How to tell if a blood clot is starting? A comprehensive guide

September 23, 2025 •

4 min read

According to the Centers for Disease Control and Prevention, up to 900,000 Americans are affected by blood clots each year, with as many as 100,000 people dying from them. Knowing how to tell if a blood clot is starting can be life-saving, as early detection is crucial for proper treatment and prevention of more serious complications like a pulmonary embolism.

Quick Summary

Recognizing a starting blood clot involves looking for specific symptoms that vary by location, including swelling, pain, warmth, or redness in a limb (DVT) or more severe signs like shortness of breath and chest pain (PE). Prompt medical evaluation is essential to confirm a diagnosis and begin treatment. Acting quickly can prevent the clot from traveling to vital organs, where it can cause a medical emergency.

Key Points

Limb Swelling and Pain: A primary sign of a deep vein thrombosis (DVT) is swelling, pain, or tenderness in one leg or arm, particularly in the calf or thigh.
Skin Changes and Warmth: Look for redness, discoloration, or warmth in the skin over the affected area, which may feel warmer than the surrounding skin.
Sudden Shortness of Breath: This is a critical indicator of a pulmonary embolism (PE), a life-threatening condition where a clot travels to the lungs. Seek emergency care immediately.
Chest Pain and Rapid Heartbeat: A PE may cause sharp, stabbing chest pain that worsens with deep breathing, accompanied by a rapid or irregular heart rate.
Don't Massage the Area: If you suspect a blood clot, do not rub or massage the affected limb, as this could dislodge the clot and cause a pulmonary embolism.
Immediate Medical Attention is Key: Never ignore potential blood clot symptoms. Only a medical professional can confirm a diagnosis and determine the right course of action.

Recognizing the early warning signs of a blood clot

While a healthy body relies on clotting to stop bleeding, the formation of an unnecessary clot can pose a serious health risk. These clots, known as thromboses, can block blood flow and, if they break loose and travel to vital organs, cause a medical emergency. The symptoms you experience depend on where the clot is forming.

Symptoms of a DVT in the legs or arms

Deep vein thrombosis (DVT) is a serious condition where a blood clot forms in a deep vein, most often in the leg, but sometimes in the arm. Recognizing these symptoms early is key.

Swelling: This is one of the most common signs. Swelling may occur in the entire leg or arm, or just in the specific area of the clot, such as the calf. It may feel tight or appear as pitting edema, where a dimple is left after pressing the skin.
Pain or Tenderness: Many describe this as a throbbing pain or cramping sensation, often in the calf or thigh. The pain can be constant or may only be felt when you stand or walk. It can feel like a muscle cramp that won't go away.
Redness or Discoloration: The affected limb may show a visible change in skin color, appearing red, purplish, or darker than usual. This can be more difficult to spot on darker skin tones.
Warmth: The area of the clot may feel warm to the touch when compared to the surrounding skin or the other limb.

Symptoms of a pulmonary embolism (PE)

If a DVT in a leg or arm breaks free, it can travel to the lungs, causing a pulmonary embolism (PE), a life-threatening condition. Immediate medical attention is necessary if you experience any of these signs.

Sudden Shortness of Breath: This is a hallmark symptom of a PE and can come on abruptly, getting worse with physical activity.
Chest Pain: A sharp, stabbing chest pain is common. It may feel worse when taking a deep breath, coughing, or bending over.
Rapid Heartbeat: You may notice your heart beating faster than usual or irregularly.
Coughing: Some people develop a cough, which may or may not produce blood or bloody mucus.
Lightheadedness or Dizziness: Feeling faint or dizzy is another possible symptom, particularly if the PE is large.

Comparison of clot symptoms by location

To help differentiate the symptoms, consider this comparison table:


Symptom	Deep Vein Thrombosis (DVT)	Pulmonary Embolism (PE)
Location of Symptoms	Typically a single leg or arm	Chest, lungs, and heart
Pain	Throbbing, cramping, often in calf/thigh	Sharp, stabbing chest pain, worse with deep breaths
Swelling	Often significant in one leg/arm	Not associated with PE
Skin Changes	Redness, warmth, discoloration	May include flushed or clammy skin
Respiratory Issues	None associated directly	Severe shortness of breath, cough
Heart Rate	No change or slight increase	Rapid or irregular heartbeat
Other Signs	Tenderness to the touch	Lightheadedness, fainting, coughing up blood

Understanding risk factors and causes

Certain factors can increase your risk of developing a blood clot. While not everyone with a risk factor will get a clot, understanding your risk profile is important for prevention.

