Application of the clearance concept to hyponatremic and hypernatremic disorders: a phenomenological analysis.

The kidney and its response to the antidiuretic hormone (ADH) are the principal protective mechanisms necessary to maintain a normal plasma tonicity (osmolality). Hence, determination of the response of the ADH-renal axis to an abnormal plasma tonicity is an important step to understanding water homeostasis. Determination of free water clearance is the most direct clinical method to measure the ability of the kidney to reabsorb or excrete water; it can be used as a sensitive method to study water metabolism, describing the abnormal water homeostasis in simple quantitative terms. A positive electrolyte-free water clearance denotes the excretion of excess free water. A negative electrolyte-free water clearance indicates reabsorption of excess free water. During hypertonicity, an increased concentration of ADH enhances renal reabsorption of free water. With diminished ADH secretion and normal renal function, a substantial volume of free water is cleared in response to hypotonic stimuli. A positive free water clearance > 0.4 L/day in hypertonic conditions or a negative free water clearance during hypotonicity confirms an abnormal ADH-renal axis response.