Peripheral Blood Smear (White Blood Cell Differential)

What is a peripheral blood smear?

A peripheral blood smear is the examination under a microscope of a drop of blood that has been spread in a thin layer on a glass slide and stained with special dyes. This examination allows the cells counted by automated analyzers to also be assessed for shape, size and appearance. The white blood cell differential, in turn, refers to the percentage distribution of white blood cells by type.

The specialist examining the smear counts white blood cells such as neutrophils, lymphocytes, monocytes, eosinophils and basophils and checks for immature or abnormal-looking cells. The size, color and shape of the red blood cells, as well as the approximate number of platelets and whether they are clumping, are also assessed. This provides important clues to the cause of the numerical changes seen on the complete blood count.

When is a peripheral blood smear ordered?

A peripheral blood smear is usually ordered when there is an unexplained change in complete blood count results or when the automated analyzer raises a flag. It is a valuable test for determining the type of anemia, getting an idea of the nature of an infection and making an initial assessment when a blood disorder is suspected. In some patients undergoing treatment, it may also be repeated to monitor changes in the cells. A similar microscopic examination is also used to look for certain blood parasites, such as malaria.

• Unexpected white blood cell, red blood cell or platelet counts on the complete blood count
• Unexplained anemia, weakness or pallor
• Prolonged fever, frequent infections or enlarged lymph nodes
• Easy bruising, or confirmation of a low platelet count
• Jaundice and suspected breakdown of red blood cells (hemolysis)
• Treatment monitoring in people diagnosed with a blood disorder

How should you prepare for a peripheral blood smear?

No fasting or special preparation is needed for this test. Cortisone-type medicines, chemotherapy, some antibiotics and a recent infection can change the cell distribution, so sharing this information makes interpretation easier. Intense exercise and marked stress can also temporarily affect white blood cells.

Blood is drawn from a vein in the arm into a purple-top (EDTA) tube and is usually analyzed from the same sample as the complete blood count. Because the appearance of the cells can deteriorate over time, the smear is preferably prepared within a few hours of the sample being taken. The blood needed for the smear can be taken at your home or workplace during a nurse visit.

What does the peripheral blood smear result mean?

This test does not give a single numerical value; the result is reported as the percentages of the white blood cell types, together with comments on the appearance of the cells. An increase in neutrophils often suggests a bacterial infection or inflammation, an increase in lymphocytes viral infections, and an increase in eosinophils allergy or parasitic infections. If the report mentions terms such as atypical lymphocytes, immature cells or blasts, your doctor may plan further tests. Platelet clumping in the tube can look like a falsely low platelet count on the analyzer; the smear helps to tell these apart.

Small, pale-looking red cells may be associated with iron deficiency or thalassemia carrier status, and large-looking ones with vitamin B12 or folate deficiency. The cell distribution changes with age; for example, the proportion of lymphocytes is higher in young children than in adults, and the assessment is made according to the laboratory's own criteria. Your doctor is the person who will evaluate what the smear findings mean for you, together with your symptoms and complete blood count results.