When to contact a doctor
New unexplained one-sided swelling or pain needs urgent, same-day assessment. Chest pain, sudden breathlessness, coughing blood or fainting may mean a clot has travelled to the lungs: call 108. A rapidly worsening, very painful or blue swollen leg also needs emergency assessment. Do not massage the leg or wait for a routine vein appointment.
What you may notice
A leg that becomes noticeably more swollen, sore, warm or discoloured than the other may have impaired venous drainage. A deep-vein clot can cause these symptoms, but may produce few visible signs. You cannot confirm or exclude a clot by pressing the calf.
One possible cause: a deep-vein clot
What could cause it?
Other explanations include a muscle injury, skin infection, a ruptured cyst behind the knee, superficial vein inflammation or lymphatic swelling. The doctor considers recent surgery, immobility, pregnancy, medicines and previous clots. Clinical assessment, an ultrasound and sometimes a blood test help decide whether a DVT is present.
How interventional radiology may help
Most DVT treatment centres on anticoagulant medicine and follow-up. IR clot-removal or clot-dissolving treatment is reserved for selected extensive or limb-threatening clots after assessing bleeding risk. A vein stent may be appropriate for a relevant obstruction. These procedures are not required for every DVT and do not replace the broader clot-treatment plan.
Further reading: detailed assessment and treatment options
Patient information sources
References are in English.
Educational information, not a diagnosis. The causes listed are examples, not a complete list. Tests and treatment depend on your individual assessment.