Gross hematuria is easy to define: the urine is colored red. However, other pigments, particularly myoglobin, may discolor the urine, or the urine may be contaminated by bleeding from a nonurinary source. It takes approximately 1 mL of whole blood per liter of urine to result in grossly visible hematuria. Gross hematuria may also result in false proteinuria, since 1 mL of whole blood equals about 5 billion RBCs or 50 mg of albumin. This must be taken into account when protein-losing diseases (e.g., nephritic or nephrotic syndromes) are considered in the differential diagnosis. About 3 percent of the general population has had an episode of gross hematuria.1 The incidence is higher in women (due to urinary tract infections), older adults (due to neoplasia), and older men (due to prostatic hyperplasia).
Microscopic hematuria is more difficult to define. Approximately 1 million RBCs pass into the urine each day. This amount is detected as 1 to 3 RBCs/hpf (400 *) in centrifuged urine sediment. While various definitions have been proposed, the most common (and arbitrary) definition of microscopic hematuria is more than 5 RBCs/hpf. Population-based studies of healthy young individuals have found at least one episode of transient microscopic hematuria in up to 40 percent of men and 13 percent of women.2,34 and 5 The incidence of microscopic hematuria in children and adolescents is up to 6 percent for one specimen and 1 to 2 percent for two positive specimens.67
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