Table 1 Translator for acid–base variables across traditional and modern approaches

pH

pH

PCO₂

PCO²

HCO₃^-

Total CO₂

Total CO₂ includes dissolved CO₂, H₂CO₃ and CO₃^2- in addition to HCO₃^-. However, for practical purposes, at physiologic pH the two variables are very similar

Buffer base

SIDe

In the absence of unmeasured anions SIDe = SIDa = SID. However, because this rarely happens, SIDe = SID = SIDa - SIG (see text for discussion)

SBE

SID_present - SID_equilibrium

For blood plasma in vivo, SBE rather than ABE quantifies the amount of strong acid (or strong base if SBE is negative) that would be needed to return the SID to its equilibrium point (the point at which pH = 7.4 and PCO₂ = 40). Note that change in SBE can brought about by a change in A^- or SID, but SBE only quantifies the change in SID required to reach equilibrium. In the case of a change in A^-, the new equilibrium for SID will be different (see text). The version of SBE that corrects for abnormalities in A^- (SBEc) is given in Eqn 5 (see text)

Anion gap

A^- + X^-

Virtually all of A^- is composed of albumin and phosphate. A^- can be approximated by 2(albumin [in g/dl]) + 0.5(phosphate [mg/dl]). The value of X^- is the actually the difference between all unmeasured anions and all unmeasured cations Because unmeasured anions are typically greater than unmeasured cations, the sign of X^- is positive. If a 'cation gap' exists then the convention is to refer to this as a negative anion gap

Anion gap - A^-

SIG

Anion gap - A^- approximates SIG, except that anion gap does not consider Mg²⁺, Ca²⁺, or lactate. Given that A^- + X^- = anion gap, it is tempting to equate SIG and X^-. However, SIG will change if unmeasured weak acids (A^-_X) are present as well, so actually SIG = X^- + A^-_X

N/A

A_TOT

A_TOT = A^- + AH