Platelet Aggregation Test


Platelets are blood cells. They are disk-shaped and are also known as thrombocytes. Blood platelets contribute to the process of clotting of blood. The platelet aggregation test is conducted with a view to gauge the functioning of the platelets. When there is a cut in the body from which blood flows out, the blood platelets collect at the opening in a clump, thereby blocking the exit of the blood, and thus stopping the bleeding.

The Mechanism of Blood Clotting

When a blood vessel gets cut, the damaged tissue attracts blood platelets which form a sticky mass around the wound. Once the platelets adhere to the surface of the wound, they become activated. The activated platelets react with the tissue which releases proteins and chemicals into the plasma. These substances formed are known as” clotting factors” which are 13 in number. They are assigned Roman numbers as factors I to XIII.

The last stage of the clotting process is the conversion of factor I, a soluble protein also known as fibrinogen into thin protein strands known as fibrin. These fibrin strands are the ones that collect the platelets and blood cells and a solid clot is formed. Clots in healthy blood vessels can lead to serious problems. Therefore there are chemicals within the blood which help to dissolve the clots. Normal-functioning blood should strike a healthy balance between clotting and prevention of clotting.   

Problems of Platelet Malfunctioning

When there is some problem in the way the blood platelets work, two problems can occur – bleeding disorders and clotting disorders.

  1. Bleeding Disorders: Excessive bleeding can be caused due to various reasons in the event a blood vessel being cut or damaged. Some of the reasons are as follows:
    • Scarcity of platelets: This can result in excessive bleeding. The condition is known as thrombocytopenia.
    • Genetic Reasons: If for some genetic reasons, the body does not manufacture particular clotting factors, the result is excessive bleeding. One of the most familiar conditions is that of hemophilia. This is caused by absence of factor VIII.
    • Vitamin Deficiency: If the body has a shortage of vitamin K, bleeding problems can occur. Vitamin K is required for the reactions that take place in the process of blood clotting.
    • Liver Disorders: As the liver is responsible for the production of most of the clotting factors in the body, if there is any problem in the liver, it can cause bleeding problems.
  2. Clotting Disorders: If insufficient clotting can cause problems, excessive clotting can also be a serious problem. This is when clotting occurs in a situation where there no cut or injury to a blood vessel. The reason for formation of blood clots are as follows:
    • Presence of Fatty Material in the Blood: Due to an incorrect diet, fatty substances known as “atheroma” can accumulate in the blood vessels. It results in clumping of platelets around the fatty masses, causing clots.
    • Deep Vein Thrombosis: Commonly occurring in the leg veins, this condition can cause the blood to flow sluggishly and is a result of clots.
    • Genetic Conditions: The genetic makeup of a person may cause them to have a tendency to blood clots due the nature of the blood since birth.
    • Medication: Particular medication can cause an imbalance in clotting as a result of which blood clotting is more often.
    • Liver Disorders: As the liver produces most of the clotting factors, in the event of liver malfunction, blood clots may form.

In the event of the above conditions, the platelet aggregation test is one of the most common tests conducted to check for the functioning of the blood platelets.

Platelet Aggregation Test Method

A blood sample is drawn from a vein, either from the back of the hand or inside of the elbow. The method for drawing the blood is the usual method of taking a blood sample – the sample site is first cleaned with liquid antiseptic and a tourniquet is applied to make the vein swell. A syringe is then inserted into the vein and blood is drawn into the syringe. The needle is withdrawn and the puncture is then sealed with a bit of cotton wool.

If the patient is an infant or a very small child, then a tool known as a “lancet” is used to puncture the vein and the blood is collected through a pipette.

The next stage of the test is the analysis stage, where a laboratory technician looks at the blood as it is spread out. The technician checks how the platelets arrange themselves in the plasma (liquid part of blood), and observes the clotting pattern of the platelets after the addition of a reagent. The change in cloudiness is measured by a machine and a printout is taken of the readings.


The platelet aggregation test is a very important blood test which is useful for all types of blood disorders. As mentioned earlier, blood disorders of either bleeding or clotting can be due to many reasons and the platelet aggregation test is one way of helping to pinpoint the cause of the problem.