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

Possible etiologies are: Diabetic ketoacidosis Renal tubular acidosis

Check urinary chloride (UCl)

UCL < 20

UCL > 20

Possible etiologies are: Vomiting Nasogastric tube

Possible etiologies are: Diuretics Bartter's Gitelman's

References

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