Metabolic acidosis has been recently recognized as an important comorbid event in the high mortality rates seen in patients with end-stage renal disease. The recognition of hypobicarbonatemia is dependent on a reliable assay for total carbon dioxide (TCO2). It is common practice for dialysis facilities to send blood samples for testing to remote laboratories, which may assay bicarbonate differently than the local hospital. We noted that serum bicarbonate concentrations from blood samples sent to our reference laboratory were significantly lower (4 mEq/L) compared with blood samples sent to our local laboratory. Blood samples were assayed for TCO2 using an enzymatic technique (in the reference laboratory) and direct measurement using an electrode (in the local laboratory). The blood test results for TCO2 sent to the reference laboratory (18.7 +/- 0.8 mEq/L) were significantly lower than samples assayed in our local laboratory (22.2 +/- 0.7 mEq/L). In conclusion, recognition of the differences in assays used in the laboratory for routine bicarbonate measurements is important in defining the magnitude of metabolic acidosis and in helping to dictate appropriate therapy.