Osmolal Gap (Urine)
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Na (mEq/L)
K (mEq/L)
Glucose (mmol/L)
Urea (mmol/L)
Measured (mOsm/KgH2O)
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Osmolal Gap (Urine)

The urine osmolal gap is defined as the difference between measured urine osmolality and the sum of the concentrations of sodium, potassium, urea and glucose.

It is used to estimate the urine ammonium concentration.

Formula:

  • Urine Osmolal Gap = Measured - Calculated
  • Calculated Osmolality = (2 × Na) + (2 × K) + Glucose + Urea

References:

  1. Dyck RF, Asthana S, Kalra J, et al. A modification of the urine osmolal gap: an improved method for estimating urine ammonium. Am J Nephrol. 1990;10(5):359-62.
  2. Halperin ML, Margolis BL, Robinson LA, et al. The urine osmolal gap: a clue to estimate urine ammonium in "hybrid" types of metabolic acidosis. Clin Invest Med. 1988 Jun;11(3):198-202.