Base excess on a blood gas: a summary of the metabolic component

Blood pH depends on two things: carbon dioxide, controlled by breathing, and bicarbonate and other buffers, controlled mainly by the kidneys. Changes in carbon dioxide move pH quickly and can mask what the metabolic system is doing. Base excess is a calculated figure that tries to isolate the metabolic contribution: it asks how much acid or base would be needed to bring the blood back to a normal pH if carbon dioxide were normal.

Lab Tests Online UK explains that blood gas analysers measure pH, PO2 and PCO2 directly, and that calculations or measurements give other values such as oxygen saturation and bicarbonate. Base excess belongs to this group of derived values, and this page relies on general acid-base principles for the definition, since no UK patient-facing source describes the figure in detail.

A value near zero indicates the metabolic component is balanced. A clearly negative number, called a base deficit, indicates metabolic acidosis, meaning acid has built up or bicarbonate has been lost. A clearly positive number indicates metabolic alkalosis. Your report prints the interval used by the hospital laboratory, and clinicians interpret the figure together with the pH, PCO2, bicarbonate, lactate and clinical story.

Because it is a summary of the metabolic component, it is most useful when the pH is abnormal and the doctor wants to know how much of the problem is respiratory and how much metabolic.

Lab Tests Online UK describes metabolic acidosis, with low pH and decreased bicarbonate, as due to a metabolic or kidney disorder, with causes including diabetes, shock, drugs and kidney failure. In its bicarbonate guide it lists raised anion gap causes such as diabetic ketoacidosis, kidney disease, lactic acidosis, alcoholic ketoacidosis, toxic alcohol poisoning and overdoses of paracetamol or salicylate, and normal anion gap causes such as diarrhoea and renal tubular acidosis.

Lactate, which rises when tissues lack oxygen, is often measured at the same time. The lactate guide says lactic acidosis can disrupt pH balance and cause symptoms such as muscular weakness, rapid breathing, feeling sick, vomiting, sweating and even coma.

Metabolic alkalosis, with raised pH and increased bicarbonate, is listed by Lab Tests Online UK with causes including low blood potassium, chronic vomiting, dehydration and sodium bicarbonate overdose. A positive value can also be a normal response to chronic lung disease, where the kidneys retain bicarbonate to offset a high carbon dioxide.

The value is generated from an arterial blood sample, usually from the radial artery, which must be capped, free of air bubbles and analysed immediately. It is therefore mainly seen in hospital settings such as emergency departments, intensive care and operating theatres. Patients rarely need to interpret it themselves.

A markedly abnormal value prompts treatment of the cause, repeat gases and often lactate, glucose, ketone, kidney function and electrolyte tests. Call 999 for rapid breathing, drowsiness, confusion or collapse; the team will use the whole set of values, not base excess alone.