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
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
- <25 meq/L: vomiting or remote diuretic use
- >40 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 | ||||||||||||||||||||||||