You can’t definitively diagnose a blood clot at home, but you can recognize the warning signs that should send you to a doctor for testing. Blood clots most commonly form in the deep veins of the legs, a condition called deep vein thrombosis (DVT), and the hallmark signs are one-sided leg swelling, pain that starts in the calf, skin that feels warm to the touch, and a color change toward red or purple. If you’re noticing these symptoms, getting a medical evaluation quickly matters because an untreated leg clot can break free and travel to the lungs.
Signs of a Blood Clot in the Leg
DVT tends to affect one leg at a time, so the most telling clue is asymmetry. If one calf or thigh is noticeably more swollen than the other, that’s a red flag. The difference doesn’t have to be dramatic. Clinically, a calf that measures more than 3 centimeters larger than the other side is considered significant, but even smaller differences paired with other symptoms warrant attention.
Beyond swelling, look for:
- Pain or cramping that often starts in the calf and feels like a deep soreness, not a surface-level muscle ache. It may worsen when you stand or walk.
- Warmth in the swollen area compared to the same spot on the other leg.
- Skin color changes such as redness on lighter skin or a purple or darkened hue on darker skin.
- Visible veins near the skin’s surface that look larger than usual.
Blood clots can also form in the arm, though this is less common. The symptoms mirror what happens in the leg: one-sided arm swelling, pain or tenderness, warmth, and discoloration. Arm clots sometimes develop after a central IV line or prolonged immobilization of the shoulder.
Why Self-Checks Are Unreliable
You may have heard of Homan’s sign, an old physical test where you flex your foot upward to see if it causes calf pain. This test is largely outdated. It only produces pain in about 50% of people who actually have a DVT, and it can also be positive in people with a herniated disc, a ruptured cyst behind the knee, or simple calf muscle spasm. Worse, forcefully flexing the foot could theoretically dislodge an existing clot. No single physical maneuver at home can reliably confirm or rule out a blood clot.
The challenge is that several common conditions look a lot like DVT. A skin infection (cellulitis) also causes redness and warmth, but it tends to give the skin a dimpled, orange-peel texture and often comes with swollen lymph nodes nearby. DVT skin is typically smooth and the redness fades when you press on it. A pulled calf muscle can cause similar pain and swelling, but usually follows a clear moment of injury. If you’re unsure which you’re dealing with, that uncertainty itself is a good reason to get checked.
What Raises Your Risk
Doctors use a scoring system called the Wells Score to estimate how likely a clot is before they even run tests. The factors that increase your probability include active cancer treatment, recent surgery (within the past four weeks), being bedridden for more than three days, a history of previous DVT, and paralysis or a cast on a lower limb. If two or more of these apply to you alongside symptoms, a clot becomes significantly more likely.
Other everyday risk factors include long flights or car rides where your legs stay still for hours, pregnancy, birth control pills or hormone therapy, obesity, and smoking. Age matters too. People over 60 are at higher risk, and clot-related lab tests become harder to interpret as you get older.
How Doctors Confirm a Clot
When you show up with suspected DVT, the evaluation typically involves two steps: a blood test and an imaging scan.
The blood test measures something called D-dimer, a protein fragment that appears in your blood when a clot is actively forming and breaking down. The standard cutoff is 500 micrograms per liter, but this threshold shifts with age. For people over 50, doctors increasingly use an age-adjusted cutoff (your age multiplied by 10) because D-dimer naturally rises as you get older. The test is excellent at ruling clots out. A negative result, combined with low clinical suspicion, means you almost certainly don’t have one. Its sensitivity is above 95%, meaning it catches the vast majority of real clots. The tradeoff is that it also comes back positive in many people who don’t have a clot, since surgery, infection, pregnancy, and inflammation can all elevate it.
If your D-dimer is elevated or your symptoms are strongly suggestive, the next step is a duplex ultrasound. This is the standard imaging test for DVT. A technician presses a probe against the skin over your veins, using sound waves to visualize blood flow and check whether veins compress normally. A vein with a clot inside it won’t flatten under pressure. The test is painless, takes 15 to 30 minutes, and gives results right away. For suspected clots in the chest or lungs, doctors use a CT scan with contrast dye instead.
Signs a Clot Has Reached the Lungs
A pulmonary embolism, where a clot travels from a vein to the lungs, is a medical emergency. The symptoms come on suddenly and feel different from DVT. The most common is shortness of breath that appears out of nowhere, even at rest, and gets worse with any movement. Chest pain is the other hallmark. It tends to be sharp, worsens when you breathe in deeply, and can stop you from taking a full breath. Some people describe it as feeling like a heart attack.
Other warning signs include a rapid or irregular heartbeat, coughing up blood (even a small amount), lightheadedness, and fainting. If you experience any combination of sudden breathlessness, chest pain, a fast heartbeat, or coughing blood, call emergency services immediately. This is true even if you haven’t noticed any leg symptoms beforehand, since a clot can travel to the lungs without ever causing noticeable leg swelling.
What to Do If You Suspect a Clot
For leg or arm symptoms without breathing difficulty or chest pain, contact your doctor the same day or go to an urgent care center that can order an ultrasound. Don’t wait days to see if the swelling resolves on its own. While you wait for your appointment, avoid massaging the swollen area, as this could potentially dislodge a clot. Staying lightly mobile (walking around rather than staying completely still) is generally fine.
For any combination of sudden chest pain, difficulty breathing, rapid heartbeat, fainting, or coughing blood, go to the emergency room or call 911. These symptoms suggest a clot may have reached the lungs, and treatment within the first few hours significantly improves outcomes. Sudden weakness or numbness on one side of your face or body, or difficulty speaking, can signal a clot affecting the brain and also requires emergency care.