What is the role of urine osmolality measurement in the diagnosis of hypokalemia (low potassium level)?

Updated: Dec 06, 2018
  • Author: Eleanor Lederer, MD, FASN; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

If the urine osmolality is high (>700 mOsm/kg), then the absolute value of the urine potassium concentration can be misleading and can suggest that the kidneys are wasting potassium. For example, suppose the serum potassium level is 3 mEq/L and the urine potassium level is 60 mEq/L. The high urine potassium level would suggest renal potassium loss. However, the final concentration of potassium in the urine is dependent not only on the quantity of potassium secreted in response to sodium reabsorption, but also on the concentration of the urine.

In the above example, if urine osmolality is 300 mOsm/kg (ie, not concentrated relative to serum), then a measured urine potassium of 60 mEq/L indeed suggests renal potassium loss.

However, if the urine osmolality is 1200 mOsm/kg (ie, concentrated 4-fold relative to serum), then the 60-mEq/L potassium concentration would, in the absence of urinary concentration due to water reabsorption, be only 15 mEq/L (ie, very low). The conclusion would then be that the kidneys are not responsible for the low serum potassium.


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