Complete Blood Count (CBC)
A complete blood count (CBC) gives important information about the
kinds and numbers of cells in the blood, especially
red blood cells ,
white blood cells , and
platelets. A CBC helps your doctor check
any symptoms, such as weakness, fatigue, or bruising, you may have. A CBC also
helps him or her diagnose conditions, such as
anemia, infection, and many other disorders.
A CBC test usually includes:
- White blood cell (WBC, leukocyte) 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 but fewer in number. When
a person has a bacterial infection, the number of white cells rises very
quickly. The number of white blood cells is sometimes used to find an infection
or to see how the body is dealing with cancer treatment.
- White blood cell types (WBC differential). The
major types of white blood cells are
neutrophils, lymphocytes, monocytes, eosinophils, and
basophils. Immature neutrophils, called band neutrophils, are also 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. Too many or too few of the different
types of white blood cells can help find an infection, an
allergic or toxic reaction to medicines or chemicals,
and many conditions, such as
- Red blood cell (RBC) count. Red blood cells carry
oxygen from the lungs to the rest of the body. They also carry carbon dioxide
back to the lungs so it can be exhaled. If the RBC count is low (anemia), the
body may not be getting the oxygen it needs. If the count is too high (a
polycythemia), there is a chance that the red blood
cells will clump together and block tiny blood vessels (capillaries). This also
makes it hard for your red blood cells to carry oxygen.
- Hematocrit (HCT, packed cell volume, PCV). This
test measures the amount of space (volume) red blood cells take up 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 made of red blood cells. Hematocrit and
hemoglobin values are the two major tests that show if
anemia or polycythemia is present.
- Hemoglobin (Hgb). The hemoglobin molecule fills up
the red blood cells. It carries oxygen and gives the blood cell its red color.
The hemoglobin test measures the amount of hemoglobin in blood and is a good
measure 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 measured by a
machine, and their values come from other measurements in 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. Red cell distribution width (RDW) can also be measured which
shows if the cells are all the same or different sizes or shapes.
- Platelet (thrombocyte) count. Platelets
(thrombocytes) are the smallest type of blood cell. They are important in blood
clotting. When bleeding occurs, the platelets swell, clump together, and form a
sticky plug that helps stop the bleeding. If there are too few platelets,
uncontrolled bleeding may be a problem. If there are too many platelets, there
is a chance of a blood clot forming in a blood vessel. Also, platelets may be
involved in hardening of the arteries (atherosclerosis ).
- Mean platelet volume (MPV). Mean platelet volume
measures the average amount (volume) of platelets. Mean platelet volume is used
along with platelet count to diagnose some diseases. If the platelet count is
normal, the mean platelet volume can still be too high or too low.
Your doctor may order a blood smear test to be done
at the same time as a CBC but it is not part of the regular CBC test. In this
test, a drop of blood is spread (smeared) on a slide and stained with a special
dye. The slide is looked at under a microscope. The number, size, and shape of
red blood cells, white blood cells, and platelets are recorded. Blood cells
with different shapes or sizes can help diagnose many blood diseases, such as
sickle cell disease.