Peripheral Smear (PS) Test

The Peripheral Smear test is used to examine the characteristics of your blood cells, including red blood cells (RBCs), white blood cells (WBCs), and platelets. The test is also called a peripheral blood film, blood film, blood smear analysis, manual differential, differential slide, or leukocyte differential test.

In a blood smear test, a drop of blood is spread onto a glass slide (two types of smears: thick & thin). After drying the slide, staining is done, and the slide is examined under a microscope to check the size, shape, and number of cells in the sample. The test helps find certain blood cell abnormalities that may not be detected by a routine blood test, called the complete blood count (CBC). Overall, this test is very important for evaluating and diagnosing blood disorders and abnormalities in your blood components.

A Peripheral Smear test is recommended if you have any of the following symptoms of blood cell disorders: 

• Unusual bleeding episodes, like nose bleeding 
• Yellowing of the eyes and skin, known as jaundice 
• Fever 
• Swollen lymph nodes 
• Ongoing tiredness or weakness 
• Skin that bruises easily 

The test is also advised for individuals with:

• Blood cell disorders – The test can be used to assess whether treatment for anaemia (deficiency of healthy RBCs) is effective. 
• Symptoms of parasitic diseases – This is especially important for people living in or travelling to regions where parasitic diseases like malaria and babesiosis (a rare infection caused by ticks, that infects RBCs) are common. 
• Abnormal CBC test results – This is a routine blood test used to evaluate the quantity and size of different blood cells. A blood smear provides a closer look at these cells by examining any irregularities in their appearance, number, and size.

The frequency of the Peripheral Smear test is mentioned in the table below: 

A Peripheral Smear test cannot alone diagnose a disease, but it can be used with other tests to assess various conditions, such as: 

• Blood and bone marrow disorders – This includes anaemia, sickle cell disease (a genetic disorder that affects haemoglobin, a protein that carries oxygen through RBCs), thalassemia (deficiency of healthy RBCs and haemoglobin), thrombocytopenia (lower than normal number of platelets), thrombocytosis (higher than normal number of platelets), and thrombocythemia (a disorder that causes the body to produce more platelets).
• Cancers affecting the blood or lymphatic system – Leukaemia comprises a group of cancers that develop in the blood and bone marrow. The blood cells could be lymphocytes (WBCs) or myeloid cells (immature cells that develop into different types of blood cells).
• Parasitic diseases – Malaria, babesiosis, and other blood-borne parasitic infections are detectable on a blood smear. 

What to Expect Before the Peripheral Smear (PS) Test

No fasting is required for a standalone Peripheral Smear (PS) test. You can eat and drink normally. However, if it is part of a broader package that includes fasting tests, your doctor may advise fasting for 8-12 hours. Avoid strenuous exercise for 24 hours before the test, as intense physical activity can temporarily raise Peripheral Smear (PS) levels and affect interpretation. Always inform your doctor about any ongoing medications.

What to Expect During the Blood Collection

A phlebotomist cleans the skin with an antiseptic, then places an elastic band around your upper arm to make the veins easier to access. A small needle is inserted into the vein; you may feel a brief pinch. Blood is collected into a labelled tube in under a minute.

What to Expect After the Peripheral Smear (PS) Test

A small cotton swab and bandage are placed over the site to stop any minor bleeding. You can return to normal activities immediately. Minor bruising is normal and disappears quickly. If dizzy, sit and rest for a few minutes.

In the Peripheral Smear test, several different measurements are taken, including: 

• RBC morphology – Cell size, shape, colour, intracellular inclusions, and pathological arrangement 
• WBC morphology – Counts, size, shape, and colour of the cells (neutrophils, lymphocytes, eosinophils, monocytes, and basophils) 
• Platelet morphology – Size and count 

The Peripheral Smear test normal values are indicated in the table below: 

RBC characteristics

WBC characteristics 

Platelet characteristics 

Note: 

• The normal range may vary slightly depending on the laboratory and individual health conditions. 

The table below provides a general interpretation of the PS test results: 

Like any other medical test, this test also has certain risks and limitations. 

Risks 

•  The PS test is generally safe. You may feel slight pain, bruising, or discomfort when the needle is inserted. However, risks like mild bruising are rare and resolve quickly on their own.

Limitations 

• A PS test is usually insufficient to diagnose a medical condition on its own; doctors need to consider the patient’s medical history and symptoms. 
• If the blood smear is poorly prepared, abnormal findings may be missed or wrongly interpreted.

Depending on your symptoms and test results, your doctor may recommend certain additional tests to better understand your condition. These include: 

• Haptoglobin test
• Bone marrow tests
• Ferritin blood test
• Haemoglobin electrophoresis
• Iron tests
• Bilirubin blood test
• Liver function test