Know Your Test - Platelet Count

Dr David Howdijam, MD *

Diagram of the internal structure of a human blood platelet :: Pix - Wikipedia/Dr Graham Beards

Platelets, also called thrombocytes, are tiny fragments of cells that are essential for normal blood clotting. They are formed from very large cells called megakaryocytes in the bone marrow and are released into the blood to circulate. The platelet count is a test that determines the number of platelets in a person’s sample of blood. The test is included in a Complete Blood Count (CBC), a panel of tests often performed as a part of a general health examination.

A platelet count may be used to screen for or diagnose various diseases and conditions that can cause problems with bleeding and clot formation. It may be used as a part of the workup of a bleeding disorder, bone marrow disease, or excessive clotting disorder, to name a few.

The test may be used as a monitoring tool for people with underlying conditions or undergoing treatment with drugs known to affect platelets. It may be used to monitor those being treated for a platelet disorder to determine if therapy is effective.

A platelet count may be performed in conjunction with one or more platelet function tests, which assess the function of platelets, and other tests that evaluate coagulation such as Prothrombin Time (PT) and Partial Thromboplastin Time (PTT). If results are not within the normal interval, a number of other tests may be performed to help give clues as to the cause.

WHEN IS IT DONE?

A platelet count may be performed when a person has signs and symptoms associated with low platelets or as a bleeding disorder, such as:

o Unexplained or easy bruising.
o Prolonged bleeding from a small cut or wound.
o Numerous nosebleeds.
o Gastrointestinal bleeding (which can be detected in stool samples).
o Heavy menstrual bleeding.
o Small red spots on the skin called petechiae – may sometimes look like a rash.
o Small purplish spots on the skin called purpura, caused by bleeding under the skin.

Testing may also be done when it is suspected that an individual has too many platelets. An excess of platelets can cause excessive clotting or sometimes bleeding if the platelets are not functioning properly. However, people with too many platelets often have no signs or symptoms, so the condition may be found only when a platelet count is done as a part of a routine health check-up or for other reasons.

SAMPLE REQUIRED

A blood sample drawn from a vein in the arm.

PREPARATION NEEDED

No special preparation is needed for the test.

WHAT DOES THE RESULT MEAN?

A low platelet count, also called thrombocytopenia, and accompanying signs and symptoms may be caused by a number of conditions and factors. The causes typically fall into one of two general categories:

1. Disorders in which the bone marrow cannot produce enough platelets.

2. Conditions in which platelets are used up (consumed) or destroyed faster than normal.

Examples of conditions causing a low platelet count include:

o Idiopathic thrombocytopenia (ITP), also known as immune thrombocytopenic purpura, is the result of antibody production against platelets.
o Viral infections such as mononucleosis, HIV or measles.
o Certain drugs and antibiotics have been associated with drug-induced decreased platelet counts.
o Heparin-induced thrombocytopenia (HIT) results in low platelets when a person who is on or received heparin therapy develops an antibody.
o Leukaemia, lymphoma or another cancer that has spread to the bone marrow.
o Aplastic anaemia.
o Sepsis.
o Cirrhosis.
o Certain autoimmune disorders.
o Chemotherapy or radiation therapy.

A high platelet count may be referred to as thrombocytosis. This is usually the result of an existing condition, such as:
o Cancer, most commonly lung, gastrointestinal, ovarian, breast or lymphoma.
o Anaemia, in particular iron-deficiency anaemia and haemolytic anaemia.
o Certain inflammatory conditions.
o Infectious diseases, such as tuberculosis.
o If a person has had their spleen removed surgically.

COMMON FOLLOW-UP TESTS AFTER AN ABNORMAL PLATELET COUNT

If the cause of the abnormal result is not apparent and cannot be determined from one’s medical history and physical examination, the doctor may choose to ask for additional tests. Depending on the suspected cause and results from a CBC and blood smear, various follow-up tests may be performed. A few examples include:

o Tests for inflammatory conditions, such as CRP, ESR, etc.

o Tests for infectious diseases, including bacteria and viruses.

o Tests for bleeding disorders such as PT, PTT, fibrinogen, etc.

o Tests for kidney diseases.

o Iron studies or vitam B12 and folate levels.

o Tests for liver diseases.

o In unexplained cases, a bone marrow aspiration/biopsy.