* After filtration from glomerulus, a drug may undergo two processes (tubular reabsorption and tubular secretion) before going out from the body i.e. renal clearance *Suppose 100 mg of a drug is filtered by glomerulus and the renal clearance is 150 mg, it means 50 mg is coming from somewhere else, i.e. tubular secretion must be present. However, we cannot say that reabsorption is not occuring because if 20 mg is reabsorbed and 70 mg is secreted, same thing can happen. * Suppose, 100 mg of a drug is filtered but renal clearance is 50 mg. Therefore, 50 mg must have gone somewhere i.e. tubular reabsorption must be occurring. Again, we cannot say that tubular secretion is not present.