hemoglobinurie

Hemoglobinurie, also known as hemoglobinuria, is the presence of free hemoglobin in urine, typically resulting from intravascular hemolysis. Clinically, the urine is often dark red or brown. A urine dipstick test may be positive for blood, but microscopic examination reveals few or no red blood cells, which helps distinguish hemoglobinurie from true hematuria. Hemoglobinurie can occur with a range of underlying conditions and may be accompanied by kidney dysfunction in severe cases.

Pathophysiology and differentiation are important. When red blood cells are lysed in circulation, hemoglobin is released into plasma. Once circulating haptoglobin is depleted, free hemoglobin is filtered by the glomerulus and may appear in urine. Intravascular hemolysis can overwhelm tubular reabsorption, leading to hemoglobinuria and, in some instances, hemoglobin-induced nephrotoxicity. Hemoglobinuria must be distinguished from myoglobinuria (muscle injury) and from true hematuria caused by bleeding in the urinary tract.

Common causes include autoimmune or alloimmune hemolytic processes, paroxysmal nocturnal hemoglobinuria, mechanical destruction from prosthetic heart valves or microangiopathic states (such as TTP, HUS, DIC), transfusion reactions, and certain infections or toxins that promote red cell destruction. Strenuous exercise and some drugs can also contribute in specific contexts.

Diagnosis relies on urinalysis and laboratory studies. A positive urine heme dipstick with few or no red cells on microscopy suggests hemoglobinuria or myoglobinuria. Additional tests show elevated lactate dehydrogenase, indirect bilirubin, and reticulocytosis, with reduced haptoglobin in intravascular hemolysis. Measuring plasma free hemoglobin and assessing renal function aid in evaluating kidney involvement.

Management involves treating the underlying cause, ensuring adequate hydration, and avoiding nephrotoxins. Transfusion may be required for symptomatic anemia. Specific therapies exist for certain conditions (for example, eculizumab for paroxysmal nocturnal hemoglobinuria). Prognosis depends on the etiology and the degree of kidney involvement.