Hypokalemia laboratory findings

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. [2]; Assistant Editor(s)-In-Chief: Jack Khouri; Rim Halaby, M.D. [3]

Urinary potassium and transtubular potassium gradient are helpful to differentiate renal loss vs gastrointestinal (GI) loss of potassium. When renal loss is suspected, the assessment of the acid/base status and urinary chloride helps in determing the underlying etiology of hypokalemia.

Shown below is a list of tests that can be useful in the evaluation of hypokalemia:

• Complete blood count (CBC)
• Blood urea nitrogen (BUN)/creatinine
• Calcium
• Magnesium
• Glucose
• Sweat chloride test
• Arterial blood gases
• Aldosterone level
• Renin levels
• Urinary sodium
• Urine potassium
  • Levels <25 meq/day (or <15 meq/L on urine spot) rule out a renal cause of hypokalemia and suggest extrarenal potassium loss or transcellular shift
  • Higher potassium excretion suggest renal losses.
• Transtubular potassium gradient (TTKG)
  • TTKG= (Urine K x Plasma osmolarity)/(Plasma K x Urine osmolarity)
  • A TTKG less than 2-3 indicates renal potassium conservation in a hypokalemic patient
  • A urine osmolality less than plasma osmolality or urine sodium <20 mEq/L, the formula is not applicable
• Urine chloride
  • <20 meq/L: vomiting or diuretic use
  • >20 meq/L: diuretics, Bartter’s, Gitelman’s, and mineralocorticoid excess

Shown below is an algorithm depicting the possible laboratory findings and their interpretation.

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