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?

Possible etiologies are: Diarrhea Laxatives Villous adenoma

Normal or ↓

↑

Check the acid/base status

Possible etiologies are: Primary aldosteronism Secondary aldosteronism Non aldosterone increase in mineralcorticoids

Acidemia

Alkalemia

Variable

Order: ❑ Aldosterone ❑ Renin

Possible etiologies are: Diabetic ketoacidosis Renal tubular acidosis

Check urinary chloride (UCl)

Hypomagnesemia

↑ Aldosterone ↓ Renin

↑ Aldosterone ↑ Renin

↓ Aldosterone

UCl < 20

UCl > 20

Primary aldosteronism

Secondary aldosteronism

Non aldosterone increase in mineralcorticoids

Possible etiologies are: Vomiting Nasogastric tube

Possible etiologies are: Diuretics Bartter's Gitelman's

References

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