Comparison of urine anion gap, urine osmolal gap and modified urine osmolal gap in assessing the urine ammonium in metabolic acidosis.

ABSTRACT

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 CO2 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 NH+4 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+ + NH+4) + urea for both spot (r2 = 0.990, p < 0.001) and 24 hour urine (r2 = 0.907, p < 0.001) collection. 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.