What Is A Peripheral Blood Smear

To diagnose an illness, a doctor might request different tests to reach a definitive diagnosis, one versatile test that can be used to help diagnose many different conditions is the peripheral blood smear. In this test, a sample of blood is drawn and spread on a glass slide to examine under a microscope and detect any abnormalities. In this article, you will learn about the peripheral blood smear, when it is required, how it is prepared, what a doctor sees and looks for, and what conditions it might help a doctor diagnose.

A doctor might ask for a peripheral blood smear (also called peripheral blood film) to visualise blood cells under a microscope and detect any changes in their shape, size, distribution, and colour, which might indicate a certain illness. These smears are prepared from a blood sample that a clinical laboratory specialist will obtain from you and test under a microscope. Although informative, peripheral blood smears are rarely used alone to make a diagnosis and are mostly coupled with a complete blood count (CBC) and other tests that together can help your doctor make a final diagnosis.¹

How is blood collected for a peripheral blood smear?

To prepare the blood smear, the blood is first collected using one of two ways:

Capillary blood: The blood sample can be taken from the small blood vessels near your skin called capillaries, this is obtained by a fingerstick. The healthcare provider will:

• Ask you if you have any phobias or allergies and if you have ever fainted during a blood draw or injection
• Explain the procedure to you
• Select a finger to puncture, usually the middle or ring finger, or heel in infants
• Apply alcohol to your finger and allow it to dry (about 30 seconds)
• Puncture the tip of your finger with a lancet
• Wipe away the first drop of blood
• Press a glass slide to the puncture site; they might gently squeeze your finger to obtain the blood

Venous blood: The blood can also be taken from your veins, this is obtained by a venipuncture.² The healthcare provider will:

• Ask you if you have any phobias or allergies and if you have ever fainted during a blood draw or injection
• Explain the procedure to you
• Extend your arm and inspect the forearm to locate a vein
• Apply a band called a tourniquet above the puncture site, this will make the vein more visible
• Apply alcohol to your skin and wait for it to dry (about 30 seconds)
• Ask you to form a fist to make the vein more prominent and then they will enter the needle into your vein and start collecting the blood
• Release the tourniquet, withdraw the needle, and then apply gentle pressure on the puncture site with a gauze or cotton ball
• Ask you to keep pressing for several minutes while your arm is still extended

How is a peripheral blood smear prepared?

The specialist will place a small drop of blood on a glass slide, spread it, stain it with a special stain to colour the blood cells, and finally, use a microscope to look at your blood cells and record any abnormalities.²

What will the examiner look at in a peripheral blood smear?

An adult human body contains around 5 litres of blood. Blood consists of plasma and blood cells; these cells are produced from the bone marrow (the spongy part in the centre of your bones) and then released into circulation. The examiner will be looking at these cells in your blood smear, and any change in their size, shape, or number can indicate a specific condition or disease.³ These cells are:

Red blood cells (RBCs), also known as erythrocytes. These cells comprise almost half of your blood’s volume and contain a red protein called haemoglobin, hence the red colour of blood. Their main function is to carry oxygen from the lungs to body tissue and carry carbon dioxide from tissue back to the lungs to be exhaled.³

Under the microscope: The normal RBCs are concave in shape but look like small round cells under the microscope. The haemoglobin is distributed in the edges of the cells which leaves a pale area in its centre.

White blood cells (WBCs), also known as leukocytes, are far fewer in number compared to RBCs. There are five different types of WBCs that differ in function and shape. WBCs are very important in fighting infection, cancers, and allergies as they are part of our immune system.³

Under the microscope: They are fewer in number compared to RBCs but are bigger in size. The five different types of WBCs each have different morphologies and distinct nuclei.¹

Platelets, also known as thrombocytes. Platelets’ main function is to form a clot at the site of a cut to stop the bleeding.³

Under the microscope: They look like small fragments, with no nucleus, and are about one-third the size of a red blood cell. They are higher in number compared to WBCs, but less than RBCs.¹

When might a peripheral blood smear be necessary?

A doctor might request a peripheral blood smear in these cases:¹

• Unexplained increase or decrease in the number of red blood cells, white blood cells, or platelets, seen from a complete blood count
• Hemolysis (breakdown of RBCs)
• Jaundice (yellowing of the skin and mucous membranes)
• Bone pain
• Splenomegaly (enlargement of the spleen)
• To test the efficacy of certain types of treatment you might be taking

What conditions can the peripheral blood smear detect?

The PBS is used in combination with other tests to reach a final diagnosis; it can help in detecting the following:

Anaemia

Anaemia is a red blood cell disorder - it occurs when there are not enough RBCs in your circulation or when there is a problem with their function.

Signs and symptoms include:

• Tiredness and dizziness
• Shortness of breath
• Pale skin
• Headache

There are many types of anaemia, and the change in the size, shape, and colour of the RBCs in the peripheral blood smear might indicate the presence of a specific type of anaemia. For example:

Iron deficiency anaemia is the most common form of anaemia and occurs due to blood loss. In the PBS, the RBCs will look smaller in size and they will have less colour as the pale centre will be wider.⁴

Vitamin deficiency anaemia occurs due to a lack of vitamins B12 and B9 in a person's diet. In the PBS, the RBCs are larger in size and the nucleus of the neutrophils (a type of white blood cells) are more segmented than usual.⁵

Sickle-cell anaemia is an inherited condition where the RBCs have abnormal haemoglobin. In the PBS, many of the RBCs will have a distinct crescent shape.¹

Leukaemia

Leukaemia is a cancer of the blood and bone marrow where the number of WBCs increases, but many are immature and cannot do their job in fighting infections properly; in addition, they affect the bone marrow’s ability to produce RBCs and platelets.

Signs and symptoms include:

• Flu-like symptoms (e.g. night sweats, chills, and fever)
• Swelling and bleeding in the gums
• Tiredness
• Weight loss

In the peripheral blood smear, the examiner will see a high number of WBCs with abnormal shapes compared to those found in the smear of a healthy person.

Lymphoma

Lymphoma is a type of cancer that affects the lymph system, which is responsible for getting rid of excess fluids from the body and for producing immune cells. A specific type of white blood cell, called lymphocytes, starts to multiply uncontrollably and accumulate in the lymph nodes (i.e. in the neck and armpits).

Signs and symptoms include:

• Swelling in the lymph nodes
• Tiredness
• Weight loss
• Difficulty breathing
• Fever

In the peripheral blood smear, the examiner will notice an increase in WBC numbers and changes in their normal shape.¹

Malaria

Peripheral blood smears are the gold standard in the diagnosis of malaria, which is a parasitic disease that is transmitted by the bite of a mosquito. You can contract the disease when you travel to areas where it is endemic (for a full list of endemic countries, click HERE).

Signs and symptoms include:

• Chills
• Fever
• Headache
• Sweats
• Muscle pain

The parasite infects the red blood cells and in a stained peripheral blood smear of an infected person, the parasite can be seen inside the RBCs. It is important that this infection is detected and treated as soon as possible as certain types of the parasite can cause death within days (CDC).

Summary

The peripheral blood smear (PBS) is a diagnostic test where a specialist obtains a blood sample and spreads a small amount of blood on a glass slide, stains it, and then examines it under the microscope to look for abnormalities in the size, shape, or number of blood cells. 

The PBS results alone are not enough for a diagnosis, and the doctor will use it alongside other tests to reach a final diagnosis. PBSs can help with diagnosing anaemias, cancers such as leukaemias and lymphomas, and blood parasites such as malaria.