Platelet Count Tests

Platelets, also called thrombocytes, are the smallest cells in the blood. These
cells do not have a nucleus, are round or oval, flattened, disk-shaped structures,
and are necessary for coagulation . Some texts refer to platelets as fragments
of cytoplasm .
Two tests that measure or count the number of platelets are the platelet
count, which measures the number of platelets in the blood, and the mean
platelet volume (MPV), which provides information about platelet size.

Platelet Count

A platelet count test often follows a decreased platelet count that was estimated
from a peripheral blood smear. The platelet count is an important
blood test because thrombocytopenia is the most common cause of bleeding
diseases. This count is used to evaluate bleeding disorders due to liver
disease, thrombocytopenia, and anticoagulant therapy. The test is also ordered
for patients who have diseases associated with bone marrow problems,
such as leukemia and aplastic anemia. The platelet count is expressed
as the number of platelets per cubic millimeter (mm3) of blood .

Normal Range

Platelets 150,000-400,000/mm3

Variations from Normal. An abnormal increase in the number of platelets is
called thrombocythemia or thrombocytosis. This increase is seen in diseases
such as malignancies, early stages of chronic granulocytic leukemia, polycythemia
vera, tuberculosis, chronic inflammatory disease, and chronic blood
loss.
A decreased platelet count is known as thrombocytopenia and can result
in significant bleeding problems. Diseases that decrease the platelet count include
pernicious and aplastic anemias, and idiopathic thrombocytopenic purpura (ITP). A low platelet count is commonly seen in AIDS cases. Exposure
to various chemicals and the toxic effects of many drugs can also lead to
thrombocytopenia . Individuals whohave serious platelet deficits often show
signs or symptoms such as petechiae, bleeding from gums, nosebleeds, and
gastrointestinal bleeding.

Interfering Circumstances . Platelet counts can show a normal increase at
high altitudes, after strenuous exercise, and in the winter . A normal decrease
occurs on the first day of an infant's life and before menstruation.

Mean Platelet Volume (MPV)

The mean platelet volume provides information about the relative size of
platelets, which is calculated by a cell analyzer and compared to what is observed
on a microscope slide. The diameter of the platelet is expressed in
micrometers (!gym) . The MPV is a useful diagnostic tool for thrombocytopenic
disorders.
The relative size of platelets varies with platelet production. When the
overall platelet count drops, functioning bone marrow produces younger
and larger platelets to compensate for the decreased number of platelets.
This process results in an increased mean platelet volume . Lack of bone
marrow function results in the decreased production of platelets, a diminished
platelet size, and a decreased mean platelet volume.

Normal Range

Platelets
2-4 gm in diameter

Variations from Normal. An increase in the diameter of the platelets occurs
in systemic lupus erythematosus, idiopathic thrombocytopenic purpura in
remission, various anemias, myeloproliferative disorders, and a variety of
chronic disease processes. A decrease in the size of platelets is associated
with aplastic anemia, megaloblastic anemia, and hypersplenism.