Blood Smear

What is a blood smear?

A blood smear is a test that's used to help diagnose and monitor a wide variety of blood disorders and other conditions that affect your blood. The test checks a sample of your blood under a microscope to see whether your blood cells look normal. It looks at your:

• Red blood cells (RBCs), which carry oxygen from your lungs to the rest of your body
• White blood cells (WBCs), which fight infection and are part of your immune system
• Platelets, which help your blood clot to stop bleeding when you have a cut or wound

To do a blood smear, a sample of blood is spread on a glass slide. The slide is treated with a special stain that makes the cells easier to see. Then a laboratory professional looks at the slide under a microscope to check the size, shape, and number of cells in your blood sample. Automatic computer programs may be used to help examine your blood smears.

Other names: peripheral smear, peripheral blood film, smear, blood film, manual differential, differential slide, blood cell morphology, blood smear analysis, leukocyte differential

What is it used for?

A blood smear alone can't diagnose a disease. But it can be used with other tests to help diagnose and monitor a variety of conditions, such as:

• Blood and bone marrow disorders
• Certain types of cancer, including those that affect your blood, bone marrow, and lymphatic system (a part of your immune system)
• Infections
• Certain parasites

Why do I need a blood smear?

You may need a blood smear if:

You had a complete blood count (CBC) with abnormal results. A CBC is a group of blood tests that measures the number and size of the different cells in your blood. It's often done as part of a routine checkup. A blood smear can confirm abnormal CBC results by checking the size and number of your cells. It can also give your health care provider more information about what your blood cells look like, including their color.

You have symptoms of a condition that affects your blood cells. Symptoms may include:

• Fatigue
• Jaundice, a condition that causes your skin and eyes to turn yellow
• Nosebleeds or other unusual bleeding
• Easy bruising
• Fever
• Bone pain
• Swollen lymph nodes, also called "swollen glands"

You are being treated for a condition that affects your blood cells, such as anemia or blood cancer. A blood smear can help find out if your treatment is working.

You have symptoms of certain diseases caused by parasites, and you live in or travel to areas where those diseases are common. These diseases include:

• Malaria, which is spread by bites from infected mosquitos
• Babesiosis, which is spread mainly by bite from infected ticks

Symptoms of these parasitic diseases can be like having the flu. They include, fever, chills, body aches, nausea, and fatigue.

What happens during a blood smear?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Sometimes, the blood sample will be taken by pricking your finger with a needle. When testing a baby, the heel is pricked.

Will I need to do anything to prepare for the test?

You don't need any special preparation for a blood smear. If your provider has ordered other blood tests, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

The results of a blood smear alone usually can't diagnose a medical condition. Your provider will use your test results combined with your medical history, symptoms, and the results of other tests to make a diagnosis.

Your blood smear results will describe what your blood cells look like and how many there are. The results will also show anything else that's unusual about your blood, for example, if any parasites were found.

Red blood cell results that aren't normal, may be a sign of conditions, such as:

• Anemia
• Sickle cell disease
• Hemolytic anemia, a type of anemia that causes your body to destroy red blood cells faster than it makes them
• Thalassemia
• Bone marrow disorders

White blood cell results that aren't normal may be a sign of conditions, such as:

• Infection
• Leukemia
• Bone marrow disorders

Platelet results that aren't normal may be a sign of conditions, such as:

• Thrombocytopenia, a lower-than-normal number of platelets. This may increase your risk of bleeding.
• Thrombocytosis, a higher-than-normal number of platelets that is caused by another disease or condition, such as anemia, or an infection. Having too many platelets can cause blood clots to form in your blood vessels (the tubes that carry blood in your body).
• Thrombocythemia, a higher-than-normal number of platelets that is caused by a problem with the cells in your bone marrow that make platelets.

Other conditions can also cause abnormal blood smear results, including:

• Severe illness
• Stress
• Blood transfusions

If your blood smear results are not normal, your provider may order more tests to help find the cause. Talk with your provider to learn more about your results.

Learn more about laboratory tests, reference ranges, and understanding results.