxygen to tissues
  • Platelets (thrombocytes) — small cell fragments that are vital to proper blood clot formation

These cell populations are produced and mainly mature in the bone marrow and are eventually released into the bloodstream as needed. The number and type of each cell present in the blood is dynamic but is generally maintained by the body within specific ranges.

The drop of blood on the slide used for a blood smear contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. Under the microscope, the stained WBCs can be easily seen and the number and type of cells present can be estimated. The laboratorian can compare their size, shape, and general appearance to the established appearance of "normal" cells. It is also possible to distinguish between the five different types of WBCs and to determine their relative percentages (manual differential). During this examination, the laboratorian can also evaluate the size, shape, and color (indicators of hemoglobin content) of the RBCs (RBC morphology) and also estimate the number of platelets present.

A variety of diseases and conditions can affect the number and appearance of blood cells. Examination of the blood smear can be used to support findings from other tests and examinations. For example, RBCs that appear larger and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of WBCs that are not fully mature may add to information from other tests to help make a diagnosis of infection, malignancy, or other conditions.

How is the sample collected for testing?

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.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.