Occult blood in urine test of 3-year-old children may be related to urinary system infection, Nutcracker syndrome, blood diseases and so on. 1. Urinary system infections: such as acute glomerulonephritis, causing damage to the glomerular filtration membrane and hematuria. This is also accompanied by edema, proteinuria and other symptoms. 2. Nutcracker syndrome: caused by mechanical compression of the left renal vein at the angle formed by the abdominal aorta and the superior mesenteric artery or at the gap between the abdominal aorta and the spine. It is often accompanied by hematuria, proteinuria, and pain in the renal region. 3. Blood diseases: such as thrombocytopenic purpura, mainly seen after vaccination, infection and other causes of thrombocytopenia, when playing collision to the kidneys, more likely than normal to cause bleeding, occult blood in the urine. There are other causes of occult blood in urine test in 3-year-old children, it is recommended to seek medical advice in time, follow the doctor’s instructions for further examination, with the help of the doctor to clarify the cause, and targeted treatment or therapy.