An easy method of mentally estimating the metabolic component of acid/base balance using the Fencl-Stewart approach

The Stewart approach defines acid/base abnormalities as resulting from changes in PCO2, strong ion difference (SID), and weak acids (mainly albumin) but needs a computer for calculation. The base excess (BE) is a measure of the net effect of changes in SID and weak acids, therefore, metabolic acid/base balance can be described as BE effects of their change from normal. We compared our mental estimation of BE effects with the more complex calculation. Acid/base abnormalities were identified in 44 critically ill patients. The BE effects of change in strong ions, change in albumin, and the BE-gap ("other species" or unmeasured anion effect) were calculated using standard equations. An estimate of the BE effects was determined. The difference between SID (using [Na+] + [K+] - ([Cl-] + lactate)) and normal SID (42 mEq/l) estimated the effect of a change in SID. The effect of a change of albumin from normal was estimated as 0.25 x (normal albumin-measured albumin). The predicted BE was defined as estimated change in SID plus albumin effect. The estimated BE-gap (BE-gap(est)) was actual BE minus predicted BE. The Bland-Altman method was used to test agreement. Calculations were made on 46 data sets from 44 individuals. The bias (limits of agreement) for calculated and estimated strong ion effects, calculated and estimated albumin effect, and calculated and estimated BE-gap were -1.26 (-3.14 to 0.66) mEq/l, 0.5 (-0.05 to 1.05) mEq/l, and 0.76 (-1.27 to 2.79) mEq/l respectively. However the bias (limits of agreement) for BE-gap and BE-gap(est) and strong ion gap were poor, being 1.1 (-4 to 14) mEq/l and 0.4 (-9.2 to 10) mEq/l respectively. The BE-gap and BE-gap(est) are unsuitable to quantify gap ions. However, our easy-to-perform estimation has a clinically acceptable bias compared to calculated BE effects and is a simple method for identifying the components of acid/base abnormalities.