Many people have a somewhat vague understanding of urine occult blood and tend to think that a positive urine occult blood test means that there is blood in the urine. In fact, a positive urine occult blood test paper reflects hemoglobin and myoglobin, not red blood cells. Urine containing more red blood cells is called hematuria. In normal people, there are no red blood cells in the urine, or occasionally there are traces of red blood cells (0 to 2 per high-powered field of view). After strenuous exercise, heavy physical labor or prolonged standing, a transient trace of red blood cells may appear in the urine; this is normal and has no clinical significance. However, if more red blood cells appear frequently in the urine, it indicates abnormality and should be closely examined. The diagnostic criteria for hematuria are: 1, fresh morning urine without precipitation, direct smear examination, red blood cells > 1 in every 2-3 high magnification field. 2, urine sediment microscopy, >3 red blood cells per high magnification field; or count red blood cells >8000/ml with cow packet waffle calculation disk, or urine red blood cell excretion rate >100,000/hr. 3, 12-hour urine sediment erythrocyte count (Addis count) > 500,000. If the urine specific gravity is too low, the urine is alkaline or the urine specimen is left for too long, the red blood cells in the urine will be destroyed, and although there is hematuria, the red blood cells can not be detected in the urine sediment. The most common causes of hematuria are urinary system diseases, among which glomerular diseases, stones, infections and tumors are more common. Urine containing free hemoglobin is called hemoglobinuria. Normal plasma contains a small amount of free hemoglobin combined with bound bead protein, which cannot be filtered through the normal glomerular filtration membrane because of its large molecular weight, so hemoglobin does not appear in the urine of normal people. When acute hemolysis occurs, a large number of red blood cells are destroyed and the destroyed red blood cells release free hemoglobin. When the free hemoglobin exceeds the binding capacity of binding bead protein, the unbound free hemoglobin is discharged from the kidney and hemoglobinuria occurs. Occult blood appears on urinalysis, but there are no red blood cells on microscopic examination. In severe cases, the urine has a saucy color. Causes of hemoglobinuria include hemolysis in the urinary tract, renal infarction, and intravascular hemolysis. The first two conditions are rare and are mainly seen clinically as hemolysis. Urine containing a large amount of myoglobin is called myoglobinuria. Myoglobinuria is caused by destruction of muscle tissue from various causes (degeneration, inflammation, extensive trauma and metabolic disorders), which produces large amounts of myoglobin that is excreted from the urine. Myoglobin has a small molecular weight and can be filtered from the glomerular filtration membrane. Therefore, the method of detecting hematuria is microscopic examination. Positive test strips for the urine dry chemical method of measuring occult blood reflect hemoglobinuria and myoglobinuria, not hematuria. Because occult blood test strips are also sensitive to ascorbic acid, the presence of a small amount of occult blood in the urine is not a cause for concern. The clinical significance of the urine occult blood test is mainly to identify hemoglobinuria, and a positive test does not mean that the person is suffering from nephritis. Urine occult blood may be positive for blood type incompatibility, blood transfusion reaction, sericosis, paroxysmal hemoglobinuria, malaria, large burns, and inflammation caused by certain drugs or toxins. Hemoglobinuria may occur during various episodes of hemolytic anemia. In addition, patients with paroxysmal sleep hemoglobinuria and allergic hemoglobinuria.