hemoglobinuria

Hemoglobinuria is a condition in which the oxygen transport protein hemoglobin is found in abnormally high concentrations in the urine. The condition is caused by excessive intravascular hemolysis, in which large numbers of red blood cells (RBCs) are destroyed, thereby releasing free hemoglobin into the plasma. Excess hemoglobin is filtered by the kidneys, which excrete it into the urine, giving urine a discolored appearance. Hemoglobinuria can lead to acute tubular necrosis which is an uncommon cause of a death of uni-traumatic patients recovering in the ICU.

== Pathophysiology == When red blood cells are lysed, cell-free hemoglobin will bind to hemoglobin binding protein known as haptoglobin in order to prevent it from reaching the kidneys' filtration system, instead forming large complexes to be targeted and cleared by macrophages. In the absence of adequate defenses to remove hemoglobin in excess, hemoglobin can cause damage to the kidneys due to direct cytotoxic effects, oxidative stress, and endothelial dysfunction. In conditions with intravascular hemolysis, the binding capacity of available haptoglobin can be fully saturated by free hemoglobin. It is subsequently filtered through the glomerulus and actively reabsorbed by the proximal tubular cells until the reabsorptive capacity is over exceeded. This results in hemoglobin appearing in the urine, or hemoglobinuria.