Blood Smear

Blood smear analysis is one of the laboratory tests used to diagnose cold agglutinin disease (CAD), a type of autoimmune hemolytic anemia.

What is a blood smear?

In cold agglutinin disease, autoantibodies called cold agglutinin produced by the immune system attack the red blood cells upon exposure to cold temperatures. This causes the red blood cells to clump together and eventually leads to their premature breakdown.

Some tests detect the levels of cold agglutinins and the number of red blood cells for CAD diagnosis. Blood smear analysis provides additional visual confirmation of the clumping (agglutination) of red blood cells that leads to their disintegration (lysis). However, it is not sufficient to diagnose CAD alone, and it cannot differentiate between other disorders that may cause red blood cell clumping.

How is a blood smear performed?

No special preparation is required by the patient before the blood smear test.

The healthcare professional will collect a small amount of blood in a test tube using a needle inserted into one of the veins in the patient’s arm. Blood sample collection may take about five minutes.

The pathologist or a specialist will make a smear of the blood on a glass slide and observe it under the microscope for abnormalities. Agglutination of red blood cells and their decreased count in the blood smear, along with findings from the tests that detect cold agglutinin levels, help in confirming CAD diagnosis.

After the test

The patient may feel a small prick when the needle is inserted. He or she may feel sore at the site, but this will subside soon.

Additional information

Other names of the blood smear test include peripheral blood film,  differential slide, and blood cell morphology.

A blood smear test also may be ordered if other blood-related disorders are suspected, such as parasitic infections, unusual bleeding, jaundice, fatigue, fever, etc. Doctors may use a blood smear test to track the effectiveness of treatments such as chemotherapy.


Last updated: Aug. 15, 2019


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