DVT (usually one leg) causes one-sided swelling, pain, warmth, and redness; a pulmonary embolism causes sudden breathlessness, sharp chest pain on deep breaths, racing heart, and sometimes fainting or coughing blood, and that is a 911 call.
The one-sided rule is the quickest home check for a leg clot: one calf swollen, warm, tender, and red while the other is normal. Risk context sharpens suspicion: recent surgery or hospitalization, long immobility like flights, estrogen-containing birth control or pregnancy, cancer, obesity, smoking, and any prior clot. The action split is absolute. Suspected DVT: same-day medical evaluation, because the danger is the clot migrating to the lungs, and do not massage or aggressively exercise the leg meanwhile. Suspected PE: sudden breathlessness, sharp pleuritic chest pain, coughing blood, or collapse is emergency-services-now, not an appointment. Caught in time, clots are treated with blood thinners and outcomes are good; the CDC's core message is that these events are often preventable and treatable, with time as the variable that changes everything.
What to do
Common questions
What are the signs of a blood clot?
Blood clot symptoms depend on where the clot sits, and knowing the two big patterns can save your life. Deep vein thrombosis (DVT), a clot in a deep vein, usually one leg: swelling in one leg or arm (compare sides; one-sided is the tell), pain or tenderness that often starts in the calf, warmth over the area, and redness or discoloration. Some people describe a cramp that will not go away. Pulmonary embolism (PE), a clot that has broken off and traveled to the lungs: sudden shortness of breath, sharp chest pain that worsens with a deep breath or cough, rapid heartbeat, sometimes coughing blood, lightheadedness, or fainting. The risk context matters for suspicion: recent surgery or hospitalization, long flights or car rides, birth control pills or pregnancy, cancer, obesity, smoking, and prior clots all raise the odds. What to do: suspected DVT is same-day medical care, not watchful waiting, because the clot can travel; suspected PE (sudden breathlessness, sharp chest pain, coughing blood, fainting) is a 911 call now, not a drive to urgent care. Clots are treatable with blood thinners when caught, and the CDC emphasizes that VTE is often preventable and treatable, but only if you show up in time. Never massage or "walk off" a suspected leg clot.