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Free hemoglobin (Hgb) in urine usually is the result of lysis of RBCs present in the urine due to bleeding into the urinary tract (kidney, ureters, bladder). Less commonly, intravascular hemolysis (eg, transfusion reaction, hemolytic anemia, paroxysmal hemoglobinuria) may result in excretion of free Hgb from blood into urine.
Injury to skeletal or cardiac muscle results in the release of myoglobin, which also is detected by this assay. Conditions associated with myoglobinuria include hereditary myoglobinuria, phosphorylase deficiency, sporadic myoglobinuria, exertional myoglobinuria in untrained individuals, crush syndrome, myocardial infarction, myoglobinuria of progressive muscle disease, and heat injury.
Screening for hematuria, myoglobinuria, or intravascular hemolysis
Free hemoglobin (Hgb), in the presence of RBCs, indicates bleeding into the urinary tract.
Free Hgb, in the absence of RBCs, is consistent with intravascular hemolysis.
Note: RBCs may be missed if lysis occurred prior to analysis; the absence of RBCs should be confirmed by examining a fresh specimen.
The test is equally sensitive to hemoglobin and to myoglobin. The presence of myoglobin may be confirmed by MYOU / Myoglobin, Urine.
Elevated specific gravity may reduce the reactivity of this test. Captopril (Capoten) also may cause decreased reactivity.
Certain oxidizing contaminants, such as hypochlorite, may produce false-positive results. Microbial peroxidase associated with urinary tract infection may cause a false-positive reaction.
Levels of ascorbic acid normally found in urine do not interfere with this test.
Appearance (internal specimens only): normal
RBCs (internal specimens only): 0-2 rbcs/hpf
Fairbanks, V.F. and Klee G.G., Textbook of Clinical Chemistry 1986, Chapter 15, p 1562