The blood in the arteries and veins remains liquid because of a fine balance between factors trying to make it clot and stop bleeding, and factors acting to prevent the blood setting. Factors which cause the blood to clot are thrombogenic, and those factors which keep the blood liquid are thrombophyllic. Just as some patients have a tendency to bleed (haemophiliacs), some patients also have a tendency to clot. A clot can occur in any vessel – arteries or veins. Clots in superficial veins seldom give rise to serious problems, although they can be associated with considerable pain. If blood sets in the deep veins – and these are the central veins, called deep veins because they run beneath an encircling layer of fibrous tissue (deep fascia) – then it may block the outflow of blood from the limb. Ninety five per cent of deep vein thromboses occur in the lower limb and pelvic vessels. An obstruction to the deep veins can cause swelling, pain, discomfort and dilatation of the superficial veins (see Figure 9.1). These signs and symptoms may or may not be present. It is possible to have very extensive deep vein thrombosis with no apparent signs or symptoms. Over half the patients who have a deep vein thrombosis have no signs or symptoms and, sometimes, to cause further difficulty, the signs which we would attribute to deep vein thrombosis may be caused by other factors. A rupture of a calf muscle, a rupture of a cyst behind the knee and minor trauma can all cause the leg to swell with redness and pain. The clinical diagnosis of a deep vein thrombosis may be extremely difficult and, for that reason, tests are relied upon in order to make the diagnosis.