Interpretation of Urine Electrolytes and Osmolality

Question 1

What is the significance of measuring urine Na⁺, Cl⁻, and K⁺?

Answer 1

Useful in the diagnostic evaluation of volume status, hyponatremia, acute kidney injury (AKI), hypokalemia, and acid–base disorders.

Question 2

What is typically adequate for the determination of urine electrolytes?

Answer 2

A spot urine sample.

Question 3

What is the purpose of measuring urine creatinine?

Answer 3

Used for the calculation of fractional excretion of Na⁺, urea, K⁺, or other electrolytes.

Question 4

When are serum and urine osmolalities requested?

Answer 4

For the differential diagnosis of hyponatremia, polyuria, and AKI.

Question 5

What does urine Na⁺ assess?

Answer 5

• Volume status
• Differential diagnosis of hyponatremia
• Differential diagnosis of AKI
• Salt intake in hypertension
• Calcium/uric acid excretion in stone-formers
• Electrolyte-free water clearance

Question 6

What is the clinical use of urine Cl⁻?

Answer 6

Differential diagnosis of metabolic alkalosis.

Question 7

What does urine K⁺ help to diagnose?

Answer 7

Dyskalemias and calculate electrolyte-free water clearance.

Question 8

What is the role of urine osmolality?

Answer 8

Differential diagnosis of hyponatremia, polyuria, AKI and calculate urine osmola gap.

Question 9

What does the urine osmolal gap estimate?

Answer 9

Urinary NH₄⁺ excretion rate.

Question 10

What is the urine anion gap (UAG) used for?

Answer 10

Distinguish distal RTA from diarrhea in metabolic acidosis.

Question 11

What does electrolyte-free water clearance estimate?

Answer 11

Solute-free water excretion and manage hypo-/hypernatremia.

Question 12

How is the fractional excretion of Na⁺ (FE_Na) calculated?

Answer 12

FE_Na (%) = (U_Na × P_Cr) / (P_Na × U_Cr) × 100.

Question 13

What does a FE_Na <1% indicate?

Answer 13

Prerenal azotemia.

Question 14

What does a FE_Na >2% indicate?

Answer 14

Acute tubular necrosis (ATN) or diuretic use.

Question 15

How is the urine anion gap (UAG) calculated?

Answer 15

UAG = [Na⁺] + [K⁺] - [Cl⁻].

Question 16

What does a positive UAG indicate?

Answer 16

Distal RTA or chronic kidney disease (CKD).

Question 17

What does a negative UAG indicate?

Answer 17

Diarrhea.

Question 18

How is the urine osmolal gap calculated?

Answer 18

Urine Osmolal Gap = Measured Osmolality - (2 × (Na⁺ + K⁺) + (BUN + Glucose) / 2.8).

Question 19

What does a urine osmolal gap >100 mOsm/kg indicate?

Answer 19

High NH₄⁺ excretion.

Question 20

How is electrolyte-free water clearance (C^e_H₂O) calculated?

Answer 20

C^e_H₂O = V (1 - (U_Na + U_K) / P_Na).

Question 21

What does a positive electrolyte-free water clearance indicate?

Answer 21

Hypernatremia (free water loss).

Question 22

What does a negative electrolyte-free water clearance indicate?

Answer 22

Hyponatremia (water retention).

Question 23

What urine Na⁺ levels indicate extrarenal Na⁺ loss in hypovolemia?

Answer 23

0–20 mEq/L.

Question 24

What urine Na⁺ levels indicate renal salt wasting in hypovolemia?

Answer 24

> 20 mEq/L.

Question 25

What does FE_Na <1% in AKI indicate?

Answer 25

Prerenal azotemia.

Question 26

What does FE_Na >2% in AKI indicate?

Answer 26

Acute tubular necrosis (ATN).

Question 27

What urine Na⁺ level and FE_UA >10% indicate in hyponatremia?

Answer 27

Syndrome of inappropriate antidiuretic hormone secretion (SIAD) or cerebral salt wasting.

Question 28

What does Cl⁻ <10 mEq/L indicate in metabolic alkalosis?

Answer 28

Cl⁻-responsive conditions (e.g., vomiting).

Question 29

What does U_K/U_Cr <1.5 mmol/mmol indicate in hypokalemia?

Answer 29

Extrarenal loss (e.g., diarrhea).

Question 30

What is the normal range for urine osmolality?

Answer 30

50–1200 mOsm/kg H₂O.

Question 31

What osmolality >400 mOsm/kg indicates in prerenal AKI?

Answer 31

Enhanced water reabsorption.

Question 32

What osmolality <400 mOsm/kg indicates in ATN?

Answer 32

Tubular injury.

Question 33

What osmolality >200 mOsm/kg indicates in SIAD?

Answer 33

Water retention.

Question 34

What osmolality ≤100 mOsm/kg indicates in central diabetes insipidus?

Answer 34

ADH deficiency.

Question 35

In Case 1, what does FE_Na <1% indicate despite ATN?

Answer 35

Na⁺ reabsorption.

Question 36

In Case 1, what does FE_Urea <35% confirm?

Answer 36

Hypovolemia.

Question 37

In Case 2, what does a positive UAG indicate?

Answer 37

Distal RTA.

Question 38

In Case 2, what does a negative UAG indicate?

Answer 38

Diarrhea.

Question 39

In Case 3, what does U_K/U_Cr <1.5 indicate?

Answer 39

Extrarenal loss.

Question 40

In Case 3, what does U_K/U_Cr >2.5 indicate?

Answer 40

Renal loss.

Question 41

Who authored the suggested reading on measurement of urinary electrolytes?

Answer 41

Harrington JT, Cohen JJ.

Question 42

What is the title of the suggested reading by Kamel KS and Halperin ML?

Answer 42

Intrarenal urea cycling.