Blood smear

The smear is a snapshot of the cells present in the blood at the time the sample is taken. To make a smear, a drop of blood is spread in a thin layer on a glass slide, dried and then stained with special dyes. Once dry, the slide is examined under the microscope by a laboratory doctor.
The smeared drop contains thousands of red blood cells, hundreds of white blood cells and numerous platelets (cell fragments involved in clotting). The slide shows a single layer of cells. Under the microscope, the cells can be examined individually so that the laboratory specialist can roughly estimate the number of each cell type present and compare their size, shape and general appearance to establish whether they are normal. The different white blood cell subpopulations can be distinguished and their relative percentages determined by counting one hundred cells. The examination can be used to assess the size, shape and haemoglobin content of red blood cells and to determine whether there is an adequate number of platelets.

How is the sample collected for testing?
A venous blood sample from a vein in the arm, or a fingerprick, earlobe or – in newborns – heel prick sample.

How is it used?
In the past, a peripheral blood smear was carried out on almost everyone having a routine check-up, as part of the full blood count. It was the standard method for assessing red blood cell morphology (size, shape and structure), identifying the type and percentage of white blood cells and determining whether enough platelets were present. This type of assessment has now been largely replaced by automation.

Today, an automated count can assess hundreds if not thousands of cells within two minutes – at least when the cells present are normal. However, when the automated count shows abnormalities and/or when the doctor suspects abnormal white cells, red cells or platelets, a blood smear examined by an expert eye is still the best way to assess and identify immaturity and abnormalities.

Many diseases, disorders and deficiencies can affect the number and type of blood cells produced, their function and their lifespan. Although normally only normal cells are released into the circulation, some circumstances can cause the bone marrow to release immature or malformed cells. When a significant number of abnormal cells are present, they may point to an undetected disease and prompt the doctor to order further tests.
A blood smear is ordered to assess whether red blood cells, white blood cells and platelets are normal and mature. It can be used to distinguish between the different white blood cell subpopulations and to determine their relative percentages. When immature white cells are present, the smear can be used to determine how many white cells are affected, which subpopulations and how immature they are. One example is the variety and number of immature lymphocytes (a type of white blood cell) found in patients with acute lymphoblastic leukaemia.

When is it requested?
A blood smear is ordered mainly to assess blood cell populations when a full blood count with differential performed on an automated analyser indicates the presence of abnormal or immature cells. It may also be carried out when the doctor suspects a deficiency, disease or disorder affecting blood cell production, such as anaemia, reduced or abnormal cell production in the bone marrow, or increased cell destruction. A smear is also ordered when a patient is being treated or monitored for a blood disorder.

What do the test results mean?
Blood smear findings are not diagnostic in themselves. They may indicate an underlying disease that is not yet evident, give information on its possible severity and suggest further diagnostic tests. Depending on the disorder present, there may be changes in one or more blood cell populations. Blood smear findings may involve:

RBCs (red blood cells)
Normal red blood cells are fairly uniform in size. They are round and flattened, rather like a doughnut with a depression on each side instead of a hole (biconcave). Because of the haemoglobin inside them, red blood cells have a pale pink colour, slightly lighter in the centre where the cell is thinnest. Although red blood cells are not all exactly alike, a significant variation in their shape indicates an abnormality. Changes are graded from 1+ (few abnormal cells) to 4+ (predominantly abnormal cells) to give an idea of the percentage of cells affected.

WBCs (white blood cells)
White blood cells have a nucleus surrounded by cytoplasm. All white blood cells derive from a precursor cell in the bone marrow. They are divided into two main groups, granulocytes and non-granulocytes, which are further divided into five subpopulations.

* Granulocytes – including neutrophils, eosinophils and basophils – have granules in their cytoplasm (like spots or dots) and a segmented nucleus (like irregular bands).
* Non-granulocytes – including lymphocytes and monocytes – have uniform cytoplasm and a round or roundish nucleus.

Together with the white blood cell count, the smear is used to assess samples with abnormalities flagged by automated analysers. Under the microscope, the laboratory specialist assesses:

* How closely the cells match normal cells, their size and shape, and any characteristic inclusions.
* The degree of maturity of the white blood cells. Immaturity means that a problem has occurred in cell production or destruction. If the body’s demand for cells exceeds the bone marrow’s ability to produce and mature them, immature cells are released into the circulation. In the case of production problems – as seen in leukaemias, where large numbers of a single cell subpopulation are produced – cells at various stages of maturity (of a single subtype) are released into the bloodstream. If there are large numbers of immature cells, or too few cells, and the doctor suspects a bone marrow disorder, a bone marrow examination may be ordered.
* If the white cell count is normal (with smudge cells present – cells with a ruptured cell wall), this may be due either to cell fragility or to a laboratory artefact.

The blood smear can also show the relative percentage of all the white blood cell subpopulations.

Platelets
Platelets are cell fragments that the body uses to seal breaks in blood vessels during clotting; they become sticky and clump together at the site of injury. There must be enough platelets to control bleeding. Sometimes the body produces too many platelets. In extreme cases this can interfere with blood flow and increase the risk of clotting. These people may also have bleeding problems, because many of the excess platelets do not work properly, even though they look normal.
Platelets are normally counted by an automated analyser, but giant platelets and platelet clumps can lead to incorrect counts. This may or may not indicate a platelet deficiency or abnormality. For example, some people whose platelets clump in vitro when blood is drawn have no platelet-related disease or problem. When the count is low and/or the analyser flags an alert, the smear is used to determine whether there is an adequate number of platelets and to check for giant platelets and/or platelet clumps.

Is there anything else I should know?
Because transfusions involve transferring red blood cells, white blood cells (whose function is altered by storage) and platelets from one person to another, a recent transfusion can affect both the automated count and the smear.

1. Why hasn’t the automated analyser completely replaced the blood smear?
Automated analysers are now routine, but they rely on assessing the shape, size and electrical properties of red cells, white cells and platelets. The cells produced by the body vary somewhat from one another, and the instruments measure and assess these variations. Sometimes the cells present are abnormal or immature but the instrument cannot assess them, or they are not classic examples of immaturity and the instrument does not recognise them. Cell fragments and platelet clumps may not be counted correctly. The laboratory specialist can see these abnormalities on the blood smear and is trained to identify and classify them.