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A blood smear is a blood test that gives information about the number and shape of blood cells.
How the Test is Performed
A blood sample is needed.
The blood sample is sent to a lab. There, the lab technician looks at it under a microscope. Or, the blood may be examined by an automated machine. The smear shows the number and kinds of white blood cells (differential), abnormally-shaped blood cells, and gives a rough estimate of white blood cell and platelet counts.
How to Prepare for the Test
No special preparation is necessary.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
This test may be done as part of a general health exam to help diagnose many illnesses. Or, your doctor may order this test if you have signs of any of the following disorders: Any known or suspected blood disorder
- Hairy cell leukemia
- Monitoring the side effects of chemotherapy
Red blood cells normally are the same in size and color and have a lighter-colored area in the center. The blood smear is considered normal if there is:
- Normal appearance of cells
- Normal white blood cell differential
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
Abnormal results mean there is an abnormality in the size, shape, color, or coating of the red blood cells (RBCs).
Some abnormalities may be graded on a 4-point scale:
- 1+ means 25% of cells are affected
- 2+ means half of cells are affected
- 3+ means 75% of cells are affected
- 4+ means all of the cells are affected
Presence of target cells may be due to:
- Breakdown of red blood cells (decreased osmotic fragility)
- Deficiency of an enzyme called lecithin cholesterol acyl transferase
- Abnormalities of hemoglobin, the protein in red blood cells that carry oxygen (hemoglobinopathies)
- Iron deficiency
- Liver disease
- Spleen removal
Presence of sphere-shaped cells (spherocytes) may be due to:
- Low number of RBCs due to body destroying them (immune hemolytic anemia)
- Low number of RBCs due to some red blood cells shaped like spheres (hereditary spherocytosis)
- Increased osmotic fragility
Presence of fragmented cells (schistocytes) may be due to:
- Artificial heart valve
- Disorder in which the proteins that control blood clotting become overactive (disseminated intravascular coagulation)
- Infection in the digestive system producing toxic substances that destroy red blood cells, causing kidney injury (hemolytic uremic syndrome)
- Blood disorder that causes blood clots to form in small blood vessels around the body and leads to a low platelet count (thrombotic thrombocytopenic purpura)
Presence of a type of immature red blood cell called a normoblast may be due to:
- Cancer that has spread to bone marrow
- Blood disorder called erythroblastosis fetalis that affects a fetus or newborn
- Tuberculosis that has spread from the lungs to other parts of the body through the blood (miliary tuberculosis)
- Disorder of the bone marrow in which the marrow is replaced by scar (fibrous) tissue (myelofibrosis)
- Removal of spleen
- Severe breakdown of RBCs (hemolysis)
- Disorder in which there is excessive breakdown of hemoglobin (thalassemia)
The presence of burr cells (echinocytes) may indicate:
- Abnormally high level of nitrogen waste products in the blood (uremia)
The presence of spur cells (acanthocytes) may indicate:
- Inability to fully absorb dietary fats through the intestines (abetalipoproteinemia)
- Severe liver disease
The presence of teardrop-shaped cells may indicate:
- Severe iron deficiency
- Thalassemia major
The presence of Howell-Jolly bodies may indicate:
- Bone marrow does not produce enough healthy blood cells (myelodysplasia)
- Sickle cell anemia
The presence of Heinz bodies may indicate:
- Alpha thalassemia
- Congenital hemolytic anemia
- Disorder in which red blood cells break down when the body is exposed to certain drugs or the stress of infection (G6PD deficiency)
- Unstable form of hemoglobin
The presence of slightly immature red blood cells (reticulocytes) may indicate:
- Anemia with bone marrow recovery
- Hemolytic anemia
The presence of basophilic stippling may indicate:
- Anemia caused by bone marrow not producing normal blood cells due to toxins or tumor cells (myelophthisic process)
- Lead poisoning
The presence of sickle cells may indicate sickle cell anemia.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Bain BJ. The peripheral blood smear. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 160.
Vajpayee N, Graham SS, Bern S. Basic examination of blood and bone marrow. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 30.
Reviewed By: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.