Anion Gap Calculator

The anion gap in brief

The anion gap is a calculated value that helps clinicians work out the cause of metabolic acidosis – a build-up of acid in the body. It is calculated from routine electrolytes:

Anion gap = sodium − (chloride + bicarbonate)

Some laboratories include potassium: sodium + potassium − (chloride + bicarbonate). All values are in mmol/L (equivalent to mEq/L for these ions).

The "gap" exists because blood contains negatively charged substances (anions) that are not routinely measured – mainly albumin, plus phosphate, sulphate and organic acids such as lactate. Electrical neutrality means positive and negative charges must balance, so the difference between the main measured cations and anions reflects these unmeasured anions.

On modern analysers, a typical anion gap without potassium is roughly 3–11 mmol/L, although each laboratory has its own range, and some still quote older ranges such as 8–16 mmol/L. When potassium is included, the range is about 4 mmol/L higher.

The calculator above gives the anion gap, an optional version including potassium, and an albumin-corrected gap, which matters because low albumin lowers the anion gap and can hide a high-gap acidosis.

What metabolic acidosis is

The body keeps blood pH in a narrow range, about 7.35–7.45, using buffers – mainly bicarbonate – together with the lungs and kidneys. Metabolic acidosis occurs when acid accumulates or bicarbonate is lost, causing bicarbonate to fall. The lungs respond by increasing breathing to blow off carbon dioxide, which partly corrects the pH. Deep, rapid breathing (Kussmaul breathing) is a classic sign of severe metabolic acidosis.

Acidosis matters because it affects heart rhythm, blood pressure, muscle function, potassium levels and consciousness. Mild, chronic acidosis – as in chronic kidney disease – can weaken bones and muscles over time. The anion gap is the first step in working out why bicarbonate is low.

How to calculate the anion gap

1. Take sodium, chloride and bicarbonate (sometimes reported as "CO₂" or "total CO₂") from the same blood sample.
2. Add chloride and bicarbonate.
3. Subtract that sum from sodium.
4. Optionally, correct for albumin: add 2.5 mmol/L for every 1 g/dL (10 g/L) that albumin is below 4.0 g/dL (40 g/L).

Worked examples

The last example shows why albumin correction matters: a gap of 10 looks reassuring, but with albumin of 2.5 g/dL the corrected gap of about 14 suggests extra unmeasured anions, such as lactate.

Why the normal range has fallen over time

When the anion gap came into wide use in the 1970s, the normal range was usually quoted as about 8–16 mmol/L, with an average of 12. In the 1990s, laboratories moved to ion-selective electrodes, which measure chloride slightly higher. As a result, the normal anion gap on many modern analysers is lower, roughly 3–11 mmol/L, with an average near 6–7. Textbooks and online resources still often quote the old range, which can cause confusion. The values used in the delta ratio and in teaching examples (such as 12) come from the older era, so they should ideally be replaced with your laboratory's own mean.

Why albumin matters

Albumin is the largest contributor to the normal anion gap, because each albumin molecule carries many negative charges at normal blood pH. When albumin falls – common in hospital patients, liver disease, nephrotic syndrome, malnutrition and critical illness – the anion gap falls too. Figge and colleagues showed in 1998 that each 1 g/dL fall in albumin lowers the anion gap by about 2.3–2.5 mmol/L. Without correction, a high-gap acidosis can appear to have a normal gap.

A step-by-step approach used by clinicians

When an electrolyte panel shows low bicarbonate, a typical approach is:

1. Confirm acidosis with a blood gas showing low pH (below 7.35) and low bicarbonate.
2. Check respiratory compensation using Winter's formula: expected PaCO₂ = 1.5 × bicarbonate + 8 (± 2) mmHg. If the measured PaCO₂ is higher, there is also a respiratory acidosis; if lower, a respiratory alkalosis.
3. Calculate the anion gap and correct it for albumin.
4. If the gap is high, measure lactate, ketones, kidney function and, if poisoning is possible, osmolality and drug levels.
5. If the gap is high, calculate the delta ratio to look for a second disorder.
6. If the gap is normal, consider gut losses, intravenous saline and kidney causes, and use the urine anion gap.

This structured approach is taught widely because it catches mixed disorders that would otherwise be missed.

High anion gap metabolic acidosis

A high anion gap with low bicarbonate means acids that are not chloride have accumulated. Common causes are often remembered with the mnemonic GOLDMARK:

GOLDMARK was proposed by Mehta and colleagues in 2008 as an update to older mnemonics such as MUDPILES. Most high-gap acidoses seen in practice are due to lactic acidosis, ketoacidosis or kidney failure.

Diabetic ketoacidosis

In diabetic ketoacidosis (DKA), a lack of insulin leads to the production of ketone acids, which lower bicarbonate and raise the anion gap. Diagnostic criteria typically include raised glucose (or known diabetes), raised ketones and a bicarbonate below about 15–18 mmol/L or pH below 7.3. The anion gap is used to track recovery: as insulin treatment clears ketones, the gap closes. DKA can occur with near-normal glucose ("euglycaemic DKA"), particularly in people taking SGLT2 inhibitors.

Lactic acidosis

Lactate accumulates when tissues are short of oxygen (as in shock or severe heart failure), when the liver cannot clear it, or with some medicines and toxins. Lactate levels are measured directly, and the anion gap rises roughly in proportion, although the relationship is imprecise.

