Hematuria by the naked eye is a blood-like or meat-wash-like urine color seen with the naked eye. If the urine is acidic, the urine is dark red or autumn black; if the urine is alkaline, it is bright red. Regardless of how the urine color changes, red blood cells must be seen in the urine to be called hematuria. If red blood cells are found only under the microscope, it is called “microscopic hematuria”. Usually, 1000ml of urine contains less than 1ml of blood, which cannot be identified by the naked eye, and the color of urine is only slightly muddy. There is no clinical significance until the number of blood cells in the urine exceeds a certain amount. Usually, after the urine is centrifuged, there are more than 3 red blood cells in each high magnification field before there is clinical significance, which is considered pathological hematuria. I would like to emphasize that: 1, although hematuria of the naked eye is a very prominent clinical symptom, the amount of hematuria is not necessarily proportional to the severity of the disease (with the exception of traumatic hematuria), and in addition to renal damage, non-renal causes (hematologic system, infection, perirenal tissue lesions) must be considered for hematuria of the naked eye. 2, Urinary occult blood may not necessarily be hematuria, it is likely that the urine contains high levels of iron. 3, Morphological characteristics of red blood cells in urine are of great clinical significance in discerning their origin. Hematuria is an important clinical manifestation of bladder and kidney cancer, and a common symptom of severe kidney disease. Depending on the amount of blood, the color of urine may vary from normal (small amount, invisible to the naked eye), red, to strong tea-like color. The earlier it is detected, the better the outcome. Therefore, it is important to clarify the cause of blood in urine. When there is blood in the urine, the possible causes are as follows: 1. Urinary tract infection (cystitis) is the most common clinical cause, especially in young women. If blood in the urine is accompanied by symptoms of cystitis and disappears after anti-infective treatment is given, it is usually attributed to the infection and no further tests are needed. Other symptoms of cystitis include: painful urination, frequent urination, and fever. 2. Kidney stones Stones may cause bleeding when they are expelled, usually with severe pain in the abdomen and groin. 3, bladder or kidney tumors Bladder and kidney tumors are becoming more common with age, so anyone over the age of 40 who develops hematuria needs further testing to confirm the diagnosis. 4, Glomerulonephritis Nephritis is the most common cause of hematuria in children and adolescents, but it should be noted that the disease can occur at any age. Some patients can be diagnosed with urine and blood tests, but most patients require a kidney biopsy to confirm the diagnosis. Because it includes many types, it may resolve spontaneously or may lead to renal failure and hypertension. 5.Polycystic kidney Polycystic kidney is a relatively common hereditary kidney disease in clinical practice. Although the disease is present at birth, it usually has few clinical symptoms until adulthood. Blood in urine (sometimes accompanied by back pain) is often the first manifestation of this group of patients. 6. Bleeding disorders Blood clotting abnormalities (such as hemophilia) can also lead to blood in urine, which is usually painless. Although only a minority of patients with blood in the urine are due to bleeding disorders, timely detection is important because it can usually be prevented by treatment. Patients taking anticoagulants (e.g., warfarin) may also develop hematuria, suggesting the need to adjust the dose of the medication. 7. Sickle cell disease Sickle cell disease is an inherited disorder of the red blood cells that can cause pain and anemia, usually in childhood, so regular checkups should be performed before the development of renal complications, but care should be taken to avoid the use of contrast agents. 8. Benign familial hematuria is clinically characterized by recurrent hematuria, normal kidney function and a positive family history, and can be detected by test strips. If there is no family history, it is difficult to identify the type of nephritis and a kidney biopsy is required to make a clear diagnosis. 9. Other Food and drugs: Some red foods (such as beet) and drugs (such as rifampicin) can also cause red urine without clinical symptoms and will not affect health. Blood mixing: menstrual contamination, bleeding hemorrhoids.