urine osmolal gap

Principle

• calculated serum osmolality is given by the formula:

2( [Na+] + [K+]) + glucose (mg/dL)/18 + urea (mg/dL)/2.8

• the urine osmolal gap is the difference between the measured vs calculated urine osmolality
• it is important to obtain [Na+], [K+], [glucose] & [BUN] on the same specimen as the measured osmolality measured vs the calculated osmolality

Clinical significance

• provides estimate of urinary ammonium excretion
  • urine ammonium is ~ 1/2 of the urine osmolal gap
  • patients with kidney defect have urine ammonium < 30 meq/L
  • patients with extrarenal losses of bicarbonate have urine ammonium > 80 meq/L

More general terms

• urine osmolality

Additional terms

• urine osmolal gap