Lifestyle

5 Signs of a Blood Clot Doctors Say Most People Ignore Until It’s an Emergency

By Mike Harper · August 10, 2026

Up to 900,000 Americans develop a blood clot every year. One in three who develop a pulmonary embolism die before they’re diagnosed.

A deep vein thrombosis — a blood clot that forms in the deep veins of the leg or pelvis — can develop silently, with up to 40% of cases showing no typical symptoms. When it breaks loose and travels to the lungs, it becomes a pulmonary embolism, which kills between 60,000 and 100,000 Americans annually. The window between warning and emergency can be as short as a few days.

Here’s what doctors say to watch for before it reaches that point.

One leg is suddenly swollen and the other isn’t. Swelling in both legs at the end of a long day is usually circulation or fluid retention. Swelling in one leg — especially if it’s accompanied by warmth, tenderness, or a slight purple discoloration — is a red flag. The asymmetry is what matters. A DVT obstructs blood flow in the affected limb, causing fluid to build up in a way that doesn’t mirror on the other side.

Your calf hurts but you didn’t exercise. DVT pain is frequently mistaken for a muscle cramp or strain. But the pain is typically deep, persistent, and concentrated in the calf or inner thigh — not the kind that resolves with stretching. The pain often worsens when you flex your foot upward. If your calf hurts and you haven’t done anything to explain it, the cause may not be muscular.

You’re suddenly short of breath without exertion. This is the symptom that signals a DVT may have already become a pulmonary embolism. Shortness of breath that comes on abruptly — whether you’re climbing stairs, sitting at your desk, or lying in bed — is a medical emergency when combined with any other symptom on this list. It means the clot may be blocking blood flow in the lungs.

You feel a sharp chest pain that gets worse when you breathe in. The chest pain associated with a pulmonary embolism is pleuritic — it intensifies with deep breaths, coughing, or sneezing. It’s often mistaken for a pulled chest muscle, a rib injury, or anxiety. Unlike cardiac chest pain, which tends to be crushing and central, PE pain is often sharp and localized to one side.

Your heart is racing for no apparent reason. A rapid or irregular heartbeat — especially when combined with shortness of breath or lightheadedness — can be the body’s response to reduced oxygen caused by a clot in the lungs. The heart speeds up to compensate for the blood flow obstruction. If your pulse jumps without exertion, caffeine, or anxiety, it deserves medical evaluation.

Risk factors include being over 60, recent surgery or hospitalization, prolonged sitting (flights over four hours, long car trips, desk-bound work), obesity, smoking, pregnancy, and a family history of clotting disorders. Diagnosis involves an ultrasound for DVT in the legs and a CT pulmonary angiogram for suspected PE. Both are fast and widely available.

The gap between a treatable DVT and a fatal pulmonary embolism can be measured in days. Recognizing the warning signs before the clot moves is the difference between a blood thinner and a body bag.