The actual base excess

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.

base excess formula

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.


Device-specific information in all these ABG pages refers to the ABG machine used in my home unit.

Other machines may have different reference ranges and different symbols.

For my ABG analyser, one can examine this handy operations manual.

There is also an even more handy reference manual, but one needs to be an owner of this equipment before one can get hold of it. Its called the "989-963I ABL800 Reference Manual"

Ole Siggaard-Andersen's website was an excellent source of information for this topic.

Nargis, W., et al. "Comparison of Conventionally Measured Serum Bicarbonate Values with Bicarbonate Values Obtained in Arterial and Venous Blood Gas Analysis." Bangladesh Journal of Medical Biochemistry 5.1 (2013): 12-15.

Siggaard-Andersen, Ole. "The van Slyke equation." Scandinavian Journal of Clinical & Laboratory Investigation 37.S146 (1977): 15-20.

Morgan, Thomas J., Christopher Clark, and Zoltan H. Endre. "Accuracy of base excess—an in vitro evaluation of the Van Slyke equation." Critical care medicine 28.8 (2000): 2932-2936.