If you can feel the hard vein is your leg, and and the doctor in the emergency room told you that you have a blood clot, then what you have is superficial thrombophlebitis. This does not have the same significance as a clot in your deep veins. There are two systems of veins in your legs. The deep veins are beneath the muscles and are more important to the circulation. The superficial veins are above. They may be visible at the surface or beneath the surface and not visible. As long as they are above the muscle, then they are considered superficial. Superficial thrombophlebitis presents as clots in the superficial veins.. These clots are initially red, tender, and hard. They are usually self limited and respond well to heat and anti inflammatory medication. The process is usually self limited ,and rarely do these clots involve the deep veins, which can be dangerous. When they occur in the arms, it is often related to a previous IV. They may also occur in varicose veins. Although in and of itself, phlebitis is usually not dangerous, it may on occasion be an indicator of an undiagnosed illness or clotting disorder.
What you describe is clot in a superficial vein also known as superficial phlebitis. Most superficial phlebitis heals with only supportive measures , however it can be associated with clotting in a deeper vein which can be serious. I am glad your sought medical attention please follow through with the follow up plan they advised.
A vein that has a tender bump is most likely a "superficial thrombophlebitis", which in English means a blood clot and inflammation in a vein under the skin. This is usually a harmless condition, but the pain can be quite intense for a few weeks. In addition to pain, tenderness, swelling, and a firm lump or cord, the skin can be red. The limb can hurt when it is extended or moved. Treatment consists of a heating pad or warm compress for 15 minutes three times a day, as well as an anti-inflammatory medication such as ibuprofen, naproxen, aspirin, or acetaminophen. A blood clot such as this can develop in a varicose vein, but a superficial vein blood clot would not cause a varicose vein to develop. Occasionally a superficial blood clot can propagate to larger veins and deeper veins, in which case the condition becomes more severe and needs more aggressive treatment, such as anti-coagulation (see below). A blood clot in deep veins is a much more serious condition, potentially life threatening, and requires physician management. Blood thinning medication (anti-coagulation) may be indicated, as well as other measures, such as compression garments, ambulation, ultrasound examination, and follow up with the treating physician.
It was not clear from your question whether the tender bump on your arm was the result of a work or sports injury or giving blood or if it just happen spontaneously. Also, not clear what was said or done in the Emergency Room. From this limited discussion it appears you may have phlebitis which is a venous blood clot in a superficial vein. Phlebitis is treated with medications like ibuprofen, heat pads and sometimes can be drained for comfort. Superficial blood clots and bruises are common and resolve over time.
Blood clots can be dangerous depending on where they occur and in which vein. If you have a blood clot near your biceps for 10 days that is unlikely to cause serious harm. If you have a blood clot in the popliteal vein or femoral vein at the knee or distal or mid thigh that could lead to other clots forming higher up the femoral or iliac vein that could travel to your lung and be lethal (pulmonary embolus).
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