Raphael KL, et al. - The correlations between urine ammonium, the standard urine anion gap, and a modified urine anion gap that includes sulfate and phosphate were determined. Researchers also compared risks of ESRD or death between these ammonium estimates and directly measured ammonium. Urine anion gap was identified as a poor surrogate of urine ammonium in CKD unless phosphate and sulfate were included in the calculation. In CKD, direct measurements of urine ammonium are preferable as the modified urine anion gap only estimates urine ammonium and requires five measurements.