The rate of excretion of potassium must be assessed in relation to the plasma level. If the patient is hyperkalaemic, potassium excretion would be expected to be maximal. This can be as high as 450 mmol/day in an adult.
To excrete potassium, there must be reabsorption of sodium in the cortical collecting duct (CCD), which creates an electrical gradient down which potassium moves. (Click here for recap). This produces a high [K+] in the lumen of the CCD. Failure to produce a high [K+] will lead to hyperkalaemia.
Aldosterone and renin
Hyperkalaemia should produce a rise in plasma
aldosterone. Failure for this to occur indicates a renin or adrenal problem. If renin levels are high, this indicates an adrenal failure or inhibition of angiotensin
converting enzyme.
If ECF volume is low and renin level not raised, the problem is in the kidneys.