What Are Blood Clots? Symptoms & Causes

Blood clots are clumps of blood cells mixed with proteins and platelets. If they become too large, blood clots can become stuck in your blood vessels. These clots can become dangerous if left untreated, causing severe symptoms depending on where the clot is located.1

Learning what puts you at risk for developing a blood clot and the signs to look for can help you get the treatment you need.

What Causes Blood Clots?

Blood clotting is a normal process that your body uses to stop bleeding from a cut or other injury.2 When a blood vessel becomes damaged, specialized cells known as platelets begin attaching to the wound. The platelets release chemical signals to bring in other platelets, eventually creating a plug that stops bleeding on the outside of the blood vessel.3

Inside, clotting factor (coagulation factor) proteins help bring together strands of fibrin, which creates a mesh to stop bleeding. After the blood vessel has healed itself, the blood clot typically dissolves on its own.

Outside of the normal healing process, blood clots can become dangerous. In some cases, they form even if you don’t have a cut or injury. If your body can’t break down the blood clot, it may travel to another part of the body, cutting off blood flow to certain organs or extremities (arms or legs).

Risk Factors for Blood Clots

There are several conditions that increase your risk of developing a blood clot. Some of these are preventable, while others aren’t. Risk factors and conditions associated with blood clots include:1,4

  • Family history of blood clots — having a parent or sibling who has had a blood clot can increase your risk
  • Prior history of DVT – having a medical history of DVT is itself a risk factor for DVT
  • Factor V Leiden — a mutation in the gene for clotting factor V (five), which increases your risk of blood clots in the lungs or legs5
  • Antiphospholipid syndrome — an autoimmune condition in which the immune system attacks your body’s tissues, causing blood clot formation6
  • Heart problems — includes having a heart arrhythmia or heart attack, or heart failure
  • Obesity — causes chronic inflammation, which can change levels of clotting factors7
  • Peripheral artery disease (PAD) — narrowing of the arteries that carry blood to the arms and legs8
  • Pregnancy — levels of clotting factors are increased to help prevent blood loss during delivery9
  • Taking certain medications — such as estrogen therapy or hormonal birth control
  • Infection with Coronavirus disease 2019 (COVID-19) — causes body-wide inflammation, which raises the levels of clotting factors in your blood10
  • Arteriosclerosis/atherosclerosis — hardening of your arteries/buildup of cholesterol and fats on artery walls, which can burst and cause blood clots11
  • Surgery — extra debris may enter your circulatory system during the operation, which clumps with blood cells and other proteins
  • Smoking — damages blood vessels and encourages platelets to stick together more
  • Polycythemia vera — a type of blood cancer caused by your bone marrow making too many red blood cells, which thickens the blood12
  • Sitting for long periods of time — typically for long car or airplane rides

Signs of Blood Clots

The signs of a blood clot depend on where it’s located. Common places that blood clots block blood flow include the lungs, arms or legs, and brain. If you notice any of these symptoms, seek medical attention immediately, as they can be life-threatening if left untreated.

Signs of Deep Vein Thrombosis (DVT)

Deep vein thrombosis (DVT) occurs when a blood clot forms in the deep veins of your body, typically in your arms or legs. Symptoms of DVT in the extremities include:13

  • Swelling of your arm or leg
  • Pain or tenderness that’s not associated with an injury
  • Warm, red or purple skin accompanied by pain and/or swelling

DVT can also develop in the abdomen, specifically in the deep veins that take blood away from the intestines and back to the heart.14 This may cause:

  • Severe stomach pain
  • Nausea
  • Vomiting

Signs of a Pulmonary Embolism (PE)

A pulmonary embolism (PE) is caused by a blood clot becoming stuck in an artery in the lung, blocking blood flow.15 Signs of a PE include:

  • Sharp chest pain that becomes worse with deep breathing; you may feel like you’re having a heart attack
  • Shortness of breath that occurs while resting and becomes worse with movement or exercise
  • Passing out from a drop in your heart rate or blood pressure (known as syncope)
  • Excessive sweating
  • Fever
  • Dizziness or lightheadedness
  • Fast, irregular heartbeat

DVT and PE occurring at the same time is known as a venous thromboembolism (VTE).

Signs of an Ischemic Stroke

An ischemic stroke occurs when a blood clot blocks oxygen and nutrients from reaching your brain.16 The Centers for Disease Control and Prevention (CDC) give the acronym F.A.S.T. for recognizing the signs of a stroke:17

  • Face — your face may droop on one side when you try to smile
  • Arms — you may not be able to lift one of your arms
  • Speech — your speech may be strange or slurred
  • Time — if you experience any of these signs, call 9-1-1 immediately

How Are Blood Clots Diagnosed?

If your doctor thinks you may have a blood clot, they’ll run some tests to confirm it while ruling out other causes. Depending on your symptoms, they may choose certain tests in order to diagnose PE, DVT, or a stroke. These include:13,15,16

  • D-dimer blood test — measures levels of the D-dimer protein that’s made by blood clots; elevated levels can rule out a PE and confirm a DVT
  • Duplex ultrasound — uses sound waves that bounce off the veins and create images used to look for blood clots; used to diagnose PE and DVT
  • Carotid ultrasound — takes images of the carotid arteries in your neck to look for any narrowing or blockages; used to diagnose stroke
  • Magnetic resonance imaging (MRI) — uses a high-powered magnetic field to take detailed images of your tissues and organs; used to diagnose PE, DVT in the abdomen, and stroke
  • Computed tomography (CT) scans — use x-rays to take images of your body from several angles to find evidence of blood clots; used to diagnose PE and stroke

Complications from Blood Clots

In some cases, blood clots and the underlying health conditions that increase your risk can lead to complications. If you start to notice symptoms, talk to your doctor about how to manage them.

One complication from DVT is PE. Often, blood clots from DVT break off and travel to the lungs, causing symptoms. If you’ve been diagnosed with DVT, it’s a good idea to become familiar with the signs of PE in case of an emergency. Another complication from DVT is post-thrombotic syndrome, or PTS (previously known as postphlebitic syndrome). In this condition, damaged blood vessels from a blood clot can’t bring enough blood to tissues, which causes pain, swelling, and skin sores.13

PE may cause high blood pressure in your lungs, known as pulmonary hypertension (PH). This condition also makes it harder for the right side of your heart to pump blood, which eventually weakens it. If left untreated, it can cause right-side heart failure.15

Preventing Blood Clots

If you’re at a higher risk of developing blood clots, you can take certain steps to help prevent them. These include:1,15,18

  • Stop smoking
  • Maintain a healthy weight
  • Maintain healthy cholesterol and blood pressure levels
  • Get up often and stay hydrated on long plane or car rides
  • Get active after surgery or being on bed rest
  • Wear compression stockings to help keep blood circulating in your feet and legs
  • Take your medication as prescribed by your doctor

Resources for Living with Blood Clots

Living with and managing treatments for blood clots and other associated health complications can feel overwhelming. Fortunately, there are resources available to help connect others who share your experiences and bring awareness to them.