Peripheral Smear (PS) Test

The peripheral smear test, though basic, is a very important test to diagnose a number of diseases. This test checks the characteristics of blood components (RBCs, WBCs and platelets) to identify any abnormality. These findings help in the diagnosis of multiple blood and blood-related disorders.

In this test, a sample of blood is spread on a specific slide and this slide is observed under a microscope to check for various blood parameters. The peripheral smear test has great significance in the diagnosis of many diseases, for it identifies abnormalities in blood components that help to relate to the presence of consequential diseases. It can also help to check the therapy response.

Peripheral blood film is advised in conditions where the disease or symptoms indicate a blood disorder or blood-related disorder. It is also prescribed in cases where the diagnosis requires blood film results as supportive evidence.

Generally, RBC abnormalities suggest indications like anaemia, thalassemia, hemolytic anaemia, bone marrow disorders, sickle cell anaemia, etc. WBC abnormalities indicate leukaemia, infections or allergies. Platelet abnormalities include thrombocytopenia (lower count of platelets) or thrombocytosis (higher count of platelets).

In India, there is an alarming status of haematological disorders (blood disorders). Acquired diseases like anaemia show greater presence as compared to inherited disorders like thalassemia. However, the incidence of thalassemia is also considerably high.

Correct diagnoses with precise physical and laboratory examinations constitute the necessary step to tackle this situation. A peripheral smear test is one of the important diagnostic tests performed for haematological disorders. Though there have been many advances in diagnostics, this test retains its function and importance.

Other names of peripheral smear test

• Manual differential or differential slide
• Blood smear analysis
• Blood cell morphology
• Peripheral blood film
• A blood film or smear examination

What does the peripheral smear test detect or measure and who is this prescribed for?

Peripheral blood smear detects any abnormality in blood cells and thus diagnoses the relevant blood or blood-related disorder. This test measures the size, shape and number of blood components, which include RBCs, WBCs and platelets. Some of the conditions it detects are:

• Anaemia: The presence of macrocytes (RBCs of large size) of oval shape indicates megaloblastic anaemia. A greater quantity of hypochromic microcytic RBCs (cells having less colour and of smaller than normal size) are found in iron deficiency anaemia, thalassemia, etc.
• Thrombocytopenia: When there is a lesser number of platelets than normal, the condition is called thrombocytopenia.
• Bacterial infections: If there are a greater number of neutrophils (neutrophilia) in comparison to the general number, a bacterial infection is indicated.

So, the alterations in the morphology of blood cells help to detect multiple disorders.

A peripheral blood smear is prescribed when:

• One shows abnormal CBC (complete blood count) results.
• There are symptoms relative to blood disorders like jaundice, fever, fatigue, bone pain, abnormal bleeding, and pale skin.
• One has a chance of being exposed to ticks.
• One has travelled to developing countries with a chance of contact with parasites like malaria.

Common indications in which peripheral smear is advised include anaemia, thrombocytopenia, unexplained jaundice, congenital haemolytic anaemias, suspected organ failure like renal failure or liver disorder, parasite infection or bacterial sepsis, malignancies, leukaemia, unexplained haemolysis, bone marrow disorders, nutritional anaemia and many more.

Normal ranges for red blood cells’ characteristics observed in the test:

Normal ranges for white blood cells’ characteristics:

Normal ranges for platelets’ characteristics:

Upon preparation of the smear, an observation is made for the morphology of blood cells, which includes the number, size, shape and colour of the cells. Below is the interpretation of abnormalities found in these cells.

Red Blood Cells:

In RBCs, various abnormalities can be found that indicate the presence of specific blood disorders. Oval macrocytes, which are oval-shaped, large RBCs are associated with iron deficiency anaemia. Teardrop-shaped RBCs are found in iron deficiency anaemia, Vitamin B12 deficiency and a few other types of anaemia. RBCs with a slightly paler shade than normal are called hypochromic, indicating lesser levels of haemoglobin. Bite cells, which are generally semi-circle in shape, show up in the case of GP6D (Glucose 6 Phosphate Dehydrogenase) deficiency. Bite cells also indicate hemolytic anaemia associated with liver disease.

Sickle cells, which are elongated with pointed ends, are found in cases of sickle-cell anaemia. Boat-shaped RBCs are also associated with sickle cell disease. RBCs are sometimes found in bunches, which is called ‘Rouleaux formation’ associated with the presence of infections, inflammations, cancer or connective tissue disorder.

There are other kinds of abnormal RBCs, too like Keratocytes, Burr cells, etc., which also indicate haematological disorders.

White Blood Cells:

There are five types of white blood cells: basophils, eosinophils, lymphocytes, monocytes and neutrophils. Various abnormalities found in the WBCs include an abnormal number and stage of development of lymphoid cells, which indicate leukaemia. Leukaemia or myelodysplasia is indicated if immature monocytes are found.

If basophils are found in significantly increasing numbers, it is indicative of myeloproliferative disorder. A greater number of lymphocytes suggest viral illness, hepatitis, HIV, rubella infection, etc.

Platelets:

A decrease in the number of platelets is called thrombocytopenia. If the number of platelets is higher, then the condition is called thrombocytosis. Thrombocytopenia indicates the presence of disorders like bone marrow failures, thrombotic microangiopathies, hypersplenism, etc.

Thrombocytosis could present due to haemorrhage, iron deficiency, major surgeries, infections, rheumatoid arthritis, etc. It could also be seen in some malignant conditions.

Large-sized platelets are also observed sometimes. A higher number of large platelets can be found in disorders like immune-related hyper destruction, myeloproliferative disorders, disseminated intravascular coagulation, megaloblastic anaemias, May–Heggelin anomaly or the Bernard–Soulier syndrome.

To carry out this test, a medical professional will collect blood from a vein of the forearm with a syringe.

Test Preparation

No particular preparation is required for this test. For this test, one needs to visit a medical professional, who will then collect the blood sample for the test. It takes about 5 minutes to collect the blood sample. Upon collection of the blood sample, a small amount is spread on a specific slide, which is then observed under a microscope.

Test inclusions: What parameters are included?

Below are the parameters measured in the peripheral smear test:

Red blood cell morphology:

• Cell size
• Shape
• Colour
• Intracellular inclusions
• Pathologic arrangement

White blood cell morphology:

• Cell counts: Neutrophils, lymphocytes, eosinophils, monocytes and basophils
• Cell size
• Shape
• Colour

Platelet morphology:

• Size
• Count

How frequently should you take the peripheral smear test?

There is no particular frequency established for this test. Based on the recommendation of a medical professional, the test has to be repeated.

What are peripheral smear findings?

Below are the findings of the peripheral smear test:

• Red blood cell morphology: Cell size, shape and colour, intracellular inclusions, pathologic arrangement, Anisocytosis.
• White blood cell morphology: Cell counts (neutrophils, lymphocytes, eosinophils, monocytes and basophils), cell size, shape and colour.
• Platelet morphology: Size and count

Anaemia, Thrombocytopenia, Bone marrow disorders, Leukaemia, Hemolytic anaemia, Parasite