What is base excess (BE)?

From the Guidelines

Base excess is a laboratory value that measures the amount of excess or deficit of buffer base in the blood, indicating the metabolic component of acid-base balance. It represents the amount of acid or base that would need to be added to a blood sample to normalize its pH to 7.4 at a PCO2 of 40 mmHg. A positive base excess (greater than +2 mEq/L) indicates metabolic alkalosis, while a negative base excess (less than -2 mEq/L), also called base deficit, indicates metabolic acidosis. This measurement is particularly useful in critical care settings to distinguish between respiratory and metabolic causes of acid-base disturbances and to guide treatment decisions. Base excess is calculated from blood gas measurements including pH and PCO2, and it helps clinicians determine the severity of metabolic acid-base disorders independently of respiratory compensation. Understanding base excess is essential for proper management of patients with conditions like diabetic ketoacidosis, lactic acidosis, or severe diarrhea, where significant acid-base imbalances can occur, as noted in studies such as 1. The traditional ATLS classification system of hypovolaemic shock now includes physiological base excess and may serve as a rough estimate for blood loss and transfusion requirements, but is not without limitations, as discussed in 1. Increased base excess (indicating chronicity of hypercapnia) is also an additional prognostic variable in patients with acute hypercapnic respiratory failure, as mentioned in 1.

Key Points

• Base excess measures the metabolic component of acid-base balance
• Positive base excess indicates metabolic alkalosis, while negative base excess indicates metabolic acidosis
• Base excess is useful in critical care settings to guide treatment decisions
• It is calculated from blood gas measurements including pH and PCO2
• Understanding base excess is essential for managing patients with significant acid-base imbalances, as supported by studies such as 1

From the Research

Definition of Base Excess

• Base excess (BE) is a diagnostic tool for several in vivo events, such as mortality after multiple trauma or shock, acidosis, bleeding, clotting, and artificial ventilation 2.
• It is calculated including pH, pCO2, sO2, and cHb, and is used to assess metabolic acidosis or alkalosis 2.

Clinical Significance of Base Excess

• Base excess has been a parameter with exceptional clinical significance since 2007 2.
• It is used to identify metabolic acidosis, which arises from an abnormal process that generates non-carbonic acid or an abnormal loss of HCO3- 3.
• Metabolic acidosis can be identified by an increase or normal anion gap, and is usually managed by treating the disorder generating the acid and enhancing the clearance of the acid anion 3.

Measurement and Calculation of Base Excess

• Base excess is measured using a modern pH-blood gas analyser, and is described in terms of the arterial pH, the arterial pCO2, and the extracellular base excess 4.
• The bicarbonate concentration may be used as a screening parameter of a non-respiratory acid-base disturbance when respiratory disturbances are taken into account 4.

Relationship between Base Excess and Metabolic Acidosis

• Metabolic acidosis is a major complication of critical illness, and base excess is used to diagnose and manage this condition 5.
• Early sodium bicarbonate therapy is often used to treat metabolic acidosis, and has been shown to be beneficial in patients with vasopressor dependency 5.
• The incidence and management of metabolic acidosis with sodium bicarbonate in the ICU is an area of ongoing research 5, 6.