D-dimer is a protein fragment produced when a blood clot dissolves in your body. Blood clots prevent you from losing too much blood when injured. Normally, your body will dissolve the clot once your injury heals. With a clotting disorder, clots form when you aren't injured or don't dissolve when they should. These conditions are serious and potentially life-threatening. A D-dimer test can show if you have one of these conditions.