Blood gas analysis is the gold standard for determining acid-base status but is often omitted in clinical practice.1 Serum bicarbonate (measured as total carbon dioxide [tCO2]) is often used to evaluate acid-base status; alkalemia and acidemia are inferred from high and low tCO2 levels, respectively.2 We recently showed in patients receiving maintenance hemodialysis that compared to pH, 15% of inferences about acid-base status using tCO2 measures were incorrect.3 Notably, tCO2 measurements were systematically lower than blood gas–calculated bicarbonate (HCO3−).