How accurate is a spot urine potassium measurement?

Updated: Jun 20, 2018
  • Author: Eleanor Lederer, MD, FASN; Chief Editor: Vecihi Batuman, MD, FASN  more...
  • Print
Answer

A spot urine potassium measurement is the easiest and most commonly obtained test; a 24-hour urine potassium measurement is rarely needed. However, an isolated urine potassium level often is misleading, because the urine potassium concentration is influenced not only by secretion by the cortical collecting tubule but also by the degree of urinary concentration. If urine osmolality is high (>700 mOsm/kg), the absolute value for urine potassium concentration can be misleading and suggest that the kidneys are disposing of potassium appropriately.

For example, if serum potassium is 6 mEq/L and urine potassium 60 mEq/L, the high urine potassium level may be taken as suggesting appropriate renal potassium excretion. However, the final concentration of potassium in the urine depends not only on how much potassium is secreted in response to sodium reabsorption but also on how concentrated the urine is.

In this example, if urine osmolality is 300 mOsm/kg—that is, not concentrated in relation to serum—then a measured urine potassium level of 60 mEq/L indeed suggests renal potassium loss. However, if urine osmolality is 1200 mOsm/kg—that is, concentrated 4-fold in relation to serum—then the urine potassium concentration, in the absence of urinary concentration due to water reabsorption, is 15 mEq/L, which is very low. In the latter case, the conclusion would be that the kidneys are not appropriately excreting potassium.


Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!