Blood-tests Explained


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This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Blood Smear


At a Glance

Why Get Tested?

To evaluate your red blood cells (RBCs), white blood cells (WBCs), and platelets, to distinguish between the different types of WBCs, and to determine their relative percentages in the blood; to help detect, diagnose, and/or monitor a range of deficiencies, diseases, and disorders involving blood cell production, function, and lifespan

When to Get Tested?

When complete blood count (CBC) and/or automated WBC differential results are abnormal or when you have signs and symptoms that a health practitioner suspects are due to a condition affecting your blood cells

Sample Required?

A blood sample drawn from a vein in your arm or by pricking a finger, ear or, in the case of an infant, a heel

Test Preparation Needed?

None

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.

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

No test preparation is needed.

The Test