Proteinuria is an important sign of kidney damage, and the amount of urine protein has a close relationship with the degree and regression of kidney disease damage. Because of this, clinical doctors and kidney disease patients need to be objective, comprehensive and accurate in judging proteinuria. The way to reflect the degree of proteinuria by the number of “+” in the routine urine test is not accurate, because it is used to screen patients, and this test is quick but not accurate. All abnormal results of “+” proteinuria in routine urinalysis must be confirmed with a quantitative proteinuria test. The reason for accurate proteinuria testing is that not all abnormal urine protein is pathological. Proteinuria can be divided by its nature into functional proteinuria and pathological proteinuria. The former is proteinuria that occurs under physiological conditions, such as after strenuous exercise, fever, or under the influence of special factors such as high temperature, cold, or mental stress, the filter membrane of the kidney can undergo a transient permeability change and leak a small amount of proteinuria. After the change of environmental factors, the normal state is quickly restored. This is also true for some pregnant women in the second trimester, where protein in the urine can increase mildly due to uterine pressure and increased renal workload, and most of them can be relieved after delivery. Physiologic proteinuria has several important features: 1, the amount of urine protein is small, usually not more than 0.5 g/24 hours; 2, the molecular weight of protein is small, small molecules of protein, which can be identified by some immunochemical methods; 3, there are usually obvious causes. Such as fever, excessive fatigue, after a large amount of high-protein diet, etc. 4. There are no symptoms related to kidney disease, such as swelling, hematuria, hypertension, severe back pain, etc. Pathological proteinuria is a consequence of increased permeability of the glomerular filter membrane due to damage caused by various reasons. Not only is the amount of proteinuria leaked high, but most of it is large molecule protein, and most of it is accompanied by other symptoms of kidney disease (swelling, hematuria, hypertension, renal function impairment, hypoproteinemia, etc…). For unexplained proteinuria, it is important to check not only the precise amount of protein in the urine, but also the size of the “head” of the protein, usually the larger the head of protein, the greater the damage to the kidney membrane. It should be reminded that there is another form of proteinuria that is becoming more and more common in clinical practice, namely: overflow proteinuria. This type of proteinuria is mostly caused by diseases that can produce abnormal protein components themselves, such as plasma cell disease, multiple bone marrow and other hematologic diseases whose abnormal plasma cells can produce excessive amounts of light chain proteins, which can be deposited throughout the body and form proteinuria. This disease is most common in middle-aged and elderly people (over 45 years old). Therefore, when checking the composition of proteinuria, don’t forget to check for light chain proteins.