Albumin-Adjusted Anion Gap Calculator (Figge 2.5)
Calculate a signed sodium-only Figge 2.5 albumin-adjusted anion gap from same-specimen or paired chemistry and albumin in g/dL or g/L, with no universal interval, diagnosis, delta ratio, or treatment output.
Content updated: View sources
QuickMedCalc is developed and maintained by an independent developer. Medical content is not independently reviewed by a physician.
Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.
About
This calculator reports three signed arithmetic values from one chemistry state: the sodium-only unadjusted anion gap, a fixed Figge 2.5 albumin adjustment, and the resulting albumin-adjusted anion gap. Potassium is excluded. Sodium, chloride, bicarbonate or chemistry total CO₂, and albumin must come from the same specimen or a documented clinically paired collection time. [1, 2]
Albumin is an important unmeasured anion, so lower albumin can reduce the observed anion gap. The Figge model adds 2.5 mEq/L for each 1 g/dL decrease from 4.0 g/dL. Feldman later estimated a slope of approximately 2.3 mEq/L per g/dL in a larger laboratory population. This page deliberately reproduces the fixed 2.5 model and does not average, switch, or claim that the factor is exact for every patient or laboratory. [1, 2, 3]
An anion gap is a calculated difference rather than a direct measurement of acid, lactate, ketones, or a toxic alcohol. Analyzer method and the local laboratory reference interval matter. This page does not diagnose an acid–base disorder, calculate a delta ratio, determine a cause, or recommend bicarbonate, fluids, medicines, dialysis, referral, or another treatment. [2, 4, 5, 6]
Formula
Interpretation
| Quantity | What it represents | Calculated here |
|---|---|---|
| Sodium, chloride, bicarbonate / total CO₂ | Submitted chemistry measurements | Inputs |
| Unadjusted sodium-only AG | Na − (Cl + HCO₃/total CO₂) | Yes |
| Figge 2.5 albumin adjustment | Fixed model relative to albumin 4.0 g/dL | Yes |
| Albumin-adjusted AG | Unadjusted AG plus the fixed adjustment | Yes |
| Potassium-including AG | A different formula including potassium | No |
| pH, PCO₂ and compensation | Components of full acid–base assessment | No |
| Lactate, ketones or toxic alcohols | Separately measured or assessed quantities | No |
The three outputs are different arithmetic layers, not three direct laboratory tests. “Adjusted” describes application of the fixed Figge 2.5 arithmetic. It does not mean the result is a directly measured or universally correct physiologic value.
Albumin-adjusted AG is not lactate, ketone, or an “unmeasured acid concentration.” Do not compare this sodium-only output with a potassium-including interval. No diagnosis, delta ratio, or treatment is generated from the table or submitted result.
What this calculator computes
The four submitted chemistry measurements are sodium, chloride, bicarbonate or chemistry total CO₂, and albumin. Albumin is first normalized to g/dL. The calculator then derives the sodium-only unadjusted anion gap, the fixed Figge 2.5 albumin adjustment, and their sum. All three outputs preserve signed arithmetic.
The page does not read pH, PCO₂, lactate, ketones, toxicology results, symptoms, or clinical history. Those missing data cannot be inferred from the submitted electrolyte values.
How the sodium-only anion gap is assembled
- Add chloride to bicarbonate or chemistry total CO₂.
- Subtract that sum from sodium.
- Do not add potassium; this is the sodium-only definition.
- Preserve a positive, zero, or negative arithmetic result.
- Do not assign a cause from the number alone.
The formula and the choice to exclude potassium must be matched to the laboratory interval used for comparison. [2, 7]
Why albumin changes the anion gap
Why the page uses 2.5 but discusses 2.3
Figge reported 0.25 mEq/L per g/L, equivalent to 2.5 mEq/L per g/dL. Feldman later estimated approximately 2.3 mEq/L per g/dL in 5,328 patients. The studies used different populations and methods. This page explicitly reproduces Figge 2.5; it does not claim universal superiority, switch factors, average them, or calculate a second model.[1, 2, 3]
Why the laboratory reference interval matters
The anion gap is derived from several measurements. Electrolyte analyzers can produce materially different reference intervals, as shown in both earlier instrument comparisons and later assay-specific work. Use the current laboratory interval for the current formula and confirm whether it includes potassium; this page does not supply a universal “normal AG.” [4, 5]
Chemistry total CO₂ versus blood-gas bicarbonate
Chemistry total CO₂ is a measured laboratory quantity composed mainly of bicarbonate. Blood-gas HCO₃ is generally calculated from pH and PCO₂. Values collected together are often close but are not guaranteed to be identical; air exposure, handling, collection site, timing, and method can affect the comparison. This page uses the submitted chemistry bicarbonate or total CO₂ context and does not reconstruct a blood gas.[6, 8]
Why potassium is excluded
Units, pairing, precision, and display
- For these monovalent ions, mmol/L and mEq/L have the same numerical value.
- Albumin entered in g/L is divided by 10 before the adjustment.
- All values must describe one chemistry state.
- Internal arithmetic is not rounded before final one-decimal display.
- The adjustment shows its sign; negative gaps and adjustments are not clamped.
- Editing a number or changing the albumin unit clears confirmation and old results.
- Software validation limits are not physiologic ranges or laboratory intervals.
Worked examples from the frozen implementation
Reference-albumin vector
Na 140, Cl 104, bicarbonate 24, albumin 4.0 g/dL → unadjusted AG 12.0, adjustment 0.0, adjusted AG 12.0.
Implementation, unit, sign, and display audit only; no universal interval, patient diagnosis, or treatment is produced.
Hypoalbumin vector
Na 140, Cl 104, bicarbonate 24, albumin 2.0 g/dL → unadjusted AG 12.0, adjustment +5.0, adjusted AG 17.0.
