Anion‑gap metabolic acidosis and acid-base derangement in the elderly
This section summarises how to use, interpret and apply the anion gap (AG) in the emergency assessment of metabolic acidosis in older and frail patients. Emphasis is on practical ED pathways, common causes in older people, albumin correction, pitfalls and immediate actions (including when to involve toxicology, nephrology or critical care).
What is the anion gap?
- AG = [Na+] - ([Cl-] + [HCO3-]).
- It is a bedside index used to detect unmeasured anions (organic acids, toxic metabolites) and to classify metabolic acidosis.
- Always confirm acidemia with a blood gas (pH, pCO2, HCO3-) - the AG is an adjunct, not a substitute for gas analysis.
Normal values and albumin dependence
- Typical laboratory reference: AG ≈ 4-12 mmol/L (local ranges vary).
- Practical thresholds:
- AG
> 30 mmol/L: very likely significant metabolic acidosis.
- AG 20-29 mmol/L: mixed results are common; confirm with blood gas and clinical context.
- Albumin is the main unmeasured anion. Hypoalbuminaemia (very common in frail/critically ill older patients) reduces the AG and can mask a high‑AG acidosis.
- Rule of thumb: each 1 g/L fall in albumin lowers AG by ≈ 0.25 mmol/L.
- Corrected AG = measured AG + 0.25 × (40 - albumin [g/L]).
- Phosphate also contributes to the physiological AG (less commonly used in routine correction).