A blood smear is a blood test used to look for abnormalities in blood cells. The three main blood cells that the test focuses on are:
- red cells, which carry oxygen throughout your body
- white cells, which help your body fight infections and other inflammatory diseases
- platelets, which are important for blood clotting
The test provides information on the number and shape of these cells, which can help doctors diagnose certain blood disorders or other medical conditions.
Irregularities in the number or shape of your red blood cells can affect how oxygen travels in your blood. These abnormalities are often caused by a mineral or vitamin deficiency, but they can also be caused by inherited medical conditions, such as sickle cell anemia.
White blood cells are an integral part of your body’s immune system, which is a network of tissues and cells that help your body fight infection. Having too many or too few white blood cells can indicate a blood disorder. Disorders affecting these cells often result in the body’s inability to eliminate or control infections or other inflammatory problems.
Abnormalities in the shape or number of white blood cells may be signs of a platelet disorder. Platelet disorders affect your blood’s ability to clot, which can lead to excessive or prolonged bleeding or blood clotting. They often occur when the body produces too many or too few platelets.
The blood smear test is often done to diagnose conditions that are causing:
- unexplained jaundice
- unexplained anemia (low levels of normal red blood cells)
- abnormal bruising
- persistent flu-like symptoms
- sudden weight loss
- unexpected or severe infection
- skin rashes or cuts
- bone pain
Your doctor may order blood smear tests on a regular basis if you’re being treated for a blood-related condition.
Before the test, it’s important to tell your doctor about any prescription or over-the-counter medications, supplements, and vitamins you’re currently taking. Certain medications can affect your test results. These include NSAIDs, some antibiotics, and glucocorticosteroids.
Additionally, if you’re regularly taking anticoagulant therapy, such as warfarin, (Coumadin), you will be at risk for increased bleeding associated with the blood draw.
You should also tell your doctor about any existing medical conditions, such as hemophilia. Certain medical disorders, regular blood product transfusions, and the presence of certain types blood cancer will produce abnormalities on the blood smear result.
It’s important to discuss these matters with your doctor before the blood smear to avoid a possible diagnostic error.
The blood smear is a simple blood test. A phlebotomist, a person specifically trained to draw blood, first cleans and sterilizes the injection site with an antiseptic. They then tie a band above the venous site where your blood will be drawn. This causes your veins to swell with blood. Once they find a vein, the phlebotomist inserts a needle directly into the vein and draws blood.
Most people feel a sharp pain when the needle first goes in, but this quickly fades as the blood is drawn. Within a couple of minutes, the phlebotomist removes the needle and asks you to apply pressure to the site with gauze or a cotton ball. They next cover the puncture wound with a bandage, after which you’re free to leave.
A blood test is a low-risk procedure. However, minor risks include:
- fainting from the sight of blood due to vasovagal syncope
- dizziness or vertigo
- soreness or redness at the puncture site
A blood smear is considered normal when your blood contains a sufficient number of cells and the cells have a normal appearance. A blood smear is considered abnormal when there’s an abnormality in the size, shape, color, or number of cells in your blood. Abnormal results may vary depending on the type of blood cell affected.
Red blood cell disorders include:
- iron-deficiency anemia, a disorder in which the body doesn’t produce enough normal red blood cells due to iron deficiency
- sickle cell anemia, an inherited disease that occurs when red blood cells have an abnormal crescent shape
- hemolytic uremic syndrome, which is commonly triggered by an infection in the digestive system
- polycythemia rubra vera, a disorder that occurs when the body produces an excessive number of red blood cells
Disorders related to white blood cells include:
- acute or chronic leukemia, a type of blood cancer
- lymphoma, a form of cancer that affects the immune system
- HIV, a virus that infects white blood cells
- hepatitis C virus infection
- parasitic infections, such as pinworm
- fungal infections, such as candidiasis
- other lymphoproliferative diseases, including multiple myeloma
Disorders affecting platelets include:
- myeloproliferative disorders, a group of disorders that cause blood cells to grow abnormally in the bone marrow
- thrombocytopenia, which occurs when the number of platelets is very low due to an infection or other disease
A blood smear can also indicate other conditions, including:
Normal and abnormal ranges can vary among labs because some use different instruments or methods to analyze the blood sample. You should always discuss your results in more detail with your doctor. They’ll be able to tell you if you need more testing.