Used to work out whether biochemical abnormalities are due to renal dysfunction. There is not really a “normal range” for sodium and potassium in the urine, because it depends whether the body is trying to retain or excrete at any given time. So urinary sodium can be undetectable in dehydration, for instance.

Since creatinine is filtered passively, you can compare how much sodium/potassium is being excreted with what you would expect, by calculating:

Sodium excretion (Urinary Na/Plasma Na), divided by creatinine clearance (urinary creatinine/Plasma creatinine). Multiply by 100 to get a percentage.

Note that creatinine in plasma is usually measured in micromoles, and in urine in millimoles. Online calculator here:
https://www.thecalculator.co/health/FENa-Calculator-309.html

If sodium low, you expect the kidneys to retain, so fractional excretion should be less than 1%. For low potassium, fractional excretion should be less than 10%. The opposite is true for high values.

Even where plasma sodium normal, fractional excretion can give you a clue to kidney disease – 1-4% suggests intrinsic renal pathology, over 4% post-renal.

Renal causes of low sodium/potassium include renal tubular acidosis (various forms), Bartter’s syndrome. Non-renal causes include GI losses (eg pyloric stenosis), Pseudo-Bartter’s syndrome (eg CF).

An alternative, possibly simpler method is transtubular potassium gradient (TTKG) :

TTKG = urine potassium/(plasma osmolality/urine osmolality)/serum potassium

For this formula to be accurate urine osmolality should be higher than plasma osmolality and urine sodium should be greater than 25 mEq/L.

Individuals with hyperkalemia should have a TTKG above 10. Values below 7 are consistent with mineralcorticoid deficiency, especially if accompanied by hyponatremia and high urine sodium concentration.

Individuals with hypokalemia should have TTKG values below 2.