At a Glance
Why Get Tested?
To find out 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 anaemia, 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 done to look for 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.
A blood sample taken from a vein in your arm or by pricking a finger, ear or, in the case of an infant, a heel.
Test Preparation Needed?
No test preparation is needed.
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 are produced and mature in the bone marrow and are released into the bloodstream when needed. WBCs’ main function is to fight infection, while RBCs carry oxygen to the whole of the body. Platelets appear as small cell fragments and, when activated, stick together to form a plug as one of the first steps to stop bleeding. The number and type of each cell present in the blood changes but is normally maintained by the body within specific ranges. Values can change at times of illness or stress; intense exercise or smoking can also affect cell counts.
A blood film is a snapshot of the cells that are present in the blood at the time that the sample is obtained. To produce 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 looked at 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, the size, shape and general appearance of the cells can be compared to that of “normal” cells. It is possible to distinguish between the five different types of WBCs and to find their relative percentages by counting 100 consecutive cells. During this examination, the size, shape and colour (indicators of haemoglobin content) of the RBCs can be measured and the number of platelets estimated.
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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
Ask a Laboratory Scientist
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.