How is urine osmolal gap calculated?

Calculated urine osmolality = 2 x (urine Na + urine K) + urine glucose/18 + urine BUN/2.8. Urine osmolal gap = measured urine osmolality minus calculated urine osmolality. The gap primarily reflects unmeasured NH4+.

What does a high urine osmolal gap indicate?

Urine osmolal gap > 100 mOsm/kg indicates adequate NH4+ excretion, suggesting an extrarenal cause of normal anion gap metabolic acidosis (e.g., diarrhoea, proximal RTA type II).

What does a low urine osmolal gap indicate in acidosis?

Urine osmolal gap < 100 mOsm/kg in acidosis indicates impaired NH4+ excretion, suggesting distal renal tubular acidosis (type I RTA) or hypoaldosteronism (type IV RTA).

Urine Osmolal Gap Calculator

Calculate urine osmolal gap (measured minus calculated) to estimate urinary ammonium excretion. Gap > 100 mOsm/kg suggests adequate NH4+ excretion in metabolic acidosis.

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