(a) The effective strong ion difference ([SID+]e; Eqn 18) can be understood as a combination of [HCO3-], the buffer value (β) and constant terms. The [HCO3-] parameter can be determined from the (b) Henderson–Hasselbalch equation, whereas (d) the buffer value is derived partly from the albumin data of Figge and Fencl (c). When noncarbonate buffers are held constant, Δ[SID+]e is equal to the base excess (BE). (e) In physiologic states with a low β, BE may be an insensitive indicator of important acid–base processes. (f) The strong ion gap (SIG), which quantifies 'unmeasured anions', can be calculated from the anion gap (AG) and β. In physiological states with a low β, unmeasured anions may be present (high SIG) even with a normal AG.