The "Base Excess" is the amount of acid or base required to titrate a blood sample (of whole blood) to a pH of 7.40, at standard temperature and pressure, with a standard PaCO2 of 40mmHg. The "Standard Base Excess" is different because it uses extracellular fluid rather than whole blood. Given that extracellular fluid is a fairly heterogeneous slurry which is inconvenient to sample, the ABG machine calculates the SBE for anaemic blood, with a Hb of 50g/L. The argument for this is the buffering ability of haemoglobin. It would be inappropriate to extrapolate whole blood findings to the total extracellular fluid, because though circulating haemoglobin buffers all extracellular fluid, it does so from the intravascular compartment to which it is confined.