Major safety warning: do not use this calculator for real patient care
The anion gap and albumin-corrected anion gap displayed here are educational formula demonstrations only. This page is not a medical device, clinical decision support system, triage tool, diagnostic system, treatment protocol, emergency screening tool, poisoning exclusion tool, or substitute for a licensed clinician. Do not use it with identifiable patient data or for individualized patient decisions. Severe metabolic acidosis, shock, sepsis, diabetic ketoacidosis, renal failure, salicylate poisoning, methanol ingestion, and ethylene glycol ingestion can be fatal. Real clinical interpretation requires validated laboratory systems, local reference intervals, arterial or venous blood gas, lactate, ketones, renal function, measured osmolality, toxicology testing, medication review, local protocols, and urgent specialist input when indicated.
Anion gap calculation at a glance
Classic formula: AG = Na - (Cl + HCO3), with sodium, chloride, and bicarbonate in mEq/L or mmol/L.
Formula with potassium: AG = (Na + K) - (Cl + HCO3), using a higher teaching reference interval.
Common normal range: 8-12 mEq/L without potassium and 12-16 mEq/L with potassium, depending on the laboratory.
Albumin correction: corrected AG = AG + 2.5 x (4.0 - albumin in g/dL).
# What the anion gap represents
The anion gap is a calculated estimate of the difference between routinely measured plasma cations and routinely measured plasma anions. Sodium is the dominant measured extracellular cation. Chloride and bicarbonate are the dominant measured anions in a basic metabolic panel. Because plasma must remain electrically neutral, the difference is not a real electrical imbalance; it is a window into ions that are present but not included in the simple formula. These unmeasured anions include albumin, phosphate, sulfate, organic acids, and other negatively charged molecules.Clinicians use the anion gap most often during the evaluation of metabolic acidosis. If bicarbonate is low and the anion gap is high, the pattern suggests accumulation of unmeasured acids. If bicarbonate is low and the anion gap is normal, the pattern usually points toward bicarbonate loss or impaired renal acid excretion. The calculation therefore helps organize the differential diagnosis, but it does not identify a single cause by itself.- Measured cations
- Positively charged ions included in the formula, primarily sodium and sometimes potassium.
- Measured anions
- Negatively charged ions included in the formula, chloride and bicarbonate.
- Unmeasured anions
- Albumin, phosphate, sulfate, lactate, ketoacids, toxins, and other anions not represented in the basic formula.
- Metabolic acidosis
- A primary fall in bicarbonate or rise in acid load, interpreted with pH and respiratory compensation.
- Albumin correction
- A correction that accounts for the fact that low albumin lowers the expected baseline anion gap.
| Formula | Inputs | Typical reference interval | Common use |
|---|---|---|---|
| Na - (Cl + HCO3) | Na, Cl, HCO3 | 8-12 mEq/L | Most common modern teaching formula |
| (Na + K) - (Cl + HCO3) | Na, K, Cl, HCO3 | 12-16 mEq/L | Older or potassium-inclusive references |
| AG + 2.5 x (4 - albumin) | Anion gap and albumin g/dL | Interpret against the selected formula range | Detect masked high-gap acidosis in hypoalbuminemia |
# Why albumin correction matters
Albumin is the largest contributor to the normal anion gap because it carries negative charges at physiologic pH. When albumin is low, the baseline gap falls. A patient with severe hypoalbuminemia can accumulate lactate, ketoacids, or other organic acids and still show a raw anion gap that looks only mildly elevated or even normal. The correction used here adds 2.5 mEq/L for each 1 g/dL that albumin is below 4 g/dL. This is a teaching approximation, not a substitute for local laboratory interpretation.Practical example: hidden high gap with low albumin
Suppose sodium is 140, chloride is 108, bicarbonate is 18, and albumin is 2.0 g/dL. The raw anion gap is 14 mEq/L. Depending on the lab, that may appear only slightly high. The albumin correction adds 5 mEq/L, giving a corrected gap of 19 mEq/L. In a patient with metabolic acidosis, that corrected value makes unmeasured anions much harder to ignore.
8-12 common range without potassium
12-16 common range with potassium
2.5 mEq/L added per 1 g/dL albumin below 4
4.0 albumin reference used in correction
Enter albumin from the same clinical episode when possible
Albumin may change during critical illness, fluid resuscitation, nephrotic syndrome, liver disease, malnutrition, and inflammation. When correcting the anion gap, use an albumin value that reflects the same episode as the electrolyte panel. If albumin is unknown and the patient is critically ill, acknowledge that a normal raw gap may underestimate unmeasured anions.# High anion gap metabolic acidosis
A high anion gap in the setting of low bicarbonate indicates that bicarbonate has been consumed while buffering an acid whose conjugate base remains in plasma. The pattern is therefore a clue to unmeasured anions. Common frameworks include lactate, ketones, kidney failure acids, and toxins. Mnemonics can be useful for memory, but they should not replace a structured review of physiology, medications, exposures, and time course.- Lactic acidosis: consider shock, sepsis, hypoxemia, seizures, severe anemia, mesenteric ischemia, liver failure, beta-agonists, metformin-associated contexts, and mitochondrial toxins.