The osmolal gap

If a toxic alcohol such as methanol or ethylene glycol is suspected, clinicians also calculate the osmolal gap – the difference between measured and calculated osmolality. A high anion gap combined with a high osmolal gap is a red flag for toxic alcohol poisoning, which needs urgent treatment. See our serum osmolality calculator for details.

Normal anion gap (hyperchloraemic) metabolic acidosis

When bicarbonate is low but the anion gap is normal, chloride usually rises to compensate. Common causes include:

• diarrhoea, which causes loss of bicarbonate from the gut
• large volumes of 0.9% saline, which contains more chloride than plasma
• renal tubular acidosis, in which the kidneys cannot excrete acid or reabsorb bicarbonate
• ureteral diversion surgery
• carbonic anhydrase inhibitors such as acetazolamide
• early kidney disease
• recovery from DKA, when ketones are lost in the urine before bicarbonate is rebuilt

The urine anion gap (urine sodium + urine potassium − urine chloride) helps distinguish gut losses from kidney causes.

The urine anion gap explained

The urine anion gap estimates how much ammonium the kidneys are excreting. Healthy kidneys respond to acidosis by excreting more ammonium, which is accompanied by chloride, making the urine anion gap negative. In diarrhoea, the kidneys respond normally and the urine gap is negative. In distal renal tubular acidosis, the kidneys cannot excrete ammonium properly, and the urine gap is positive. The test is less reliable when urine sodium is very low, when unusual anions such as ketones are present in the urine, or when the urine is very dilute.

Low anion gap

A low anion gap is uncommon. The most frequent cause is low albumin. Other causes include laboratory error, very high levels of positively charged proteins in some cases of multiple myeloma, severe hypercalcaemia or hypermagnesaemia, lithium toxicity, and bromide or iodide exposure, which falsely raise measured chloride.

The delta ratio

When a high-gap acidosis is present, clinicians sometimes compare the rise in the anion gap with the fall in bicarbonate – the delta ratio (or delta-delta):

Delta ratio = (anion gap − 12) ÷ (24 − bicarbonate)

The normal values (12 and 24) should ideally be adjusted to the laboratory's own reference values. Roughly:

The delta ratio is a rough guide, and its interpretation is debated, but it can reveal mixed disorders that a single number would miss.

Tracking the anion gap in diabetic ketoacidosis: an example

A 24-year-old with type 1 diabetes arrives with vomiting and rapid breathing. Blood tests show sodium 134, chloride 96, bicarbonate 8 mmol/L and albumin 4.2 g/dL. The anion gap is 134 − (96 + 8) = 30 mmol/L – markedly high, consistent with ketoacidosis.

After eight hours of intravenous fluids and insulin, sodium is 138, chloride 108 and bicarbonate 15. The anion gap is now 138 − (108 + 15) = 15 mmol/L: the gap is closing as ketones clear. Bicarbonate is still low, partly because the large volume of saline has raised chloride – a temporary normal-gap acidosis that resolves over the following day.

Many hospital DKA protocols use a closing anion gap, falling ketones and rising bicarbonate to judge when to change from intravenous to subcutaneous insulin. This example is illustrative; treatment follows local protocols.

The anion gap in chronic kidney disease

As kidney function declines, the kidneys become less able to excrete acid and regenerate bicarbonate. In moderate CKD, acidosis is often of the normal-gap type, because the main problem is reduced ammonium excretion. In advanced CKD (eGFR below about 15–20), phosphate, sulphate and other anions accumulate and the gap rises. KDIGO suggests treating persistent low bicarbonate in CKD – often with oral sodium bicarbonate or dietary changes such as more fruit and vegetables – because chronic acidosis is associated with faster kidney decline, bone loss and muscle wasting.

Other approaches to acid–base analysis

The anion gap belongs to the traditional "bicarbonate-centred" approach. Some intensive care specialists prefer the physicochemical (Stewart) approach, which uses the strong ion difference and total weak acids, including albumin and phosphate. A related tool is the base excess, reported on blood gas analysers. Each approach has strengths, and in practice clinicians often combine the anion gap, albumin correction and base excess. For most purposes outside intensive care, the corrected anion gap is simple and sufficient.

Reference ranges

Variation by age and sex

The anion gap does not differ meaningfully between men and women. Children have similar ranges to adults, though some neonatal ranges are slightly higher. Older adults may have lower values if albumin is lower. Because analysers differ in how they measure chloride, reference ranges vary more between laboratories than between groups of people – always use the range on your report.

Factors that affect the result

• Albumin – the most important factor; correct for it when it is low.
• Analyser method – ion-selective electrodes used since the 1990s give higher chloride readings and lower anion gaps than older methods.
• Sample handling – delayed processing or underfilled tubes can lower bicarbonate.
• Severe hyperlipidaemia or hyperproteinaemia – can affect sodium measurement on some analysers.
• Bromide, iodide or salicylate – can interfere with chloride measurement.

When to see a doctor

The anion gap is mostly used in hospital to evaluate acutely unwell people. If a routine blood test shows a high anion gap or low bicarbonate, discuss it with your clinician, who will look at the full picture.

Seek emergency care for rapid or deep breathing, confusion, drowsiness, severe abdominal pain, vomiting, very high blood sugar with ketones, or any suspected poisoning (for example, antifreeze, methanol or an aspirin overdose).