A peripheral blood smear is a laboratory test that provides a microscopic window into the components of your blood. This examination is insightful for genetic conditions like sickle cell disease, which is characterized by an abnormality in hemoglobin, the oxygen-carrying protein in red blood cells. The disease causes red blood cells to change from flexible discs into a rigid, curved shape under certain conditions. A blood smear allows a trained professional to visually inspect the size, shape, and characteristics of these cells, offering direct evidence of the disease’s physical manifestation.
The Blood Smear Procedure
The process begins with collecting a small blood sample, obtained through a finger prick or a venipuncture from a vein in the arm. A single drop of this blood is carefully placed on a clean glass microscope slide. Another slide is used to spread the drop, creating a thin layer of blood that is ideally one cell thick. This technique, called a wedge smear, separates the cells so they can be examined individually.
After the blood has air-dried on the slide, it is treated with a special stain to make the different cellular components visible. The most common stain used for this purpose is the Wright-Giemsa stain, which gives various parts of the blood cells distinct colors. A trained laboratory scientist or pathologist then places the stained slide under a microscope. They systematically scan the smear to evaluate the appearance and quantity of red blood cells, white blood cells, and platelets.
Identifying Sickle Cells Under the Microscope
A blood smear from a healthy individual reveals uniform red blood cells. They appear as round, biconcave discs with a pale area in the center, known as central pallor. This shape provides a large surface area for oxygen exchange and allows the cells to be flexible enough to squeeze through the body’s narrowest capillaries. In contrast, the blood smear from a person with sickle cell disease is different.
The defining feature is the presence of drepanocytes, which are the sickled red blood cells. These cells are elongated, lack central pallor, and are curved into a crescent or sickle shape with pointed ends. This deformation is a direct result of a genetic mutation that produces an abnormal form of hemoglobin called Hemoglobin S (HbS). When oxygen levels are low, HbS molecules clump together into rigid polymers, forcing the red blood cell to contort into the sickle form. These misshapen cells are the primary visual confirmation of the disease on a smear.
Additional Cellular Findings
A pathologist examining a sickle cell blood smear often finds more than just the characteristic sickled cells, providing a broader picture of the disease’s impact. One common finding is the presence of Howell-Jolly bodies, which are small, dark remnants of nuclear DNA inside red blood cells. In a healthy person, the spleen filters these remnants out, so their presence on a smear indicates poor spleen function, a frequent complication of sickle cell disease.
Another observation may be an increase in target cells, which are red blood cells that resemble a bullseye. The smear may also show nucleated red blood cells, which are immature red cells released prematurely from the bone marrow. Their appearance signals that the bone marrow is working overtime to produce more red blood cells to compensate for the chronic anemia caused by the premature destruction of fragile sickle cells.
Diagnostic Role and Limitations
While a peripheral blood smear is a screening tool, its findings are suggestive, not definitively diagnostic of sickle cell disease. The presence of numerous sickled cells provides strong evidence, but a conclusive diagnosis requires more specific tests. The primary diagnostic test is hemoglobin electrophoresis, a lab technique that separates different types of hemoglobin to identify and quantify abnormal Hemoglobin S. Genetic testing for the specific gene mutation is also used for a definitive diagnosis.
The blood smear has limitations. For instance, individuals with sickle cell trait, who carry only one copy of the sickle cell gene, have a normal-looking peripheral blood smear because their red blood cells do not sickle under normal oxygen conditions. The smear is also used to monitor the disease, as a significant increase in the percentage of sickled cells can indicate a developing vaso-occlusive crisis, where the rigid cells block blood flow.