The Platelet Count

Platelets, also called thrombocytes, are the smallest elements found in the blood.

They are not complete cells, but actually fragments of cytoplasm (from a cell found in the bone marrow, called a megakaryocyte).

They are produced in the bone marrow and removed by the spleen.

Platelets main function is to prevent blood loss by causing it to clot.

To make life interesting both increases and decreases in the platelet count point to problems of excess bleeding or clotting.

For example when there are too many platelets you may get blood clots, but you may also get excessive bleeding because the platelets interfere with each other.

Normal values
These can be expressed in a number of ways, but they all mean the same.
150,000 - 350,000/µl ( platelets per microlitre)
150,000 - 350,000/ mm3 (platelets per cubic millimetre)
150 - 350 x 10*9/liter.

What abnormal results mean

Decreased platelets

Decreased platelets, known as thrombocytopenia, can be due to :

1. A lack of production
a) The bone marrow is impaired by -
Aplastic anemia,
Radiation, Chemotherapy or toxic chemicals
Drugs - Anti-HIV drugs can have a negative effect on the way the bone marrow produces platelets.
Thiazide diuretics slow the production of megakaryocytes.
Large amounts of alcohol can seriously slow platelet production.

b) The bone marrow which produces the platelets is damaged by:-
Acute leukaemia
Multiple myeloma

c) Megaloblastic anemia caused by:-
B12 and/or folic acid deficiency

d) Congenital diseases eg:-
Wiskott-Aldrich Syndrome (WAS) - an inherited immune deficiency disease, one of whose main symptoms is a tendency to bleed due to a low platelet count.
Bernard-Soulier (BSS) - a rare inherited disorder a congenital disorder where the platelets lack receptors to adhere to the walls of the blood vessels.
May-Hegglin anomaly - a rare, inherited disorder characterised by abnormally large platelets.

2. Increased platelet destruction
a) Autoimmune diseases
Idiopathic Cytopenic Purpura (ICP) - a condition possibly related to viral infection, autoimmunity or chemical toxin.
Systemic lupus erythematosus (SLE)
Polyarteritis - a disease where the immune cells attack the arteries.
Dermatomyositis - an inflammatory muscle disease.
AIDS - People with AIDS often develop thrombocytopenia before other symptoms.

b) A reaction to a medication such as:-
Valproic acid (Depakote) - an anticonvulsant.
Gold salts -anti-inflammatories used in patients with rheumatoid arthritis.
Chemotherapy drugs
Heparin (Calciparine, Liquaemin) - an anticoagulant.

c) Dilution due to overhydration.

d) Coagulation disorders such as:-
Disseminated intravascular coagulation (DIC)
Hemolytic-uremic syndrome - A severe blood disease, usually as a result of E-coli infections. There is destruction of platelets and red blood cells and the kidneys shut down.
Thrombotic thrombocytopenic purpura (TTP) - a rare blood condition. Small clots (thrombi) form within the bloodstream, using up platelets and thus causing a low platelet count.
Hemangioma-Thrombocytopenia Syndrome (also known as Kasabach-Merritt Syndrome) - a rare abnormal blood condition with a low platelet count and a benign tumour consisting of large, blood-filled spaces.

e) Physical trauma
Injury or destruction caused by platelets passing through an artificial heart valve. This can also occur when they pass through a blood vessel graft or through the machines used in blood transfusions or bypass surgery.

f) Eclampsia - a condition that shows up in pregnant women, The main symptom being convulsions.

g) Hypersplenism - a disorder in which the spleen rapidly and prematurely destroys blood cells and thus taking too many platelets out of circulation.

h) Too many platelets remain in the spleen.
Usually a third of the platelets are found in the spleen, with and the rest in the circulating blood. However in certain diseases the spleen 'holds on' to it's platelets, leaving fewer to circulate in the blood. The three main causes of this are:-
Severe liver disease
Gaucher's disease - an inherited metabolic disorder.

i) Platelet aggregation or large platelets

j) Rubella (German measles)

k) Liver dysfunction (cirrhosis)

l) Idiopathic thrombocytopenia purpura ITP - a disorder in which the blood does not clot as it should due to a low number of platelets.

m) Infections
Cytomegalovirus (CMV) infection.
Infectious mononucleosis caused by the Epstein-Barr Virus(EBV)

Increased platelets

Increased platelets, known as thrombocytosis, can be divided into two - Secondary and Primary Thrombocytosis.

Secondary, or reactive, thrombocytosis, is a high platelet count that is able to be traced to secondary cause. Examples of these are:-

a) Infections, more common in children than adults, but seen in diseases like
osteoporosis and tuberculosis.

b) Inflammatory diseases such as :-
Rheumatoid arthritis
Kawasaki's disease -a rare condition in which there is widespread inflammation of the arteries.
Crohn's disease
Ulcerative colitis

c) Acute blood loss (haemorrhage) - the loss of blood stimulates the bone marrow to produce more platelets.

d) Haemolytic anaemias

e) Strenuous activity that can lead to dehydration

f) Chronic iron deficiency

g) Tissue damage from accident or surgery.

h) Medications
Vincristine (Oncovin) - a chemotherapy medication that is given as treatment for certain types of cancer.

i) Certain cancers eg.
Bowel cancer
Disseminated carcinoma
Hodgkin's disease

j) Splenectomy.
Unlike other secondary causes of thrombocytosis, which occur due to an increase in the production of platelets, the removal of the spleen causes thrombocytosis because it is no longer available to remove platelets, so they build up.

Primary thrombocytosis (or essential thrombocythemia) occurs on it's own, not as a result of a secondary disorder. It is found in:-

a) Myeloproliferative disorders:-
Essential thrombocythaemia - a rare condition where the megakaryocyte levels in the bone marrow become elevated which in turn causes an increase in the platelets in the blood.
Polycythaemia vera - a type of blood disorder characterised by the production of too many red blood cells, including platelets.
Myelofibrosis with Myeloid Metaplasia - a chronic form of leukaemia. It causes secondary scarring or "fibrosis" in the bone marrow.

b) Chronic myeloid leukaemia - a form of cancer that starts in the stem cells within the bone marrow. These develop into the myeloid cells, of which platelets are one type.

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