Actual base excess is the concentration of titrable base when the blood is titrated back to a normal plasma pH of 7.40, at a normal pCO2 ( 40 mmHg) and 37° C, at the actual oxygen saturation.
It is reported as cBase(B)c.
This base excess represents the metabolic contribution to the change in base excess. In essence, this is what the base excess should be if all the non-metabolic influences were corrected.
It answer the question, "how much would my patient's base excess be if I were ventilating them properly?".
Why the actual base excess is adjusted to pH, CO2 and temperature
The adjustment of base excess to normal pH, CO2 and temperature values eliminates the respiratory contribution to the level of bicarbonate, essentially limiting the meaning of this value to metabolic acid-base disturbances. Raw untreated "base excess" unadjusted for these values would suffer from the same failings as the actual bicarbonate concentration (i.e. who the hell knows whether it is a respiratory acid-base disturbance or a purely metabolic one - each would have an effect). Therefore, these days the ABG machines dont even bother to report the normal base excess.
In contrast to the standard base excess (SBE), actual base excess does not correct for the buffering of extracellular fluid by haemoglobin.
Calculation of the actual base excess
The explanation of base excess is quite a simple one, but arriving at an actual value for the base excess (without titrating the actual blood sample manually) is a pain in the arse. Observe: this is how the Radimeter ABL800 FLEX calculates the actual base excess.
There, that's all clear now.
Armed only with the wise words of Siggaard-Andersen, one can summarise by saying that the base excess (which S-A calls "ctH+", or the concentration of titratable hydrogen ions) can be calculated with the use of the Van Slyke equation. This equation was the subject of Siggaard-Andersen's doctoral thesis, and he proposed to name it in honor of Donald D. Van Slyke. The process of calculation takes into account the distribution of buffering between plasma and erythrocytes (this is why ctHb crops up).
The involvement of ctHb in this equation is significant. It plays a role in calculating the standard base excess, which is corrected not only for the plasma-erythocyte shared buffering, but also for the fact that serum haemoglobin plays a role in buffering all the extracellular fluid.
Validity of actual base excess
Is this derived parameter an accurate representation of the "actual" actual base deficit? What would happen if you performed the titration like a chemistry undergrad? Well, somebody did just that, titrating with lactic acid. It turns out that the Van Slyke equation "accurately quantifies metabolic (nonrespiratory) acid-base status in blood in vitro". The researchers put the equation through its paces, test-driving it in perverse environments (eg. in a sample artificially fizzed up with 200mmHg of CO2, or diluted to an insanely low haemoglobin) - and still it worked.
Of course, this is all in vitro stuff. In the end of the chapter on standard base excess, one may see a critique of the Van Slyke equation when it is applied to the critically ill, with their wildly deranged fluid compartments and electrolytes.