Platelets – Part 2 – Platelets Count (Thrombocyte count)
Sample
- This can be done on EDTA blood.
- This is stable for 5 hours at 23 °C and 24 hours at 4 °C.
- Can take capillary blood and make a direct dilution, this sample is stable for 3 hours.
- Fetal blood is collected from the umbilical area percutaneous.
- The platelets can be assessed on the DLC slide.
Indication
- To diagnose the cause of petechial hemorrhage in the skin.
- To find the cause of spontaneous bleeding.
- In women with heavy menses.
- This is advised in a patient on chemotherapy.
- This is advised in case of bone marrow failure.
- Platelets count is of value in thrombocytopenia seen in:
- Uremia.
- Liver diseases.
- malignancies.
Pathophysiology
- Platelets are the smallest form of formed elements of blood.
- Platelets are formed by the megakaryocytes in the bone marrow. Basically, these are the detached portion of the megakaryocytes
- Platelets are the smallest of the formed elements of blood.
- Platelets measures 3.0 x 0.5 µm in diamter.
- These are non-nucleated, small, round or oval, flattened disk-shaped structures.
- The glycoprotein of the platelet surface is important in the platelets reactions of:
- Adhesion.
- Aggregation.
- Leading to plug formation during hemostasis.
- Glycoproteins are:
- Glycoprotein 1a.
- Glycoprotein 111a.
- Glycoprotein 1b.
- Glycoprotein 11b.
- The binding site for 11b/111a is a receptor for the fibrinogen which is important in platelet aggregation.
- The membrane phospholipids known as factor 3 is important in the conversion of coagulation factor X to Xa and prothrombin (factor 11) to thrombin (factor 11a).
- The lifespan of platelets is roughly 7.5 days.
- Platelets two-third are in the blood and one-third in the spleen.
- Platelets activity needed for:
- blood clotting.
- vascular integrity.
- vasoconstriction.
- In hemostasis the steps are:
- Platelets aggregation that occlude the breaks in the vessel wall.
- Initiates the clotting mechanism.
-
The platelets contents are:
- The glycocalyx is the outer membrane.
- α-Granules.
- Lysosomes contain neutral proteases, bactericidal enzymes, and acid hydrolase.
- Dense granules contain ADP, ATP, and Calcium.
- β-thromboglobulin functions are:
- Inhibits the heparin.
- It is chemotactic.
- Promotes smooth muscle growth for the repair of vessels.
- Platelet factor 4, inhibit the heparin.
- Platelet-derived growth factor (PDGF) functions are:
- It promotes smooth muscle growth.
- It takes part in atherosclerosis and lipid metabolism.
- Thrombospondin functions are:
- It mediates cell to cell interaction.
- It promotes platelets to platelet interaction.
- Fibrinogen leads to fibrin formation.
- Von Willebrand factor (VWF) promotes platelet adhesion.
- Factor V is a cofactor in fibrin formation.
- Factor VIII is also a cofactor for fibrin formation.
- Fibronectin functions are:
- It is a cellular adhesion molecule.
- It promotes platelets spreading.
- Plasminogen is the precursor to plasmin and has a function in the fibrinolysis.
- High molecular-weight Kinogen has a role in the activation of the intrinsic pathway via contact.
- α2-Antiplasmin inhibits plasmin.
- Platelets activation:
- This may be transient, reversible, or irreversible.
- After activation the changes are:
- Initially, there is a pseudopod formation.
- There is a contraction.
- Adhesion.
- Change in the shape.
- There is a stickiness.
- There is aggregates formation.
- There is a platelet plug formation.
- There is the release of chemical secretions.
- The last step is stabilizing the platelet plug by the formation of fibrin mesh over the platelet aggregates.
NORMAL
Source 1
Age | x 103/µL |
Fetal blood | |
18 to 20 weeks | 242.1 ± 34.5 |
21 to 22 weeks | 258.2 ± 53.6 |
23 to 25 weeks | 259.4 ± 42.4 |
26 to 30 weeks | 253.5 ± 36.6 |
Adult | 150 to 400 |
- To convert into SI unit x 106 = x 109/L
Source 2
Age | cmm (mm3) | x 109/L (SI unit) |
Adult/elderly | 150,000 to 400,000 | 150 to 400 |
Premature infants | 100,000 to 300,000 | 100 to 300 |
Newborn | 150,000 to 300,000 | 150 to 300 |
Infants | 200,000 to 475,000 | 200 to 475 |
Children | 150,000 to 400,000 | 150 to 400 |
- Thrombocytopenia when The count is less than 100,000 /cmm
Thrombocytosis when the count is more than 400,000 /cmm.
Thrombocythemia when the count is above one million /cmm.
Method to count platelets
- By automated hematology analyzers.
- Direct smear also gives information about the size, shape, and clumping of platelets.
- Direct count from the peripheral blood smears.
- Count platelets on oil objective in 10 fields and multiply with 2000, will give a rough idea about the count.
- Platelets in 10 field X 2000 = total platelets.
- Count platelets on oil objective in 10 fields and multiply with 2000, will give a rough idea about the count.
- The manual method of platelet count:
- Take 20 µL of blood.
- Add 1.8 mL of 1% ammonium oxalate.
- Ammonium oxalate will lyse the RBCs and WBCs and Platelets will remain intact.
- Leave for 15 minutes for complete lysis of RBCs.
- Mount the Neubauer chamber.
- Leave the chamber for 15 minutes in the high humidity.
- Count the central large square.

Neubauer Chamber for Counting the Platelets
-
- The average number of platelets counted in 1 mm² = X
- The dilution of the blood is 1:100.
- The depth of the chamber is 0.1 mm.
-
- Note: Can use the constant factor of 1000.
- To minimize the error to count platelets manually:
- Avoid the platelets clumping.
- Avoid the microclot formation.
- Run the test in duplicate and then get the average of the two results.
- If taking blood from the finger then don’t squeeze the finger.
What are the mechanism leading to thrombocytopenia:
- This may be due to decreased production of the bone marrow. This may be due :
- Bone marrow failure.
- Infiltration of the bone marrow by tumors or fibrosis.
- Destruction or sequestration of the platelets by hypersplenism.
- Antibodies destroying the platelets.
- destruction of the platelets by infection or drugs.
- Increased utilization of disseminated intravascular coagulopathy.
- In severe hemorrhage which leads to loss of platelets.
- A large blood transfusion leads to a dilutional effect.
Causes of thrombocytopenia or Decreased platelets:
- Idiopathic thrombocytopenia ITP.
- Hypersplenism.
- Anemias like pernicious, aplastic, and hemolytic.
- After a massive blood transfusion
- An infection like viral, and bacterial,
- Chemotherapy treatment.
- HIV infection.
- Leukemias, carcinoma, and myelofibrosis. This is due to the infiltration of the bone marrow.
- D I C.
- Toxemia of pregnancy, eclampsia.
- Antiplatelets antibody.
- Renal failure.
- Inherited diseases like Wiskott-Aldrich syndrome.
- An autoimmune disease like systemic lupus erythematosus
Thrombocytosis or increased platelets:
- malignant tumors like leukemia, and lymphoma,
- Polycythemia vera.
- Splenectomy.
- Iron deficiency anemia.
- Autoimmune diseases like Rheumatoid arthritis, and SLE.
- Hodgkin’s lymphoma
- Chronic pancreatitis, and inflammatory bowel disease.
- Tuberculosis.
Critical value
- The patient may develop spontaneous bleeding when the count is < 20,000 /cmm.
- Platelets counts > 50,000 /cmm usually show no bleeding.