Urinary NAG is a lysosomal enzyme and its elevation indicates kidney damage. Urinary NAG is widely found in various tissues throughout the body and cannot be filtered through the glomerulus because of its large molecular weight. The enzyme is also abundant in kidney tissues, especially in the epithelial cells of the renal tubules, and its elevated concentration indicates kidney damage, either in the glomerulus or in the tubules. Tubular injury is the most common, such as acute tubular necrosis, tubular acidosis, obstructive nephropathy and so on. In contrast, glomerular injury is mainly caused by various types of glomerulonephritis. Urinary NAG is earlier than creatinine and urea in predicting kidney damage, so clinically, urinary NAG can be tested to help determine when early kidney damage is suspected.