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Hemosiderinuria (syn. haemosiderinuria), "brown urine", occurs with chronic intravascular hemolysis, in which hemoglobin is released from RBCs into the bloodstream in excess of the binding capacity of haptoglobin. (Haptoglobin binds circulating hemoglobin and reduces renal excretion of hemoglobin, preventing tubular injury.) The excess hemoglobin is filtered by the kidney and reabsorbed in the proximal convoluted tubule, where the iron portion is removed and stored in ferritin or hemosiderin. The tubule cells of the proximal tubule slough off with the hemosiderin and are excreted into the urine, producing a "brownish" color. It is usually seen 3-4 days after the onset of hemolytic conditions.

Hemoglobinuria is another indicator of intravascular hemolysis, but disappears more quickly than hemosiderin, which can remain in the urine for several weeks; therefore, hemosiderinuria is a better marker for intravascular hemolysis.