By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 25 July 2026
The anion gap is a calculated value that estimates the 'unmeasured' acids in blood. Its main job is to sort metabolic acidosis into two categories — a high gap means acid has been added (ketones, lactate, toxins), a normal gap means bicarbonate has been lost — which points to very different causes.
Blood must be electrically neutral: positive and negative charges balance exactly. But a chemistry panel only measures some of those charges — chiefly sodium and potassium on the positive side, chloride and bicarbonate on the negative. Subtracting the measured negatives from the measured positives leaves an apparent shortfall: the anion gap, which represents the negatively-charged substances the panel doesn't measure directly (proteins, phosphate, sulphate, organic acids).
The anion gap estimates the unmeasured acids in blood — sorting metabolic acidosis into 'acid added' versus 'bicarbonate lost'.
That sounds abstract, but it does one genuinely useful job.
The one thing it's for: splitting acidosis in two
When bicarbonate is low (metabolic acidosis), the anion gap sorts the cause into two families:
HIGH anion gap acidosis — acid has been added. Some unmeasured acid has accumulated, taking up the space bicarbonate used to occupy. The classic causes are ketones (diabetic ketoacidosis), lactate (poor perfusion, shock, sepsis), uraemic acids (kidney failure), and toxins — above all ethylene glycol (antifreeze) poisoning, where a markedly raised anion gap is an important early clue in a rapidly-fatal, time-critical poisoning.
NORMAL anion gap acidosis — bicarbonate has been lost. Bicarbonate has drained away and chloride has risen to fill the electrical space (hence "hyperchloraemic" acidosis). The classic causes are severe diarrhoea and certain kidney tubular disorders.
These two branches point to completely different diagnoses and treatments, which is why a calculated value with no direct physical meaning still earns its place.
Reading a low result
A low anion gap is uncommon and is most often caused by a low albumin — albumin is itself a significant unmeasured negative charge, so losing it shrinks the gap. It is rarely pursued in its own right, but it is worth knowing that hypoalbuminaemia can mask a high anion gap, so a "normal" gap in a low-albumin patient may actually be inappropriately normal.
In chronic disease
The anion gap is used mainly in acute and critical settings rather than as a longitudinal marker. In diabetic ketoacidosis it is followed alongside bicarbonate as the crisis resolves; in chronic kidney disease it may drift up modestly as uraemic acids accumulate.
Cat vs dog
The calculation and interpretation are identical in both species, with similar reference intervals. Ethylene glycol poisoning — a leading reason the anion gap is checked urgently — is a serious risk in both dogs and cats, in whom even very small ingested volumes can be fatal.
About the reference range
This page shows no range strip. The anion gap is a derived value with no single agreed veterinary reference interval, and it depends on which electrolytes a given panel includes in the formula. Pawline draws a band only where it holds a verified interval. Interpret it against the underlying electrolytes and bicarbonate on your own report.
A telling illustration of that method-dependence: Cornell University's veterinary laboratory reports an anion gap of roughly 17–27 mEq/L in dogs and 18–29 mEq/L in cats — noticeably higher than the textbook 12–24 mEq/L many sources quote, purely because the calculation and the electrolyte methods differ. That gap between "the textbook number" and "one real lab's number" is exactly why your own report's figure is the one to read.
What to do next
The anion gap is not a number to act on directly — it is a classifier. If your pet has a metabolic acidosis, it tells your vet which family of causes to pursue: a high gap sends the search towards ketones, lactate, kidney failure and toxins (with antifreeze an emergency consideration), while a normal gap points towards bicarbonate loss from the gut or kidneys. Interpreted alongside albumin, glucose and the clinical picture, it usefully narrows the field.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
