Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (240 page)

   pH < 5.1

Suggested Readings

Cohen RA, Brown RS. Clinical practice. Microscopic hematuria.
N Engl J Med.
2003;348:2330–2338.

Grossfeld GD, Litwin MS, Wolf JS, et al. Evaluation of asymptomatic microscopic hematuria in adults: the American Urological Association best practice policy—part I: definition, detection, prevalence, and etiology.
Urology.
2001;57:599–603.

HEMOGLOBINURIA

   Definition

   Hemoglobinuria refers to the presence of free hemoglobin (Hb) in urine. The condition is often associated with hemolytic anemia, wherein intravascular red cell destruction increases levels of free plasma Hb. The excess Hb is filtered by the kidneys and excreted into the urine where it is visibly detected. The renal threshold for hemoglobinuria is 100–140 mg Hb/dL plasma.

   Although free Hb directly passing the glomeruli in the ultrafiltrate is relatively uncommon (usually, RBCs enter the urinary tract and undergo various amounts of lysis), nevertheless, conditions resulting in intravascular hemolysis have the potential of producing hemoglobinuria once all available plasma haptoglobin is bound by Hb. Hb is readily absorbed by the renal proximal tubules as dissociated dimers and catabolized to ferritin. In turn, ferritin is denatured to hemosiderin that can be found in urine in cases of severe, prolonged hemoglobinuria.

   Who Should Be Suspected?

   Candidates include patients with red urine but no red cells in urinary sediment, especially if there is a history suggesting intravascular hemolysis. The classic patient with hemolysis may have many of the following findings: rapid onset of pallor, anemia, jaundice, a history of pigmented (bilirubin) gallstones, splenomegaly, the presence of circulating spherocytic or fragmented red cells on the peripheral blood smear, and/or a positive direct antiglobulin test (Coombs test).

   Inciting causes of hemoglobinuria fall into several categories:

   Hemolytic anemias with intravascular hemolysis

   Paroxysmal nocturnal hemoglobinuria

   Paroxysmal cold hemoglobinuria

   Microangiopathic hemolytic anemias (thrombotic thrombocytopenic purpura/hemolytic uremic syndrome), prosthetic heart valves, severely damaged natural valves (especially aortic)

   Severe autoimmune hemolytic anemias