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
Many labs can be helpful. The transtubular potasium gradient (TTKG), urine potassium and urine chloride levels can help define the 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
- 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 mEq/L TTKG < 3 | UK > 25-30 mEq/L TTKG > 7 | ||||||||||||||||||||||||
| GI loss of potassium | Renal 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 | |||||||||||||||||||||||
| Possible etiologies are: Diabetic ketoacidosis Renal tubular acidosis | Check urinary chloride (UCl) | Hypomagnesemia | |||||||||||||||||||||||
| UCl < 20 | UCl > 20 | ||||||||||||||||||||||||
| Possible etiologies are: Vomiting Nasogastric tube | Possible etiologies are: Diuretics Bartter's Gitelman's | ||||||||||||||||||||||||