Doctors use different clinical models to assess how likely it is that you have deep vein thrombosis. They all take into consideration your medical history, symptoms, other findings from the physical examination, your major and minor risk factors, and whether there is another obvious diagnosis. For example:

• If you have one or more major risk factors (such as previous deep vein thrombosis, pulmonary embolism, or cancer) and certain symptoms of deep vein thrombosis, and no other diagnosis seems obvious, you will probably be considered high risk for deep vein thrombosis.
• If you have a minor risk factor (such as pregnancy) or minor symptoms, you will probably be considered lower risk for deep vein thrombosis.
• If you have several minor risk factors (pregnancy and smoking), or a combination of major and minor risk factors, and symptoms, your risk may be medium or high risk.

Major risk factors for developing deep vein thrombosis include:

• Prolonged (more than 3 days) bed rest.
• Abnormal blood clotting (hypercoagulable state), usually a result of inherited genes from one or both parents. Protein S deficiency, protein C deficiency, antithrombin III deficiency, and factor V Leiden are examples.
• Major trauma.
• Surgery, particularly major hip or knee surgery, neurosurgery, and abdominal or chest surgery associated with cancer.
• Cancer and its treatment.
• Paralysis due to spinal cord injury.

Minor risk factors for developing deep vein thrombosis include:

• History of certain medical conditions, such as varicose veins, heart attack, heart failure, or stroke.
• A long airplane flight or car trip.
• Pregnancy, especially immediately after giving birth or after a cesarean section.
• Increasing age. People older than 40 have a greater risk of developing deep vein thrombosis.
• Being overweight (obesity).
• Using birth control pills, birth control patches, or hormone therapy.
• Smoking.

After your doctor establishes your risk level for deep vein thrombosis through the medical history and physical exam, an ultrasound is usually done. Your ultrasound results will help your doctor either diagnose deep vein thrombosis or decide how aggressively to pursue further testing.

If your doctor thinks you should have further tests, you may have two or three additional ultrasounds over the next 7 to 10 days (called serial ultrasound), or you may have a venogram.

Your doctor will use test results to either:

• Diagnose you with deep vein thrombosis and start treating you with anticoagulants.
• Rule out deep vein thrombosis and look for other explanations of your symptoms and signs.
• Decide that further tests are needed.

Testing may vary by the individual and by the doctor's specific experience, expertise, and preferences.