Direct urine ammonium measurement: time to discard urine anion and osmolar gaps.

BACKGROUND

A failure of urine ammonium to increase during acidosis indicates impaired renal acidification, and the urinary ammonium concentration is therefore a useful investigation in determining the cause of a metabolic acidosis. However, urine ammonium measurements are not widely available in routine diagnostic laboratories. This has led to the use of urine anion or osmolar gaps, which are unsatisfactory as surrogates for urine ammonium measurement.

METHODS

We evaluated the adaptation of two widely available automated plasma ammonium assays for measurement of urinary ammonium.

RESULTS

Both assays showed good recovery and linearity in urine samples spiked with ammonium chloride, and acceptable precision. Urine ammonium concentrations estimated from urinary anion and osmolar gaps showed poor agreement with measured urine ammonium concentrations.

CONCLUSIONS

Direct urine ammonium measurements are easily performed with modern autoanalysers by simple adaptation of routine plasma ammonium assays. The use of urine anion and osmolar gaps should be abandoned where direct measurement is available.