Base excess or base deficit is referred to as the whole blood buffer base and is the sum of the concentration of buffer anions (in milliequivalents per liter) contained in whole blood. These buffer anions are the bicarbonate ion (HCO₃⁻) present in plasma erythrocytes and the hemoglobin, plasma proteins, and phosphates in plasma and red blood cells. This measurement quantifies the patient’s total base excess or deficit so that clinical treatment of acid–base disturbances (specifically those that are nonrespiratory in nature) can be initiated.

The total quantity of buffer anions is 45–50 mEq/L (45–50 mmol/L), or about twice that of HCO₃⁻ alone (22–26 mEq/L or 22–26 mmol/L). Therefore, the quantity of HCO₃⁻ ions accounts for only about half of the total buffering capacity of the blood. The base excess or deficit measurement provides a more complete picture of the buffering that is taking place and is a critical index of nonrespiratory versus respiratory changes in acid–base balance.

1. Make calculations from the measurements of pH, PaCO₂, and the hematocrit.

2. Plot these values on a nomogram and read the base excess or deficit.

1. A negative value (lower than −2 mEq/L or −2 mmol/L) reflects a nonrespiratory or metabolic disturbance or true base deficit or a nonvolatile acid accumulation caused by:

   1. Dietary intake of organic and inorganic acids

   2. Lactic acid

   3. Ketoacidosis

2. A positive value (higher than +2 mEq/L or +2 mmol/L) reflects a nonvolatile acid deficit or true base excess.

Normal

• Normal values are between −2 and +2 mEq/L (−2 and +2 mmol/L).

• A positive value indicates a base excess (i.e., nonvolatile acid deficit).

• A negative value indicates a base deficit (i.e., nonvolatile acid excess).