Hemosiderinuria "brown urine", occurs with chronic intravascular hemolysis

Hemolysis

Hemolysis —from the Greek meaning "blood" and meaning a "loosing", "setting free" or "releasing"—is the rupturing of erythrocytes and the release of their contents into surrounding fluid...

, in which hemoglobin is released from RBCs into the bloodstream in excess of the binding capacity of haptoglobin

Haptoglobin

Haptoglobin is a protein that in humans is encoded by the HP gene. In blood plasma, haptoglobin binds free hemoglobin released from erythrocytes with high affinity and thereby inhibits its oxidative activity. The haptoglobin-hemoglobin complex will then be removed by the reticuloendothelial system...

. The excess hemoglobin is filtered by the kidney and reabsorbed in the proximal tubule, where the iron portion is removed and stored in ferritin

Ferritin

Ferritin is a ubiquitous intracellular protein that stores iron and releases it in a controlled fashion. The amount of ferritin stored reflects the amount of iron stored. The protein is produced by almost all living organisms, including bacteria, algae and higher plants, and animals...

 or hemosiderin

Hemosiderin

thumb|Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderinHemosiderin or haemosiderin is an iron-storage complex. It is always found within cells and appears to be a complex of ferritin, denatured ferritin and other material...

. 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

Hemoglobinuria

In medicine, hemoglobinuria or haemoglobinuria is a condition in which the oxygen transport protein hemoglobin is found in abnormally high concentrations in the urine. The condition is often associated with hemolytic anemia, in which red blood cells are destroyed, thereby increasing levels of free...

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.