Blot clots occur when the blood flow through veins changes, either because of an obstruction in the vein or a thickening of the blood. Clots that form in deep veins, called deep vein thrombosis, are a serious condition, because the clots may break loose and travel to the lungs, brain or heart, causing life-threatening complications. Blood clots that form above the knee in the leg are more likely to break loose than clots formed below the knee, according to the ClotCare Online Resource. Blood clots cause recognizable symptoms and require immediate medical attention.
Swelling
Large blood clots can completely block the flow of blood through a vein, causing blood to pool on the side clot that is on the other side of the clot from the heart. When the clot occurs above the knee, the entire leg below the knee will start to swell. Swelling due to a blood clots can be distinguished from other types of swelling because the swelling occurs suddenly and only in one leg, according to the University of Southern California Center for Vascular Care. If the clot occurs in a smaller vein close to the surface of the skin, the swelling may be centered along the vein.
Pain
A blood clot occurring above the knee can cause pain in the leg. In some cases, the pain may only be noticed when standing or walking, reports the National Heart Lung and Blood Institute, and often worsens if all of the weight is placed on one leg. Tenderness in the leg may also develop from a blood clot. If the clot is in a small, superficial vein, the vein may feel like a hard cord running underneath the skin.
Warmth
Blood clots often cause the affected leg to feel warm to the touch while the other leg remains normal. The warmth is often felt in the same area that is swollen. For example, if the clot forms above the knee in the leg, the length of the leg below the clot will feel warmer than the other leg, explains the National Heart Lung and Blood Institute. In some cases, a patient with a blood clot may develop a mild fever.
Redness
Along with swelling, warmth and tenderness, the loss of blood flow to a leg may cause the leg to turn red in color, explains MedlinePlus. Clots that form above the knee may cause all of the leg below the clot to turn red.
