Blood Film

A blood film was once prepared on nearly everyone who had a full blood count (FBC). With the automated blood cell counting instruments currently used, an automated differential is also provided. However, if the presence of abnormal WBCs, RBCs or platelets is suspected, a blood film, examined by a trained eye, is still the best method for identifying immature and abnormal cells.

There are many diseases, disorders and deficiencies that can have an effect on the number and type of blood cells produced, their function and their lifespan. Although usually only normal mature cells are released into the bloodstream, circumstances can force the bone marrow to release immature and/​or malformed cells into the blood. When a significant number of abnormal cells are present, they can indicate disease and prompt the doctor to do further testing.

The blood film is primarily used when a FBC with differential, performed with an automated blood cell counter, shows the presence of abnormal or immature cells. It may also be used when a doctor suspects a deficiency, disease or disorder that is affecting blood cell production, such as an anaemia, decreased or abnormal production of cells in the bone marrow, or increased cell destruction. A blood film may also be requested when a patient is being treated or monitored for a blood cell-related disease.

Findings from the blood film test do not always give a diagnosis but can provide information indicating the presence of an underlying condition and its severity and the need for further diagnostic testing. Blood film findings may include:

RBC (Red blood cells)
Normal, mature red blood cells are uniform in size (7 µm). Unlike most other cells, they do not have a nucleus. They are round and flattened like a doughnut but with a depression in the middle instead of a hole (biconcave). With routine staining, due to the haemoglobin inside the RBCs, they appear pink to red in colour with a pale centre. While not every RBC will be perfect, the presence of many cells that are different in shape or size may indicate a more severe problem. There may be one or more irregularities present and may include:

• Anisocytosis – variable sizes of red cells. The presence of smaller RBCs (<7 µm) is referred to as microcytosis and RBCs larger than 7 µm is macrocytosis.
• Poikilocytosis – various shapes of red cells. These may include echinocytes, acanthocytes, elliptocytes, keratocytes, sickle cells, target cells, teardrop cells (dacrocytes), smear cells (also known as smudge or basket cells), and schistocytes.

See the section below for Details on Red Blood Cell Irregularities.

WBC (White Blood Cells)
White blood cells have a nucleus surrounded by cytoplasm. All WBCs come from bone marrow stem cells. In the marrow, they change into two groups: myeloid and lymphoid cells. They then mature into five different types of WBCs.

• Granulocytes – the three types of cell making up this group are distinguished by granules of characteristic size and colour in their cytoplasm.
  • Neutrophils – these cells have nuclei with multiple lobes and have pink or purple granules in their cytoplasm. They compose the majority of WBCs in a healthy adult. They increase in episodes of inflammation, such as with bacterial infections or in rheumatoid arthritis.
  • Eosinophils – are easily recognised in stained smears with their large, red-orange granules and nuclei usually with 2 lobes (bi-lobed). They are normally present in relatively low numbers (1–3%) but increase in number with allergies and parasitic infections.
  • Basophils – all have large, dark purple-black granules and are the least often seen type of WBC (1%). Like eosinophils they usually have bi-lobed nuclei. Increased numbers of basophils are not often encountered but may be found with certain haematological diseases most notably some leukaemias and myeloproliferative disorders. Elevated numbers of basophils can also be found associated with chicken pox, ulcerative colitis, or after an immunisation.
  • Monocytes – are usually the largest of the WBCs (12–20 µm) and are often referred to as scavenger cells (phagocytes). They can ingest particles such as cellular debris, bacteria, or other insoluble particles.
  • Lymphoid cells – lymphocytes are smaller in size (10–12 µm) and many have a smooth, round nucleus which is not lobated. Normally most lymphocytes seen on a smear don’t have visible granules or have just a few granules in their cytoplasm. Lymphocytes are responsible for the production of antibodies (immunoglobulins) or have complicated roles in organising the responses of other white blood cells or in recognising and attacking germs or even cancer cells

See the section below for more Details on White Blood Cells.

Platelets
These are cell fragments that come from large bone marrow cells called megakaryocytes. Upon release from the bone marrow, they appear as fragments in the peripheral blood. When there is blood vessel injury or other bleeding, the platelets become activated and begin to clump together to form aggregates. This is the first step in making a blood clot. You must have a sufficient number of platelets to control bleeding. If there are too few, the ability to form a clot is impaired and can be life-threatening. In some people, too many platelets may be produced, which interferes with the flow of blood and increases a person’s risk of developing a blood clot. These same people may also experience bleeding because many of the extra platelets may be dysfunctional even though they appear normal.

Details of platelet number and size is usually part of a FBC. An abnormally low number or high number of platelets may be further evaluated by preparing a peripheral blood film to visualise any anomalies in shape or size directly.

Some examples of situations or conditions that may affect or invalidate results of a blood film include:

• The patient has received a recent blood transfusion
• The patient has increased levels of protein
• The blood specimen has a blood clot in it
• Blood collected in the wrong specimen tube
• Not enough blood collected into the specimen tube
• Specimen tube not kept at the correct temperature
• Blood film not prepared or stained correctly
• Sample too old for preparation of film

Automated blood cell counters are used for routine testing of FBCs. These machines provide information based on the shape, size, and electrical or photometric properties RBCs, WBCs and platelets. A variety of physiological and external stimuli can lead to some variation in the numbers of cells the body produces and the proportions of the different cell. Automated instruments can often identify the presence of abnormal cells but lack the sophistication to subclassify them definitively. Cell fragments and platelet clumps, particularly if they are large in size, can be mistakenly counted as WBCs, thus falsely elevating a white cell count. Healthcare scientists and haematologists have been trained to identify and classify these abnormalities by examining a blood film.