[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Anion gap and delta ratio

[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Anion gap and delta ratio

The anion gap estimates unmeasured acids and splits metabolic acidoses into two broad groups.

Updated 2026-11-01

## Key facts

- AG = Na⁺ − (Cl⁻ + HCO₃⁻). It is mostly albumin, phosphate and sulfate; its normal range depends on the laboratory and analyser.
- Corrected AG: add about 2.5 mmol/L for each 1 g/dL that albumin is below normal (Figge correction).
- High AG (normochloraemic): lactate, ketones, uraemia, methanol, ethylene glycol, salicylates (mnemonic GOLD MARK).
- Normal AG (hyperchloraemic): diarrhoea, renal tubular acidosis, ureterosigmoidostomy, acetazolamide, excess saline.
- Delta ratio = (measured AG − normal AG) / (normal HCO₃⁻ − measured HCO₃⁻): 2 suggests an added metabolic alkalosis.

## In detail

- Plasma is electrically neutral: measured cations (Na⁺) exceed measured anions (Cl⁻ and HCO₃⁻) because there are unmeasured anions, chiefly albumin.
- When a strong acid and its anion (lactate, β-hydroxybutyrate, formate, glycolate) enter plasma, the H⁺ uses up HCO₃⁻ and the anion stays unmeasured: the AG rises.
- When HCO₃⁻ is lost (stool, urine) the kidney retains Cl⁻ to keep neutrality: HCO₃⁻ falls, Cl⁻ rises and the AG does not change.
- Hypoalbuminaemia (cirrhosis, nephrotic syndrome, malnutrition, critical illness) lowers the AG and can hide a lactic acidosis; that is why it is corrected.
- Causes of a low AG: hypoalbuminaemia, a cationic paraprotein (IgG myeloma), lithium or bromide poisoning (falsely high chloride).
- The delta ratio compares how much the AG rose with how much HCO₃⁻ fell. In a pure high-AG acidosis both change by similar amounts (ratio 1–2); in ketoacidosis it tends towards 1 because ketones are lost in the urine.

## Memory aid

GOLD MARK: Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal, Ketoacidosis.

**Also called**
AG, Delta–delta

## Clinical notes

- GOLD MARK: glycols (ethylene, propylene), oxoproline (chronic paracetamol), L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.
- In diabetic ketoacidosis treated with large volumes of saline the AG closes but a hyperchloraemic acidosis may remain.
- The AG is calculated from three measured values: an error in any of them (for example Cl⁻) changes it.

## Related conditions

- [High anion gap metabolic acidosis](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/)
- [Normal anion gap (hyperchloraemic) metabolic acidosis](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/)
- [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/)
- [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/)

## Related

- [Acid–base interpretation (step by step)](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/)
- [Serum osmolality and osmolar gap](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/)
- [Lactate](https://durgical.com/en/biochemistry/molecules/lactate/)
- [β-Hydroxybutyrate](https://durgical.com/en/biochemistry/molecules/hydroxybutyrate/)
- [Serum albumin as a lab test](https://durgical.com/en/biochemistry/topics/serum-albumin-as-a-lab-test/)

## Test yourself

Think of the answer, then tap the question to see it.

## How is the anion gap calculated?

Na⁺ − (Cl⁻ + HCO₃⁻); its normal value depends on the laboratory and it should be corrected when albumin is low.

## Why does profuse diarrhoea cause a normal-anion-gap acidosis?

Bicarbonate is lost in stool and the kidney retains chloride to keep neutrality; no unmeasured anion is added.

## What does a delta ratio above 2 suggest?

HCO₃⁻ fell less than the AG rose: a metabolic alkalosis (for example from vomiting) coexists.

## Why can hypoalbuminaemia hide a lactic acidosis?

Albumin is the main unmeasured anion; when it falls the baseline AG falls, so a 'normal' AG may in fact be raised.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/anion-gap-and-delta-ratio/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Anion Gap Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/anion-gap-blood-test/) (MedlinePlus, US National Library of Medicine, 2026); [Metabolic acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000335.htm) (MedlinePlus, US National Library of Medicine, 2026); [Chloride Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/chloride-blood-test/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

[← All of biochemistry](https://durgical.com/en/biochemistry/)

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/