Interpretation of Urine Electrolytes and Osmolality Flashcards
What is the significance of measuring urine Na⁺, Cl⁻, and K⁺?
Useful in the diagnostic evaluation of volume status, hyponatremia, acute kidney injury (AKI), hypokalemia, and acid–base disorders.
What is typically adequate for the determination of urine electrolytes?
A spot urine sample.
What is the purpose of measuring urine creatinine?
Used for the calculation of fractional excretion of Na⁺, urea, K⁺, or other electrolytes.
When are serum and urine osmolalities requested?
For the differential diagnosis of hyponatremia, polyuria, and AKI.
What does urine Na⁺ assess?
- 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
What is the clinical use of urine Cl⁻?
Differential diagnosis of metabolic alkalosis.
What does urine K⁺ help to diagnose?
Dyskalemias and calculate electrolyte-free water clearance.
What is the role of urine osmolality?
Differential diagnosis of hyponatremia, polyuria, AKI and calculate urine osmola gap.
What does the urine osmolal gap estimate?
Urinary NH₄⁺ excretion rate.
What is the urine anion gap (UAG) used for?
Distinguish distal RTA from diarrhea in metabolic acidosis.
What does electrolyte-free water clearance estimate?
Solute-free water excretion and manage hypo-/hypernatremia.
How is the fractional excretion of Na⁺ (FENa) calculated?
FENa (%) = (UNa × PCr) / (PNa × UCr) × 100.
What does a FENa <1% indicate?
Prerenal azotemia.
What does a FENa >2% indicate?
Acute tubular necrosis (ATN) or diuretic use.
How is the urine anion gap (UAG) calculated?
UAG = [Na⁺] + [K⁺] - [Cl⁻].
What does a positive UAG indicate?
Distal RTA or chronic kidney disease (CKD).
What does a negative UAG indicate?
Diarrhea.
How is the urine osmolal gap calculated?
Urine Osmolal Gap = Measured Osmolality - (2 × (Na⁺ + K⁺) + (BUN + Glucose) / 2.8).
What does a urine osmolal gap >100 mOsm/kg indicate?
High NH₄⁺ excretion.
How is electrolyte-free water clearance (CeH₂O) calculated?
CeH₂O = V (1 - (UNa + UK) / PNa).
What does a positive electrolyte-free water clearance indicate?
Hypernatremia (free water loss).
What does a negative electrolyte-free water clearance indicate?
Hyponatremia (water retention).
What urine Na⁺ levels indicate extrarenal Na⁺ loss in hypovolemia?
0–20 mEq/L.
What urine Na⁺ levels indicate renal salt wasting in hypovolemia?
> 20 mEq/L.
What does FENa <1% in AKI indicate?
Prerenal azotemia.
What does FENa >2% in AKI indicate?
Acute tubular necrosis (ATN).
What urine Na⁺ level and FEUA >10% indicate in hyponatremia?
Syndrome of inappropriate antidiuretic hormone secretion (SIAD) or cerebral salt wasting.
What does Cl⁻ <10 mEq/L indicate in metabolic alkalosis?
Cl⁻-responsive conditions (e.g., vomiting).
What does UK/UCr <1.5 mmol/mmol indicate in hypokalemia?
Extrarenal loss (e.g., diarrhea).
What is the normal range for urine osmolality?
50–1200 mOsm/kg H₂O.
What osmolality >400 mOsm/kg indicates in prerenal AKI?
Enhanced water reabsorption.
What osmolality <400 mOsm/kg indicates in ATN?
Tubular injury.
What osmolality >200 mOsm/kg indicates in SIAD?
Water retention.
What osmolality ≤100 mOsm/kg indicates in central diabetes insipidus?
ADH deficiency.
In Case 1, what does FENa <1% indicate despite ATN?
Na⁺ reabsorption.
In Case 1, what does FEUrea <35% confirm?
Hypovolemia.
In Case 2, what does a positive UAG indicate?
Distal RTA.
In Case 2, what does a negative UAG indicate?
Diarrhea.
In Case 3, what does UK/UCr <1.5 indicate?
Extrarenal loss.
In Case 3, what does UK/UCr >2.5 indicate?
Renal loss.
Who authored the suggested reading on measurement of urinary electrolytes?
Harrington JT, Cohen JJ.
What is the title of the suggested reading by Kamel KS and Halperin ML?
Intrarenal urea cycling.
