The glomerulus filters 180 liters of fluid per day, about 99% of which is reabsorbed by the tubules, such as water, potassium, sodium, glucose, amino acids, uric acid, phosphate, bicarbonate and other substances needed by the body. Damage to the tubules necrosis, diabetic nephropathy, etc. can cause tubular reabsorption dysfunction. Tests for renal tubular reabsorption impairment: (1) Free water clearance The damage to the renal tubules is estimated by measuring the molar concentration of osmolality in blood and urine to calculate free water clearance (CH2O). (2) Fraction of sodium excretion (FENa) Reference value <1%. Determine the ability of renal tubules to retain sodium. In dehydration, sodium excretion fraction is generally lower than 1% due to increased renin and aldosterone, which maximizes sodium retention by the renal tubules, and in acute tubular necrosis, sodium reabsorption capacity decreases and sodium excretion fraction increases to more than 1%. (3) Urine glucose measurement Blood glucose is normal, and the presence of glucose in the urine is usually caused by renal tubular damage leading to renal tubular reabsorption dysfunction, which is renal diabetes. (4) Analysis of amino acids in urine The amino acids filtered through the glomerulus are normally reabsorbed by the renal tubules. When the renal tubular damage can make the urinary amino acids increased. (5) N-acetyl-β-D amino glucosidase (NAG) (reference value: upper limit 507 U/L) Widely present in body fluids of various tissues and organs, red blood cells, white blood cells and platelets, is an acidic hydrolase in lysosomes, the organ is most abundant in the kidney, urinary NAG is mainly derived from renal proximal tubular epithelial cells, and is a sensitive indicator of renal tubular injury. Increased: acute and chronic nephritis, renal failure, epidemic hemorrhagic fever, toxic nephropathy, renal tumor, rejection of kidney transplant patients, is an indicator of early damage in diabetic nephropathy. (6) β2-microglobulin (β2M) (reference value: 77-186 μg/L in serum) β2-microglobulin is a protein with low molecular weight, widely present on the surface of all cells, and is a smaller subunit of human leukocyte antigen (HLA'S) after filtration by the glomerulus, almost all of which is reabsorbed, and increased urinary β2-microglobulin reflects The increase of β2-microglobulin in serum may be related to the increase of synthesis or the decrease of glomerular function. Studies have shown that there is a good correlation between β2-microglobulin in serum and creatinine in serum, with a correlation coefficient of 0.985. The ratio of β2-microglobulin and creatinine can be used to judge the prognosis of kidney transplantation; the normal ratio range of β2 M/Cr: 0.25 to 2.5; when β2 M/Cr<2 5="" 2-="" 4="" 2m="" >2.5, indicating poor outcome. Malignancies such as myeloma, lymphoma, chronic lymphocytic leukemia and other B-lymphocyte malignancies are also monitored for increased β2-microglobulin in the serum. (7) Urinary alglucanase (URT) (reference value: 4~19 µmol/H-1G-1 ) Urinary alglucanase is an extracellular enzyme produced in the epithelial cells of the proximal renal tubule and small intestinal mucosa, and the urinary alglucanase mainly comes from the epithelial cells of the proximal renal tubule. Its activity can be used as an early, sensitive and specific marker of brush border damage in proximal renal tubular epithelial cells.