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A complete blood count (CBC) provides important information about the kinds
and numbers of cells in the blood: red blood cells, white blood cells and
platelets. A CBC can help you and your health professional evaluate symptoms (such as
weakness, fatigue, or bruising) and diagnose conditions (such as anemia,
infection, and many other disorders).
A CBC test includes:
- White blood cell (WBC) count. White blood cells protect the body
against infection. If an infection develops, white blood cells attack and
destroy the bacteria, virus, or other organism causing it. White blood cells
are bigger than red blood cells and normally fewer in number. When a person
has a bacterial infection, the number of white cells can increase
dramatically. The white blood cell count shows the number of white blood cells
in a sample of blood. The number of white blood cells is sometimes used to
identify an infection or monitor the body's response to cancer treatment.
- White blood cell types (WBC differential). There are five major
kinds of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils,
and basophils. Immature neutrophils, called band neutrophils, are also
included and counted as part of this test. Each type of cell plays a different
role in protecting the body. The numbers of each one of these types of white
blood cells give important information about the immune system. An increase or
decrease in the numbers of the different types of white blood cells can help
identify infection, an allergic or toxic reaction to certain medications or
chemicals, and many conditions (such as leukemia ).
- Red blood cell (RBC) count. Red blood cells carry oxygen from the
lungs to the rest of the body. They also help carry carbon dioxide back to the
lungs so it can be exhaled. The red blood cell count shows the number of red
blood cells in a sample of blood. If the RBC count is low, the body may not be
getting the oxygen it needs. If the count is too high (a condition called
polycythemia), there is a risk that the red blood cells will clump together
and block tiny blood vessels (capillaries).
- Hematocrit (HCT, packed cell volume, PCV). This test measures the
amount of space (volume) red blood cells occupy in the blood. The value is
given as a percentage of red blood cells in a volume of blood. For example, a
hematocrit of 38 means that 38% of the blood's volume is composed of red
cells.
- Hemoglobin (Hgb). Hemoglobin is the substance in a red blood cell
that carries oxygen. The hemoglobin test measures the amount of hemoglobin in
blood and is a good indication of the blood's ability to carry oxygen
throughout the body.
- Red blood cell indices. There are three red blood cell indices:
mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean
corpuscular hemoglobin concentration (MCHC). They are not measured directly
but are determined from other measurements noted during a CBC. The MCV shows
the size of the red blood cells. The MCH value is the amount of hemoglobin in
an average red blood cell. The MCHC measures the concentration of hemoglobin
in an average red blood cell. These numbers help in the diagnosis of different
types of anemia.
- Manual Differential Review
- In this test, a drop of blood is spread (smeared) on a
slide and stained with a special dye. The slide is then examined under a
microscope. The numbers, size, and shape of red blood cells, white blood
cells, and platelets are recorded. Blood cells with unusual shapes or sizes
can help diagnose many blood diseases, such as leukemia, malaria, or sikle
cells anemia.
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