Implementation, unit, sign, and display audit only; no universal interval, patient diagnosis, or treatment is produced.
g/L equivalence
Albumin 20 g/L produces the same unrounded and displayed results as 2.0 g/dL with the same electrolytes.
Implementation, unit, sign, and display audit only; no universal interval, patient diagnosis, or treatment is produced.
Albumin above reference
Na 140, Cl 104, bicarbonate 24, albumin 5.0 g/dL → adjustment −2.5 and adjusted AG 9.5. A negative adjustment is formula arithmetic, not a diagnosis.
Implementation, unit, sign, and display audit only; no universal interval, patient diagnosis, or treatment is produced.
Negative-to-zero boundary
Na 130, Cl 110, bicarbonate 25, albumin 2.0 g/dL → unadjusted AG −5.0, adjustment +5.0, adjusted AG 0.0.
Implementation, unit, sign, and display audit only; no universal interval, patient diagnosis, or treatment is produced.
Low or negative anion-gap limitations
Adjusted AG is not a direct acid concentration
Why this page does not calculate delta ratio or mixed disorders
Delta-ratio arithmetic requires a reference anion gap, reference bicarbonate, an acid–base context, and additional assumptions. Fixed values such as 12 and 24 may not match the local analyzer, potassium convention, or reference interval. This page therefore does not restore delta gap, delta ratio, compensation, or mixed-disorder interpretation, and its adjusted result cannot establish a mixed disorder.[4, 5, 6]
The cited Berend review has a formal published correction. This page does not reproduce the corrected article’s other acid–base formulas. See the formal correction.
When the arithmetic may be less reliable
- Inputs do not represent the same or a reasonably paired chemistry state.
- A different analyzer or laboratory reference interval is used.
- A potassium-inclusive interval is paired with this sodium-only equation.
- Blood-gas bicarbonate is mixed with chemistry total CO₂.
- The sample was exposed to air or processing was delayed.
- Sodium, chloride, total CO₂, or albumin has analytic interference or method differences.
- The clinical state is changing rapidly.
- The fixed 2.5 factor is treated as an exact personal correction.
- The adjusted gap is treated as a direct measurement or independent diagnosis.
What the result cannot determine
- Whether metabolic acidosis exists.
- The type or mixture of an acid–base disorder.
- Lactate or ketone concentration.
- Toxic-alcohol exposure.
- The cause of a low or negative gap.
- Delta ratio or compensation.
- Bicarbonate or fluid requirements.
- Medicine, dialysis, referral, admission, discharge, or individual treatment.
References
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810. PMID 9824071. DOI 10.1097/00003246-199811000-00019.
- Kraut JA, Nagami GT. The Serum Anion Gap in the Evaluation of Acid-Base Disorders: What Are Its Limitations and Can Its Effectiveness Be Improved? Clin J Am Soc Nephrol. 2013;8(11):2018–2024. PMID 23833313. PMCID PMC3817910. DOI 10.2215/CJN.04040413.
- Feldman M, Soni N, Dickson B. Influence of hypoalbuminemia or hyperalbuminemia on the serum anion gap. J Lab Clin Med. 2005;146(6):317–320. PMID 16310513. DOI 10.1016/j.lab.2005.07.008.
- Roberts WL, Johnson RD. The serum anion gap. Has the reference interval really fallen? Arch Pathol Lab Med. 1997;121(6):568–572. PMID 9199620.
- Wu AHB, Jankowski T, Zhang Y, Lynch KL. The Reference Range for the Anion Gap Is Dependent on the Instrument’s Electrolyte Assay. J Appl Lab Med. 2021;6(6):1697–1699. PMID 34534316. DOI 10.1093/jalm/jfab091.
- Berend K, de Vries APJ, Gans ROB. Physiological Approach to Assessment of Acid–Base Disturbances. N Engl J Med. 2014;371:1434–1445. PMID 25295502. DOI 10.1056/NEJMra1003327. Formal correction: N Engl J Med. 2014;371:1948. DOI 10.1056/NEJMx140053.
- Haber LA, Dhaliwal G, Lo L, Rizzuto G. Evaluating a low anion gap: A practical approach. Cleve Clin J Med. 2023;90(10):619–623. PMID 37783490. PMCID PMC11924111. DOI 10.3949/ccjm.90a.23035.
- Centor RM. Serum Total Carbon Dioxide. In: Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Chapter 196. NCBI Bookshelf.
FAQ
Unadjusted AG = sodium − (chloride + bicarbonate or chemistry total CO₂); adjustment = 2.5 × (4.0 − albumin in g/dL); adjusted AG = unadjusted AG + adjustment. Intermediate arithmetic is not rounded.
Sources: [1]
No. Figge-style 2.5 arithmetic is a fixed model, not a universal physiologic constant. Other data have estimated a value closer to 2.3, and results can depend on the population and laboratory method. This page explicitly reproduces 2.5 without claiming it is exact for every person.
A routine chemistry panel commonly reports total CO₂, which mainly reflects bicarbonate in many settings. Blood-gas bicarbonate is calculated differently, and sample handling, timing, ventilation, and analytic method can matter. Use the value specified by the current laboratory or protocol without mixing samples or time points.
Related Calculators
Anion Gap
Calculate serum anion gap from sodium, chloride, and bicarbonate or total CO₂ using the formula without potassium, with an input-specific calculation audit.
Serum Osmolality
Calculate serum osmolality and effective osmolality (tonicity), with optional measured osmolality, osmolal gap, and separate ethanol assumptions; no toxic-alcohol or HHS diagnosis.
Corrected Ca
Reproduce the conventional 0.8 albumin-adjusted total-calcium estimate in conventional or SI units, with an input-specific calculation audit and current guidance against routine reporting.
Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.