Definition & Overview

A blood smear is a diagnostic procedure used in the evaluation and diagnosis of a variety of medical conditions. It is a standard procedure that if doctors suspect a disease or infection, a blood smear is typically the first diagnostic test to be performed.

During a blood smear, laboratory technicians will examine three main components of blood:

  • Platelets
  • White blood cells
  • Red blood cells

Platelets are cells that enable blood clot formation to prevent blood from flowing freely in case of a wound, playing an important role in the healing process. Meanwhile, white blood cells are an important part of the immune system as they fight off foreign invaders and infectious diseases. Red blood cells, on the other hand, deliver oxygen to various organs enabling them to function normally.

Technicians will view these components under a microscope to check for any abnormalities, such as in the form of the cells or their amount. Any abnormality of these cells results in complications in the delivery of oxygen, in the immune system, or in blood clotting.

Who Should Undergo & Expected Results

A blood smear is an integral part of the diagnostic process. In fact, almost every patient will require a blood smear. However, some of the most common reasons why a doctor would request such a procedure are to identify the reasons why a patient is experiencing anemia, jaundice, abnormal bruising, bone pain, rashes, infections, weight loss, or recurring flu symptoms.

It’s important to understand that the results of blood smear can be affected by multiple factors. It’s important for you to inform the doctor of any medications, vitamins, or supplements you’re taking if you’re requested to undergo such a procedure.

Some of the most common types of medications that will affect test results are anticoagulants and medications for other diseases. If you’ve been diagnosed with a certain disease prior to undergoing a blood smear, make sure that your doctor is informed to avoid erroneous results.

How Does the Procedure Work?

A blood smear will require a sample of your blood. A phlebotomist is a person who is trained in drawing blood samples and will be performing this part of the procedure. The technician will first place a tourniquet above the location where blood will be drawn. He or she will then inject a needle directly into a vein and begin drawing blood. You might feel a stabbing pain when the needle is inserted, but this will last just a couple of seconds. As soon as blood is being drawn, the pain will recede significantly.

Once the technician has obtained an adequate amount of blood for the test, the needle will be withdrawn and a cotton ball will be placed over the puncture site. Applying a small amount of pressure on the cotton ball will force the blood to clot in a matter of second. After a minute or two, you’ll be able to remove the cotton ball, as the bleeding would have stopped by that time.

The blood sample will then be sent to a laboratory for examination. The results of the test will be forwarded to the diagnosing physician who will interpret and discuss the results with the patient.

If there are abnormalities in white blood cells, these could indicate conditions, such as malaria, hepatitis C, fungal infections, parasitic infections, HIV, lymphoma, acute leukemia, and lymphoproliferative disorders. To confirm the findings, the diagnosing physician will combine the results of a blood smear with other test results to confirm a particular disorder.

Platelets abnormalities could indicate myeloproliferative disorders, thrombocytopenia, liver disease, hypothyroidism, or a kidney disease.

Abnormal red blood cell result can indicate anemia, sickle cell disease, polycythemia, vitamin B12 deficiency, or hemolytic-uremic syndrome.

Blood smear result alone will not confirm the presence of a particular disease or disorder. However, the results will point the doctor in the right direction. More tests such as a bone marrow biopsy, Complete Blood Count (CBC), hemoglobin variant, absolute lymphocyte count, and a delayed hypersensitivity skin test will be needed for the doctor to confirm a certain disease.

Possible Complications and Risks

A blood smear is an extremely safe procedure, but there are risks involved. If the patient is not accustomed to seeing blood, he or she may faint during the procedure. If a good amount of blood is drawn, it’s possible to feel a little light headed, but this should pass after a few minutes rest.

Other risks include excessive bleeding, hematoma, and an infection. However, infections are rare because hospitals use sterile instruments when withdrawing blood.

Additionally, it’s important to realize that people have different sized veins and arteries. It may be easier to draw blood in some people, but others may find it more difficult, especially if they’re dehydrated. Do not be surprised if a technician needs to insert the needle several times before finding a suitable vein to draw blood from.


  • 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

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