What to Know About a Platelet Aggregation Test

Medically Reviewed by Dan Brennan, MD on June 11, 2021

The platelet aggregation test is just one of a series of tests called a blood clotting panel. Your doctor might order this test if they think you have a clotting problem.   

What Is the Platelet Aggregation Test?

A platelet aggregation test is a blood test that looks at how well your blood cells called platelets clump together and clot. It is sometimes called a platelet function test.‌

Platelets are important for inflammation, immune system response, tissue growth, and controlling bleeding. When you get hurt in a way that breaks your skin, your body releases different chemicals that activate platelets and help to stop the bleeding.‌

They stick to the lining of your blood vessels and clump together to make a plug that stops bleeding. A protein mesh forms around it and makes the plug stronger. ‌

Sometimes people have problems with platelets, which can be serious and life-threatening. This can include conditions where your blood clots too much and causes a dangerous blockage, or where it doesn’t clot enough and you can’t stop bleeding.‌

These can include:

Who Should Get the Platelet Aggregation Test

Your doctor will order a platelet aggregation test if you have signs of bleeding or blood clots or have a risk for heart disease.‌

Signs of not enough platelets include:

  • Nosebleeds
  • Bleeding for a long time after a minor cut
  • Heavy or long periods
  • Bleeding gums
  • Blood in your urine
  • Blood in your stool
  • Unexplained bruises
  • Tiny red spots on your skin called petechiae
  • Purple spots on your skin‌

Signs of blood clotting include:

  • Headache
  • Weakness
  • Dizziness
  • Numbness in your hands and legs
  • Throbbing or cramping in your leg or arm
  • Redness or swelling in your leg or arm
  • Trouble breathing
  • Coughing up blood‌

You may have a platelet aggregation test if you’re in the hospital or before surgery as part of routine testing for patients. 

A doctor might also order it if you have a family member who has an inherited platelet disease like haemophilia where blood doesn’t clot.

Other Tests

Your doctor may use other routine blood tests along with a platelet aggregation test. This can include other blood clotting tests that look at proteins on your platelets, how long it takes to clot, and the strength of the clot.‌

You may undergo a complete blood count called a CBC that takes a look at the number of your red and white blood cells and platelets. A low platelet count might be a sign of another condition like cancer or kidney disease.‌

A lipoprotein panel will test for your cholesterol and triglyceride levels as well. This will show your risk for heart disease, which can affect your blood clotting. 

What to Expect From a Platelet Aggregation Test

The platelet aggregation test is a simple blood test. Your blood is drawn with a needle and sent to the lab. ‌

The lab looks at how the platelets spread out in the liquid part of your blood, called plasma. They will add a chemical to the blood to see how your platelets clump together. If the blood sample stays cloudy after the chemical is added, it can be a sign of a platelet clotting problem. 

Platelet Aggregation Test Results

Normal test results depend on the temperature and can vary between labs. A normal result means that your blood appears to clot normally.‌

Abnormal results can mean the platelets have certain proteins or don’t work properly. This can be a sign of inherited platelet diseases. ‌

Some medications or herbal supplements can change your results. Your doctor might ask you to stop taking some medications or supplements temporarily before your test.‌

These can include:

  • Antihistamines for allergies
  • Antibiotics
  • Statins for cholesterol
  • Nonsteroidal anti-inflammatory drugs, or NSAIDs
  • Antidepressants
  • Blood thinners like warfarin or aspirin‌

Make sure to discuss your medication with your doctor, but don’t stop taking anything unless your doctor says you should. 

Risks of the Platelet Aggregation Test

A blood draw has very little risk. In most cases, you will feel a sting and some discomfort, but it will pass quickly. Some people might have a bruise afterwards.‌

Some people have other side effects in rare cases. These include:

  • Feeling faint 
  • Lightheadedness
  • Dizziness
  • Passing out
  • Sweating 
  • Excessive bleeding‌

If you have a risk for bleeding, you might need to be careful during a blood test. Generally, the risk is low though and can be an important test to find any problems. ‌

Make sure you talk to your doctor if you feel unwell. If you have excessive bleeding or signs of a blood clot, this is a medical emergency and you should go to the hospital right away. 

WebMD Medical Reference



Fountain, J., Lappin, S., Physiology, Platelet., StatPearls Publishing.

JOHNS HOPKINS MEDICINE: “What are Platelets and Why are They Important?”

Medline Plus: “Platelet Tests.”

Medscape: “Platelet Disorders Overview of Platelet Disorders.”

NHS: “Blood clots.”

NIH National Heart, Lung, and Blood Institute: “Blood Tests.”

UCSF Benioff Children’s Hospital: “Platelet aggregation test.”

University of Florida Health: “Platelet aggregation test.”

© 2020 WebMD, LLC. All rights reserved.