Hypokalemia laboratory findings
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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]
Overview
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.
Laboratory Tests
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
Diagnostic Algorithm
Shown below is an algorithm depicting the possible laboratory findings and their interpretation.
| Hypokalemia [K+] < 3.5 | |||||||||||||||||||||||||||||||||||||||||||||||
| Order: ❑ 24 hours urinary K+ (UK) ❑ Transtubular potassium gradient (TTKG) | |||||||||||||||||||||||||||||||||||||||||||||||
| UK > 25-30 mEq/L/day TTKG > 7 | UK < 25 mEq/L/day TTKG < 3 | ||||||||||||||||||||||||||||||||||||||||||||||
| Renal loss of potassium | GI loss of potassium | ||||||||||||||||||||||||||||||||||||||||||||||
What is the blood pressure? | |||||||||||||||||||||||||||||||||||||||||||||||
| Normal or ↓ | ↑ | ||||||||||||||||||||||||||||||||||||||||||||||
| Check the acid/base status | Possible etiologies are: Primary aldosteronism Secondary aldosteronism Non aldosterone increase in mineralcorticoids | ||||||||||||||||||||||||||||||||||||||||||||||
| Acidemia | Alkalemia | Variable | |||||||||||||||||||||||||||||||||||||||||||||
| Check urinary chloride (UCl) | Hypomagnesemia | ↑ Aldosterone ↓ Renin | ↑ Aldosterone ↑ Renin | ↓ Aldosterone | |||||||||||||||||||||||||||||||||||||||||||
| UCl < 20 | UCl > 20 | Primary aldosteronism | Secondary aldosteronism | Non aldosterone increase in mineralcorticoids | |||||||||||||||||||||||||||||||||||||||||||