. 2005 Aug 5;9(5):500–507. doi: 10.1186/cc3789

Table 1.

Translator for acid–base variables across traditional and modern approaches

'Traditional' variable	Physical Chemical variable	Comment
pH	pH
PCO₂	PCO²
HCO₃^-	Total CO₂	Total CO₂includes dissolved CO₂, H₂CO₃and 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

Note that the translation from traditional to physical chemical variables is not a one to one exchange. Rather, the variable in the traditional column corresponds to a similar variable in the physical chemical column (see comments for further explanation). Adapted with permission from Kellum [10]. A^-, nonvolatile weak acid buffers; ABE, actual base excess; AH, nondissociated weak acid; A_TOT, total weak acids; PCO₂, partial carbon dioxide tension; SBE, standard base excess; SID, strong ion difference; SIDa, apparent strong ion difference; SIDe, effective strong ion difference; SIG, strong ion gap; X^-, unmeasured anions - unmeasured cations.