With the improvement of medical conditions, especially the popularity of medical checkups, the number of patients coming to the clinic with positive urine occult blood is increasing. What is occult blood in urine? What exactly is the relationship with hematuria? Let’s talk about this topic today. The urine occult blood test is a method of detecting the presence or absence of blood in the urine through the principle of color development by chemical reaction. This method increases the speed of detection, but is not specific. In addition to intact red blood cells, components of destroyed blood cells (hemoglobin) or components from muscle tissue damage (myoglobin) can also react with the test reagent and develop a color that indicates a positive result. In some cases, even if there are no blood cells or their components in the urine, the test may still be positive, i.e., a false positive, such as when hypochlorite or some oxidizing substances are present in the urine. The false positive rate of the urine occult blood test can vary from 10% to 49%, and there is a certain probability of misidentification. Therefore, a positive urine occult blood does not mean that there is blood in the urine. In this case, it should be combined with the results of a urine sediment microscopy or a urine flow-through red blood cell count (with the exception of specific diseases – hemoglobinuria and myoglobinuria). The normal values for urine sediment microscopy are less than 3 red blood cells per high-powered view and a 12-hour urine red blood cell count of less than 500,000. Exceeding these thresholds is diagnostic of hematuria. The normal range of urine flow red blood cell count can be found in each hospital. This kind of hematuria, which can only be diagnosed under a microscope, is called microscopic hematuria; its counterpart is naked eye hematuria, which means that blood can be seen in the urine with the naked eye, and the color of the urine can be seen as slightly red, washed water-like, or blood clots, etc. The source of hematuria needs to be further determined by using a phase contrast microscope to observe whether the red blood cell morphology is normal or distorted and whether the red blood cell size is consistent. If the red blood cells show multiple morphologies and vary in cell size, especially if the proportion of spiny red blood cells is above 2%, it is highly suggestive that the red blood cells in the urine are glomerulonephritic in origin, i.e., nephritis may exist. If normal or crinkled is predominant, with few deformed red blood cells and a more uniform cell size, it is more likely to suggest an extrarenal source. In the case of microscopic hematuria of purely renal origin, with no protein in the urine and good renal function, this microscopic hematuria usually has no adverse effects on the kidneys, and some people even maintain this condition throughout their lives without developing renal insufficiency and without the need for medication. However, hematuria may also be a pre-existing form of nephritis and regular urine tests are needed to prevent changes in the condition. If it is combined with proteinuria or impaired kidney function, a detailed examination must be performed at a hospital to clarify the diagnosis and then provide targeted treatment. If it is of extra-renal origin, i.e. the red blood cells are of the same size, predominantly normal or crinkled, it is necessary to first exclude whether it is blood contamination from neighboring organs, such as female menstruation, bleeding hemorrhoids, etc.; it is also necessary to be alert to systemic diseases, such as low platelets, coagulation dysfunction, and the effect of taking anticoagulant drugs; other common causes are mainly diseases of the urinary system, stones or tumors. Related tests can be performed to assist in the diagnosis. In conclusion, positive urine occult blood is not equal to hematuria and has a certain false positive rate. Only urine sediment microscopy or urine flow red blood cell count beyond the normal range is hematuria, and hematuria requires further determination of the source to clarify the diagnosis. Microscopic hematuria of purely renal origin is benign and usually does not lead to renal insufficiency, but it needs to be reviewed regularly.