How does a doctor differentiate between phlebitis and dvt?

Size of vein: Phlebitis generally refers to inflammation, frequently associated with a clot, in a superficial small vein. It often comes after having an IV in the affected area. A DVT is a Deep Vein Thrombosis and occurs, by definition, in a deep vein. The deep veins are bigger diameter and when a clot forms there, can cause problems in the extremity and break off to go to the lung. They need eval/Rx.

DVT AND PHLEBITIS: There are two kinds of phlebitis-superficial and deep. Superficial phlebitis occurs in superficial veins under the skin and is usually easily felt by examination and is painful. Deep phlebitis is not easily seen and is usually associated with leg swelling. The best way to distinguish between the two is with a venous duplex ultrasound.