Many patients may ask me, “Is it a problem to find positive urine occult blood on physical examination? In fact, urinary occult blood and hematuria are two different things. First of all, there are three long-standing methods for determining hematuria: the dry method (i.e., urinary occult blood), the microscopic method (red blood cell count under high magnification), and the instrumental method (automatic urine sediment analyzer determination). Remember, the $10 urine routine contains only the first two methods of measurement, to have a third must add $20 for urine sediment. So for the detection of hematuria, the routine sediment simultaneous test is necessary. The principle of the urine dry chemical method for the determination of urine red blood cells is that the hemoglobin in urine or the free hemoglobin released by its destruction are ferric hemoglobin, which catalyzes the peroxidation and changes the colorless o-toluidine to blue o-toluidine, and the shade of its color is proportional to the number of red blood cells. The principle shows that: when urine contains heat-unstable enzymes, myoglobin or bacteria, the dry chemical method for determining urinary RBC will give false positives; when there is a large amount of vitamin C in urine, it can competitively inhibit the reaction resulting in false negatives from the dry chemical method; in patients with kidney disease, red blood cells are destroyed in the kidney or urinary tract, or the specific gravity of urine is too low or the pH of urine is high, which can easily cause the destruction of red blood cells and the release of hemoglobin into the urine. This causes the false positive phenomenon of the so-called erythrocyte stem chemistry test; certain oxides such as hypochlorite, myoglobin and peroxidase in microorganisms of urinary tract infection can cause false positive results. While the instrumental and microscopic methods can only detect intact erythrocytes, the principle of the instrumental method is more scientific, when the specimen is diluted and fluorescently stained, relying on hydraulic pressure through the sheath flow cell surrounded by particle-free sheath fluid, the cells pass through the central vertical axis of the flow cell in a single longitudinal column, where each cell is irradiated by an argon laser beam and each cell and other associated particles have different intensities of light scattering and fluorescence, which is detected through the two-color filter area. If the urine is positive for occult blood, but negative for red blood cells by microscopy and urine sediment, then hematuria is ruled out and is a false positive, so we must remember this. In a word, a simple positive urine occult blood is normal, except for some rare hemoglobinuria and myoglobinuria, which are generally of little significance.