Hematuria is defined as the presence of red blood cells in the urine. When visible to the patient, it is termed gross hematuria, while microscopic hematuria is not visible to the naked eye but rather detected by the microscopic examination of the urinary sediment.
The dipstick method to detect hematuria depends on the ability of hemoglobin to oxidize a chromogen indicator with the degree of the indicator color change proportional to the degree of hematuria. Dipsticks have a sensitivity of 95% and a specificity of 75% and positive results need to be confirmed with a microscopic examination of the urine as patients may test positive when there is not true blood there. Free hemoglobin, myoglobin and certain antiseptic solutions (povidone- iodine) will give rise to these false positive readings. Knowing the serum myoglobin level and results of the microscopic urinalysis will help differentiate these confounders. The presence of significant proteinuria (2+ or greater) suggests a nephrologic origin for hematuria. The presence of many epithelial cells suggests skin or vaginal contamination.
Microscopic examination of urine is performed on 10 mL of a midstream, clean-catch specimen that has been centrifuged for 10 minutes at 2000 rpm or for 5 minutes at 3000 rpm. The sediment is resuspended and examined under high power magnification. With this method, microscopic hematuria is defined as > 3 red blood cells per high powered field (rbc/hpf) on a single specimen.