Prolonged Inactivity: Sitting for long periods, like during a long plane flight or car trip, or being on bed rest after surgery, slows blood flow and can cause clots.
Surgery and Hospitalization: Major surgeries, especially orthopedic procedures on the hip or knee, significantly increase the risk.
Medical Conditions: Conditions like cancer, chronic inflammatory diseases, and obesity raise your risk. Inherited clotting disorders can also play a role.
Hormones: Estrogen from birth control pills, hormone replacement therapy, and pregnancy can increase blood clotting factors.
Age: The risk of DVT and PE increases with age, particularly after 55.
Smoking: Smoking damages the lining of blood vessels, making clot formation more likely.

What to do if you suspect a blood clot

If you experience any of the symptoms associated with a DVT or PE, it is crucial to seek medical attention immediately. Do not wait to see if the symptoms go away. A doctor can perform diagnostic tests to confirm or rule out a clot.

Do not rub or massage the affected area, as this could cause the clot to break free and travel to the lungs.
Elevate the affected limb if possible, but do not move it excessively.
Provide detailed information to the medical staff about your symptoms, medical history, and any recent travel or surgery.

Seeking professional help and prevention

Medical diagnosis for a blood clot often involves imaging tests, such as an ultrasound for DVT or a CT scan for PE. A blood test known as a D-dimer test may also be used to help rule out a clot. Treatment depends on the severity and location of the clot and can include blood-thinning medications (anticoagulants), clot-busting drugs (thrombolytics), or in rare cases, surgery.

Prevention strategies are especially important for those with risk factors. For more in-depth information, the National Blood Clot Alliance provides valuable resources on recognizing symptoms, understanding risks, and taking preventative measures. Some simple preventative steps include staying active, moving around during long periods of sitting, staying hydrated, and following your doctor's recommendations for managing risk factors.

Conclusion: Act fast, don't ignore the signs

Knowing how to tell if a blood clot is starting is a critical piece of health knowledge. The symptoms can vary depending on the location of the clot, from swelling and pain in a limb (DVT) to sudden chest pain and shortness of breath (PE). If you or someone you know exhibits these signs, do not delay seeking medical help. Quick diagnosis and treatment are essential for a positive outcome. By being aware of the warning signs and taking preventative steps, you can significantly reduce your risk and protect your health.

Frequently Asked Questions

The very first sign often depends on the location. For a deep vein thrombosis (DVT), a common initial sign is localized pain, tenderness, or swelling in one leg or arm. For a clot in the lungs (PE), sudden shortness of breath or sharp chest pain may be the first symptom.

You cannot physically feel a blood clot directly with your hand. However, you might feel warmth or tenderness over the area where the clot is forming, and you can see visible symptoms like swelling, redness, or discoloration.

No, not always. Some blood clots, especially smaller ones, may not cause any noticeable symptoms. In some cases, a person may have a blood clot and not know it until it causes a more severe issue, like a pulmonary embolism.

While you can't officially 'check' for a blood clot at home, you can monitor yourself for the common symptoms: swelling, pain, warmth, and redness in a limb. If these signs appear, especially in one arm or leg, you should see a doctor for a proper diagnosis.

If a deep vein thrombosis (DVT) goes untreated, the clot can enlarge or, more dangerously, break loose and travel to the lungs, causing a potentially fatal pulmonary embolism (PE). It can also lead to chronic pain and swelling in the affected limb, a condition known as post-thrombotic syndrome.

Yes. It is possible to have a blood clot, particularly a DVT, without experiencing any symptoms. This is why knowing your personal risk factors and discussing them with your doctor is important, especially after surgery or periods of inactivity.

You should seek immediate medical attention or go to the emergency room if you suspect a blood clot and experience severe symptoms like sudden shortness of breath, chest pain, rapid heart rate, fainting, or coughing up blood. These could be signs of a life-threatening pulmonary embolism.

No. If you suspect a blood clot, you should not stand, walk, or massage the area. This can dislodge the clot. Instead, elevate the limb and seek immediate medical attention. For prevention, frequent movement during prolonged sitting is recommended, but not after a clot is suspected.