What is a Blood Smear?

A blood smear is a sample of blood that's spread on a glass slide which is treated with a special stain. In the past, all blood smears were examined under a microscope by laboratory professionals. Now automated digital systems may be used to help examine blood smears.

The purpose of examining a blood smear is to check the size, shape, and number of three types of blood cells:

• Red blood cells, which carry oxygen from your lungs to the rest of your body
• White blood cells, which fight infection
• Platelets, which help your blood to clot

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

What is it used for?

A blood smear is used to help diagnose and monitor many conditions, such as blood disorders, sudden kidney failure, and treatment for certain cancers.

Why do I need a blood smear?

You may need a blood smear if you have abnormal results on a complete blood count (CBC). A CBC is a routine test that measures many different parts of your blood.

Your health care provider may order a blood smear if you have symptoms of a blood disorder, such as:

• Fatigue
• Jaundice, a condition that causes your skin and eyes to turn yellow
• Unusual bleeding, including nosebleeds
• Fever that lasts, or comes and goes
• Bone pain
• Anemia
• Easy bruising
• A spleen that's larger than normal

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.

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 results combined with your medical history, symptoms, and other test results to make a diagnosis.

Your blood smear results usually describe the appearance and number of your red blood cells, white blood cells, and platelets. Your results will describe anything unusual about your blood.

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

• Anemia
• Sickle cell anemia
• Hemolytic anemia, a type of anemia in which the body destroys red blood cells faster than they are replaced
• Thalassemia
• Bone marrow disorders
• Liver disease
• Cancer that has spread to the bone

White blood cell results that aren't normal may be a sign of:

• Infection or inflammation
• Allergies
• Leukemia
• Bone marrow disorders

Platelet results that aren't normal may be a sign of:

• Thrombocytopenia, a condition in which your blood doesn't have enough platelets, which increases the risk of bleeding
• Inherited platelet disorders (uncommon), such as Bernard-Soulier syndrome

If you have been very ill or stressed, or you have had a blood transfusion, the shape and number of your blood cells may be different than usual. So, a blood smear may not provide enough information for your provider to make a diagnosis. If any of your blood smear results are not normal, your provider will likely order more tests. Talk with your provider to learn more about your results.

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

Is there anything else I need to know about a blood smear?

A blood smear may be used to help find certain types of parasites in your blood which cause diseases, such as:

• Malaria, spread by bites from infected mosquitos
• Babesiosis, spread mainly by bites from infected ticks
• Chagas disease, spread mainly by bites from "kissing bugs" (triatomine)

Your provider may order the test if you live in or travel to areas where you might have been infected, and you have symptoms. Symptoms depend on the type of parasite. Common symptoms may include fever, fatigue, body aches, rash, and problems with digestion.