- Ketoacidosis: diabetic ketoacidosis, alcoholic ketoacidosis, starvation ketoacidosis, pregnancy-associated ketoacidosis, and sodium-glucose cotransporter 2 inhibitor associated euglycemic ketoacidosis.
- Renal failure: retention of sulfate, phosphate, organic acids, and other uremic anions as kidney function declines.
- Toxins and drugs: methanol, ethylene glycol, diethylene glycol, propylene glycol, salicylates, pyroglutamic acid from chronic acetaminophen exposure, and selected medication solvents.
- Mixed disorders: vomiting, diuretics, respiratory alkalosis, chronic hypercapnia, and saline resuscitation can change bicarbonate and chloride enough to mask or modify the gap.
A high anion gap does not identify the acid
The anion gap flags a pattern; it does not distinguish lactate from ketoacids, salicylate, glycolate, formate, renal acids, or mixed causes. In concerning cases, clinicians generally pair the gap with lactate, beta-hydroxybutyrate, renal function, salicylate level, measured osmolality, osmolar gap, urine microscopy when appropriate, toxic alcohol testing where available, and poison center or specialist input.
# Normal anion gap metabolic acidosis
A normal anion gap metabolic acidosis is often called hyperchloremic metabolic acidosis because chloride rises as bicarbonate falls. The typical mechanism is direct bicarbonate loss or reduced renal acid excretion without accumulation of a large load of unmeasured anions. Diarrhea, pancreatic or biliary drainage, ureteral diversions, renal tubular acidosis, early kidney dysfunction, acetazolamide, and large-volume chloride-rich fluid administration are common considerations.Gastrointestinal bicarbonate loss
Diarrhea, fistulas, and drainage can remove bicarbonate-rich fluid, often with compensatory chloride retention.
- Urine ammonium response may be appropriate
- Volume status and potassium pattern help interpretation
Renal tubular acidosis
The kidney fails to acidify urine or reclaim bicarbonate appropriately, producing a normal-gap acidosis pattern.
- Urine pH and potassium pattern are important
- Medication and autoimmune history may matter
Chloride rich fluids
Large chloride loads can lower strong ion difference and bicarbonate, especially in perioperative or critical care settings.
- Review fluid type and volume
- Consider timing relative to lab draw
Strengths and limits of anion gap classification
Advantages
- Fast calculation from routine chemistry values.
- Separates many high-gap from normal-gap acidosis patterns.
- Helps trigger targeted tests such as lactate, ketones, osmolality, and salicylate.
- Useful for serial trend review during treatment.
Disadvantages
- Sensitive to albumin, lab method, and specimen timing.
- Mixed disorders can make a dangerous pattern look deceptively ordinary.
- Does not identify the exact acid or toxin without additional data.
- A falling gap can reflect dilution, albumin change, chloride rise, or true improvement.
# Low anion gap: uncommon but important to verify
A low anion gap is less common than a high gap and often reflects low albumin or analytical factors. Because albumin is a major unmeasured anion, hypoalbuminemia can lower the measured gap. Laboratory artifact, severe hyperlipidemia, hyperviscosity, paraproteinemia, bromide or iodide interference, and lithium can also be relevant. A very low or negative gap should usually prompt verification rather than immediate assumption of a rare diagnosis.| Pattern | Possible explanation | Useful next check |
|---|---|---|
| Low raw gap with low albumin | Reduced baseline unmeasured anions | Correct the gap and review albumin trend |
| Very low or negative gap | Analytical issue or unusual cation/anion interference | Repeat electrolytes and discuss with laboratory |
| Low gap with high total protein | Paraproteinemia or hyperviscosity can alter measured ions | Review total protein, SPEP context, and lab method |
| Low gap after exposure | Lithium, bromide, or iodide may affect apparent balance | Medication and toxicology review |
Clinical bottom line
The anion gap is a map, not the territory. A high or albumin-corrected high value points toward unmeasured anions; a normal value during metabolic acidosis points toward bicarbonate loss or impaired renal acid handling; a low value deserves verification and albumin review. The safest interpretation combines the calculation with pH, pCO2, bicarbonate, lactate, ketones, renal function, osmolality, medication exposure, and the patient in front of the clinician.