Urine net acid excretion can be estimated by calculating ammonium (NH4+) plus measured urine titratable acidity (TA) minus serum icarbonate (HCO3).[reference|url=https://pubmed.ncbi.nlm.nih.gov/20072112|title=Amelioration of metabolic acidosis in patients with low GFR reduced kidney endothelin production and kidney injury, and better preserved GFR|published=2010 Apr|authors=Sorot Phisitkul, Apurv Khanna, Jan Simoni, Kristine Broglio, Simon Sheather, M Hasan Rajab, Donald E Wesson|journal=Kidney Int|]