Calculation of the Transtubular Potassium Gradient (TTKG) may be useful in the investigation of potassium disorders. 

Transtubular K⁺ Gradient (TTKG) = (Plasma Osmolality x Urine K⁺)  /  (Plasma K⁺ x Urine Osmolality)

TTKG is only valid if Urine Osmo > Plasma Osmo and Urine Sodium > 25mmol/L)

In Hyperkalaemia

TTKG < 5: Suggests renal cause (low eGFR, Hypoaldosteronism, Tubular defect)

TTKG >10: Suggests non renal cause (Cell shift, haemolysis)

In Hypokalaemia

TTKG <3: Suggests non renal loss (low intake, cell shift, GI loss)

TTKG >7: Suggests renal loss (diuretics, hyperaldosteronism, RTA)