Base excess

Base excess is a value derived from a blood gas that captures, in a single number, the metabolic contribution to the blood's acid-base balance — that is, the part driven by bicarbonate and acids rather than by breathing (carbon dioxide). It answers the question: how much acid or base would have to be added to bring the blood back to a normal pH, after allowing for the breathing component? A value around zero is normal. A negative base excess (often called a base deficit) means there is too much acid or too little bicarbonate — a metabolic acidosis — and the more negative the number, the more severe it is. This is important because a growing base deficit is a red flag in acutely unwell patients: it commonly reflects a build-up of lactic acid from poor tissue blood flow (as in shock or sepsis) or ketones (as in diabetic ketoacidosis), and is used to track how sick someone is and whether treatment is working. A positive base excess means the opposite — a metabolic alkalosis, or compensation for a chronically high carbon dioxide, seen for example after prolonged vomiting. Base excess is read together with the pH, carbon dioxide, bicarbonate and lactate from the same blood gas.

• Metabolic alkalosis — a positive base excess; from prolonged vomiting, diuretics, or too much bicarbonate.
• Compensation for chronically high carbon dioxide — as in long-standing lung disease, where the kidney retains bicarbonate.

• Metabolic acidosis — a negative base excess (base deficit); the more negative, the more severe.
• Lactic acidosis — poor tissue blood flow in shock, sepsis or severe illness.
• Diabetic ketoacidosis — ketone build-up.
• Kidney failure or certain poisonings — retained or ingested acids.
• A worsening (more negative) base deficit — a warning that a critically ill patient is deteriorating.