Blood film

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Also known as: Peripheral blood smear; manual differential; red blood cell morphology
Formal name: Peripheral blood film
Related tests: FBC, differential, RBC, WBC, platelet count, reticulocyte count, haemoglobin, haemoglobin variants

At a Glance

Why Get Tested?

To determine if red blood cells, white blood cells, and platelets are normal in appearance and number; to distinguish between different types of white blood cells and to determine their relative percentages in the blood; to help diagnose a range of deficiencies, diseases, and disorders involving blood cell production, function and destruction; to monitor cell production and cell maturity in diseases such as leukaemia, during chemo/radiation therapy, or in the evaluation for haemoglobin variants

When to Get Tested?

When FBC results are abnormal, a blood film with manual WBC differential is performed to determine the presence of abnormal or immature cells; when a doctor suspects a deficiency, disease, or disorder that can affect blood cell production; when you are being treated for a disease with medications that may have an affect on blood cell production

Sample Required?

A blood sample drawn from a vein in your arm or by pricking a finger, ear or, in the case of an infant, a heel

The Test Sample

What is being tested?

A blood film allows the evaluation of white blood cells (WBCs, leucocytes), red blood cells (RBCs, erythrocytes), and platelets (thrombocytes). These cell populations are produced and mature in the bone marrow and are eventually released into the bloodstream as needed. WBC’s main function is to fight infection, while RBCs carry oxygen to the tissues. Platelets appear as small cell fragments and, when activated, form a plug as one of the first steps in blood clotting. The number and type of each cell present in the blood is dynamic but generally maintained by the body within specific ranges. Values can fluctuate at times of illness or stress; intense exercise or smoking can also affect cell counts.

A peripheral blood film is a snapshot of the cells that are present in the blood at the time that the sample is obtained. To create a blood film, a single drop of blood is spread in a thin layer across a glass slide, dried, and then stained with a special dye. Once the stain has dried the slide is evaluated under a microscope by a medical scientist or haematologist.

The drop of blood on the slide contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. Under the microscope, the stained WBCs can be easily seen and counted to estimate the number of each type of cell present. In addition, one can compare their size, shape and general appearance to the established appearance of “normal” cells. It is possible to distinguish between the five different types of WBCs and to determine their relative percentages by counting 100 consecutive cells. During this examination, one can also evaluate the size, shape,and colour (indicators of haemoglobin content) of the RBCs and also estimate the number of platelets present.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a finger, ear or, in the case of an infant, a heel.


Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.