Can serum bicarbonate be used as a substitute for base excess?

Assessment of patient acid-base status is one of the principal uses of arterial blood gas analysis. Disturbance of acid-base, usually the result of abnormal accumulation of metabolic acids, most notably lactic acid, is a relatively frequent finding among critically ill patients. 

Base deficit (BD), a blood gas parameter routinely calculated from measured parameters (pH, pCO₂) during blood gas analysis, has proved particularly useful in such circumstances. It reflects the number of mEq or mmoles of base that need to be added to blood to return pH to normal, i.e. correct the acidosis. It is thus both a measure of the severity of acidosis and an important guide of therapeutic effect.

Metabolic acidosis also causes a reduction in serum bicarbonate, a parameter normally measured as part of the routine biochemical profile of venous blood. This study was designed to discover if venous serum bicarbonate can reliably and accurately be substituted for arterial base deficit in intensive care patients. 

Inspection of case records of 2291 ICU patients admitted over an 8-year period revealed 26,063 paired data sets for analysis. A paired data set comprised the serum bicarbonate result and BD result. A necessary inclusion rule was that the venous blood for bicarbonate estimation had to be taken at the same time as the arterial blood was sampled for blood gases. 

There was significant correlation (r = 0.85) between the paired results, allowing construction of a regression equation that enables prediction of BD from serum bicarbonate. From this equation a base deficit of 5 mmol/L (defined as the cutoff for significant metabolic acidosis) equates to a serum bicarbonate of 18 mmol/L. 

Further analysis of the paired data using receiver operator characteristic curve (ROC) revealed that serum bicarbonate accurately and reliably predicts significant metabolic acidosis (i.e. BD > 5 mmol/L): area under curve (AUC) was 0.95. In this regard it proved more accurate and reliable than pH (AUC 0.80), anion gap (AUC 0.70) and serum lactate (AUC 0.7). 

The authors conclude that serum bicarbonate derived from venous blood provides clinical information equivalent to that which is provided by base deficit, the gold standard test of metabolic acidosis.