Anion Gap Calculator
Compute the serum anion gap with albumin correction and normal range guide.
What it does
The anion gap is bookkeeping for blood charges. Sodium is the main measured cation while chloride and bicarbonate are the main measured anions, and the gap between them estimates the unmeasured anions such as albumin, lactate, and ketoacids. A widened gap flags acids that the basic panel does not name directly.
The tool above does the subtraction plus the albumin correction in one pass. Enter sodium, chloride, bicarbonate, and albumin from the same draw. It returns the gap, the corrected gap, and a low, normal, or high read against the 8 to 12 band. Defaults describe a normal panel at 140, 105, and 24, so the page opens solved at 11.
Albumin is the quiet trap in this test. It supplies most of the normal gap, so when albumin falls the printed gap falls with it and a real acidosis can hide behind a normal number. Correcting adds back about 2.5 per missing gram: a printed 18 at albumin 2.0 is really 23, which changes the workup completely.
High gap and normal gap acidosis split the causes cleanly. High gap means extra acids arrived: lactate from shock, ketoacids from diabetes or starvation, uremic acids from kidney failure, or toxic alcohols. Normal gap means bicarbonate left or chloride arrived, as in diarrhea or renal tubular acidosis, with chloride filling the hole bicarbonate left.
Use the table to fix the bands and the correction in memory, then run the panel. A gap of 27 is never watchful waiting territory; it needs lactate, ketones, kidney function, and a clinician the same day.
How to use
- Copy sodium, chloride, and bicarbonate from the same blood draw.
- Enter albumin from the same lab set.
- Read the gap first, then the corrected line.
- Compare against 8 to 12 for the no potassium form.
- Rejudge with the corrected value when albumin is low.
- Pair a high gap with lactate, ketones, and kidney labs.
Formula
- Na
- Serum sodium in mEq/L.
- Cl
- Serum chloride in mEq/L.
- HCO3
- Serum bicarbonate (CO2) in mEq/L.
- Albumin
- Serum albumin in g/dL, used only for correction.
All electrolytes in mEq/L and albumin in g/dL. The 2.5 factor reflects albumin as the main unmeasured anion: each gram below 4.0 lowers the printed gap by about 2.5. Status bands use the uncorrected gap; read the corrected line whenever albumin is low.
Worked example
Example: basic metabolic panel with normal albumin
Sodium 140, chloride 105, bicarbonate 24, albumin 4.0 g/dL. Find the gap.
- Write the formula: gap = sodium minus (chloride plus bicarbonate).
- Add the anions: 105 + 24 = 129.
- Subtract: 140 - 129 = 11 mEq/L.
- Correct for albumin: 11 + 2.5 x (4.0 - 4.0) = 11 mEq/L.
- 11 sits inside 8 to 12, so the status is Normal.
- Had albumin been 2.0, correction would add 5 and unmask a gap of 16.
Reference table
| Gap | Band | Usual direction |
|---|---|---|
| Below 8 | Low | Lab error, low albumin, or paraproteins: repeat the draw |
| 8 to 12 | Normal | No extra unmeasured acids on this draw |
| Above 12 | High | Lactate, ketoacids, kidney acids, or toxins: work up promptly |
| Printed 18 at albumin 2.0 | Corrected 23 | Correction unmasks a high gap: 18 + 5 = 23 |
Limits
- Screening arithmetic, not a diagnosis by itself.
- Analyzer and lab ranges shift bands by a point or two.
- Needs lactate, ketones, and ABG to name the cause.
- Albumin correction is an estimate near 2.5 per gram.
Common mistakes
- Skipping albumin and trusting a falsely normal gap.
- Adding potassium without shifting the normal band up.
- Mixing venous CO2 with arterial bicarbonate casually.
- Reading a single gap without the bicarbonate trend.
- Chasing the gap while ignoring the underlying cause.
Try it yourself
Quick checks. Answers sit tucked under each question.
Na 140, Cl 100, HCO3 22, albumin 2.0?
Gap = 140 - (100 + 22) = 18. Corrected = 18 + 2.5 x 2.0 = 23, High.
Na 145, Cl 100, HCO3 18, albumin 4.0?
Gap = 145 - (100 + 18) = 27. Albumin is normal so corrected stays 27, High.
Gap 10 with albumin 4.2 instead of 4.0?
10 + 2.5 x (4.0 - 4.2) = 10 - 0.5 = 9.5, still inside 8 to 12.
Questions
What is a normal anion gap?
8 to 12 mEq/L without potassium in the formula, or about 12 to 16 with potassium added. Newer analyzers read chloride higher, so some labs print 3 to 11.
Should potassium go in the formula?
Add potassium to the cation side: (Na + K) minus (Cl + HCO3). The result runs about 4 higher, so shift the normal band up by 4 as well.
Why correct for albumin?
Each 1 g/dL that albumin falls below 4.0 hides about 2.5 of gap. Correct first, then judge: a printed 10 at albumin 2.0 is really 15.
What raises the gap most often?
Lactate, ketoacids, kidney failure acids, methanol, ethylene glycol, and salicylates. The delta gap and osmolar gap narrow it further.
Does diarrhea raise the gap?
No. Chloride rises as bicarbonate falls, which keeps the gap flat. Diarrhea and renal tubular acidosis are the classic pair.
What if my gap is 27?
Treat it as an emergency signal, not a home diagnosis. High gap acidosis needs venous blood gas, lactate, ketones, and toxic alcohols checked promptly.
Sources
- StatPearls: Anion gap and non-anion gap metabolic acidosis (NCBI)
- StatPearls: Biochemistry, anion gap (NCBI)