The<unk>  <unk>  is an index reflecting the conservation of<unk>  in the cortical collecting ducts (CCD) of the kidneys. It is useful in diagnosing the causes of hyperkalemia or hypokalemia. The<unk>  estimates the ratio of<unk>  in the lumen of the CCD to that in the peritubular capillaries. The following is the formula for calculating the<unk> : Note that this formula is valid only when Uosm >300 and UNa >25 The validity of this measurement falls on three assumptions: * (1) Few solutes are reabsorbed in the medullary collecting duct (MCD) * (2)<unk>  is neither secreted nor reabsorbed in the MCD * (3)The osmolality of the fluid in the terminal CCD is known Significant reabsorption or secretion of K in the MCD seldom occurs, except in profound K depletion or excess, respectively. A typical<unk>  in a normal person on a normal diet is 8-9. During hyperkalemia or high<unk>  intake, more<unk>  should be excreted in the urine and the<unk>  should be above 10. Low levels (<7) during hyperkalemia may indicate mineralocorticoid deficiency, especially if accompanied by hyponatremia and high urine Na. During<unk>  depletion or hypokalemia, the<unk>  should fall to less than 3, indicating appropriately reduced urinary excretion of<unk> .
Trans-tubular potassium gradient
Anion gap
Plasma osmolality
Net acid excretion
Osmol gap
Free water clearance
Band 3 anion transport protein
Lactate threshold