Base Excess vs Standard Base Excess

OVERVIEW

• base excess is used as an indicator of the degree of metabolic disturbance

BICARBONATE

The problem with bicarbonate

• Bicarbonate levels are not an ideal indicator of either metabolic or respiratory components of acid-base disturbance because it is affected by both
• Furthermore the relationship between metabolic acidosis and bicarbonate is neither consistent nor linear
• the concentration of the bicarbonate ion (HCO₃^–) (in mEq/L) is not measured, it is calculated from the PCO₂ and pH
• Standard base excess (SBE) is the best measure of the metabolic disturbance

BASE EXCESS AND STANDARD BASE EXCESS

Base Excess

• introduced in 1958
• base excess is dose of acid or alkali to return in vitro blood to normal pH (7.40) under standard conditions ( at 37C at a PCO2 of 40 mm Hg)
• Normal Base excess is between -3 and +3mEq/L

Standard Base Excess

• standard base excess is dose of acid or alkali to return the ECF to normal pH (7.40) under standard conditions ( at 37C at a PCO2 of 40 mm Hg)
• this is the base excess calculated for anaemic blood (Hb = 50g/L)
• based on the principle that this closely represents the behaviour of the whole body, as Hb effectively buffers the plasma as well as the ECF

EXAMPLES

Abnormal base excess with normal anion gap

• normal anion gap metabolic acidosis (e.g. acetazolide, hypercholoremia, GI losses of HCO3, renal tubular acidosis)

Normal base excess with abnormal anion gap

• lactic acidosis (or other high anion gap metabolic acidosis) with pre-existing metabolic alkalosis
• HAGMA masked by hypoalbuminemia (if anion gap is uncorrected)
• salicylate toxicity – respiratory alkalosis plus in increased anion gap metabolic acidosis