In human physiology, base excess and base deficit refer to an excess or deficit, respectively, in the amount of base present in the blood. The value is usually reported as a concentration in units of mEq/L, with positive numbers indicating an excess of base and negative a deficit. A typical reference range for base excess is −2 to +2 mEq/L.The predominant base contributing to base excess is bicarbonate. Thus, a deviation of serum bicarbonate from the reference range is ordinarily mirrored by a deviation in base excess. Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg. A base deficit (i.e., a negative base excess) can be correspondingly defined in terms of the amount of strong base that must be added.

Base excess beyond the reference range indicates•metabolic alkalosis if too high (more than +2 mEq/L)•metabolic acidosis if too low (less than −2 mEq/L)

The anion gap is the difference in the measured cations and the measured anions in serum, plasma, or urine. The magnitude of this difference (i.e. "gap") in the serum is often calculated in medicine when attempting to identify the cause of metabolic acidosis. If the gap is greater than normal, then high anion gap metabolic acidosis is diagnosed.

The concentrations are expressed in units of milliequivalents/liter (mEq/L) or in millimoles/litre (mmol/L). It is calculated by subtracting the serum concentrations of chloride and bicarbonate (anions) from the concentrations of sodium and potassium (cations):

= ([Na+] + [K+]) − ([Cl−] + [HCO3−])

However, the potassium is frequently ignored because potassium concentrations, being very low, usually have little effect on the calculated gap. This leaves the following equation:

= [Na+] − ([Cl−] + [HCO3−])

Modern analyzers make use of ion-selective electrodes which give a normal anion gap as <11 mEq/L. Therefore according to the new classification system a high anion gap is anything above 11 mEq/L and a normal anion gap is often defined as being within the prediction interval of 3–11 mEq/L, with an average estimated at 6 mEq/L.

A high anion gap indicates that there is loss of HCO3− without a

concurrent increase in Cl−. Electroneutrality is maintained by the elevated levels of anions like lactate, beta-hydroxybutyrate and acetoacetate, PO4

−, and SO4−. These anions are not part of the anion-

gap calculation and therefore a high anion gap results. Thus, the presence of a high anion gap should result in a search for conditions that lead to an excess of these substances.