Comparision ofUrineAnion Gap, Urine Osmolal Gap and Modified Urine Osmolal Gap in Assessing the Urine Ammonium in Metabolic Acidosis
CHALEOMSRI TAPANEYA-OLARN, M.D.*, RATIKORN PHUAKSUNGNERN, M.D.*, WIWAT TAPANEYA-OLARN, M.D.*, THAN OM PETCHTHONG, B.Sc. **
Affiliation : * Department of Pediatrics, ** Research Center, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok 10400, Thailand.
Twenty-four hour urine and spot urine samples from 29 patients with metabolic
acidosis were collected for evaluation of urine ammonium in relation to urine anion gap, urine
osmolal gap (OG) and modified urine osmolal gap (MOG).
Their underlying diseases included SLE in 8, RTA in 7, CRF in 6, RPGN in 2 (one with
SLE), Lowe syndrome in 2, on acetazolamide in 2, gastroenteritis in 2, and CAH in one. Twenty-
three patients had normal serum anion gap(< 14 mmol/L). Their mean C02 was 13.77 (9.4-17.9)
mmol/L, net acid excretion (NAE) was 33.18±35.36 mmol/24 hour, NH+4 excretion was
29.16±31.97 mmol/24 hour.
Neither the 24-hour urine nor spot urine anion gap correlated with corresponding urine
NH+4 with or without adding urine HCo-3 in the calculation.
Spot urine NW4 correlated well with urine OG (r2 = 0.82, p < 0.001) and less with MOG
(r2 = 0.339, p < 0.006). The urine osmolality was well correlated with the sum of 2 (Na+ + K+ +
NW4) +urea for both spot (r2 = 0.990, p < 0.001) and 24 hour urine (r2 = 0.907, p < 0.001) collec-
tion. Twenty-four hour urine NH+4 did not correlate with the OG or the MOG.
There was no correlation between spot urine NH4/Cr ratio and 24 hour urine NH4/Cr
ratio (r2 = 0.243, p = 0.53) nor between spot NAE/Cr ratio and 24 hour urine NAE/Cr ratio
(r2 = 0.380, p = 0.014).
Therefore in the presence of low urine NH+4 (< 100 mmol/L), urine osmolal gap may be
used to determine urine NH+4 indirectly with good correlation. Twenty-four hour urine collection
is still necessary to assess renal acidification.
Keywords : Metabolic Acidosis, Urine Anion Gap, Urine Osmolal Gap, Urine Ammonium
