A blood clot in the leg may occur in the veins, or in the arteries. A blood clot in the vein swells up the leg, but cutting off the leg is rarely needed. A blood clot in the artery is a different matter. If this happens, the patient develops pain, and the leg quickly turns black. If treatment is not started within eight hours of development of the clot, presuming it is a large enough clot, an amputation becomes a high probability. This amputation may be small, involving one toe, or it may be large, involving the leg. A well equiped, specialized center can save the leg provided the patient arrives quickly, say within two to four hours after the onset of pain. If a patient goes to a less equiped center, this hospital will probably refer the patient to a center more specialized for arterial work. Such a referral loses precious time.