Search
urine osmolal gap
Principle:
1) calculated serum osmolality is given by the formula:
2( [Na+] + [K+]) + glucose (mg/dL)/18 + urea (mg/dL)/2.8
2) the urine osmolal gap is the difference between the measured vs calculated urine osmolality
3) 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
Related
urine osmolal gap
General
urine osmolality
References
- Medical Knowledge Self Assessment Program (MKSAP) 16,
American College of Physicians, Philadelphia 2012