Arithmetic output
The displayed value is the result of Na − (Cl + HCO₃) using the entered values, rounded to one decimal place.
Educational reference only. Not a diagnostic tool. See full disclaimer.
General MedicineAnion Gap
Serum Anion Gap Calculator (Without Potassium)
Calculate serum anion gap from sodium, chloride, and bicarbonate or total CO₂ using the formula without potassium, with an input-specific calculation audit.
Content updated: View sources
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Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.
Input
mmol/L or mEq/L
mmol/L or mEq/L
mmol/L or mEq/L
Serum anion gap (without potassium)
—
Arithmetic result; compare with the same laboratory’s interval
Formula: Na − (Cl + HCO₃)
- Potassium is not included.
- No albumin correction is applied.
- No universal low, normal, or high category is assigned.
- The calculation uses the full entered precision.
- The displayed arithmetic output is rounded to one decimal place.
About
The serum anion gap is a derived calculation, not an independently measured analyte. It compares measured sodium with measured chloride plus chemistry total CO₂/bicarbonate. Plasma remains electrically neutral; the gap reflects the net difference between anions and cations omitted from this simplified formula. [1, 2]
This page runs only the formula without potassium and applies no albumin correction. It does not diagnose acidemia, metabolic acidosis, cause, or severity. A result within the reporting laboratory's interval cannot exclude an increase in a particular unmeasured anion. [3, 4]
Interpretation
Laboratory-specific comparison
Do not assign a universal low, normal, or high category. Compare with the interval reported by the laboratory that measured the same electrolyte panel. [3, 6]
No albumin correction
Albumin is an important unmeasured anion. Hypoalbuminemia may lower the observed gap and conceal other unmeasured anions. [3, 4]
Not a diagnosis or direct analyte
The anion gap does not directly measure lactate, ketones, toxic alcohols, or another specific acid and does not by itself diagnose metabolic acidosis. [3]
Worked examples
Positive arithmetic result
Na = 140; Cl = 104; HCO₃/total CO₂ = 24
140 − (104 + 24) = 12.0 mEq/L
This is an arithmetic example, not a universal classification.
Finite negative result
Na = 130; Cl = 110; HCO₃/total CO₂ = 24
130 − (110 + 24) = −4.0 mEq/L
The calculator preserves a finite negative result. A negative value does not identify its cause.
Why potassium changes the value
Na = 140; K = 4; Cl = 104; HCO₃ = 24
Without potassium: 140 − (104 + 24) = 12
With potassium: 140 + 4 − (104 + 24) = 16
This page reports only the without-potassium result. Potassium-inclusive values require a potassium-inclusive reference interval. [1, 6]
Why the reference interval must come from the laboratory
Published examples—not universal cutoffs:
| Published context | Example interval |
|---|---|
| Historical commonly cited interval | 8–16 mEq/L |
| Some ion-selective-electrode settings | Approximately 3–9 mEq/L |
| Chionh et al. 2022 specific laboratory study, without potassium | 10–18 mmol/L |
| Chionh et al. 2022 potassium-inclusive interval | 14.6–22.5 mmol/L |
These intervals come from different methods, laboratories, and studies. Do not select the most convenient interval to classify a patient. Use the interval from the laboratory that produced the result; a person's stable baseline may add context. This calculator does not output a low, normal, or high category. [3, 6, 8]
A 2025 review found substantial variability across published anion-gap reference intervals: reported upper reference limits ranged from 10–19 mmol/L, and interval widths ranged from 2–11 mmol/L. [7] This variability is another reason not to impose one universal interval on this calculator.
Chemistry total CO₂ versus blood-gas bicarbonate
A chemistry panel usually measures serum or plasma total CO₂, of which bicarbonate is the largest component. Blood-gas HCO₃ is generally calculated from pH and PCO₂. The values are usually related but are not completely interchangeable. Do not combine values from different samples, collection times, or measurement methods in one anion-gap calculation. [5]
Albumin and the observed gap
Albumin is an important unmeasured anion. In the Figge study, the observed anion gap decreased by approximately 2.5 mEq/L for each 1 g/dL decrease in albumin. Low albumin may therefore mask an increase in other unmeasured anions. This page does not perform an albumin correction or treat an adjustment as a perfect value across all laboratories and patients. [3, 4]
The separate Albumin-Adjusted Anion Gap calculator applies its own defined adjustment and keeps that output distinct from this uncorrected result.
What an elevated or normal gap cannot prove
- An elevated gap can support further evaluation but does not independently prove acidemia or metabolic acidosis.
- A gap within the laboratory interval cannot exclude a mild or early lactate increase.
- Studies in critically ill patients found limited sensitivity for an elevated gap to identify hyperlactatemia.
- The gap does not replace direct lactate measurement or other cause-specific testing.
- The retained anion, albumin, and mixed acid–base disorders can alter the relationship between the gap and bicarbonate.
[3]
Low, zero, or negative results
A low or negative result may first reflect specimen collection, transport, or laboratory measurement issues. The 2023 review describes repeat electrolyte measurement as the first step in confirming an unexpected low result. Other documented contexts include hypoalbuminemia, positively charged monoclonal proteins, lithium or other unmeasured cations, chloride overestimation, and sodium underestimation. The number itself does not diagnose any of these causes. [8]
Appropriate and unsupported uses
Appropriate uses
- Reproduce serum anion-gap arithmetic without potassium.
- Check a manual calculation.
- Compare one panel with that laboratory's interval.
- Teach why potassium and albumin change interpretation.
Unsupported uses
- Diagnose metabolic acidosis.
- Exclude lactic acidosis or ketoacidosis.
- Identify a toxic exposure, cause, or severity.
- Determine treatment, medication, fluid, dialysis, or referral.
- Combine electrolytes from different samples or times.
Related calculations answer different questions
- Albumin-Adjusted Anion Gap applies a separate albumin adjustment to an observed gap and must remain distinct from this uncorrected result.
- Calculated Serum Osmolality estimates serum osmolality and related gaps using a different formula and different unmeasured substances.
- Urine Anion Gap uses urine sodium, potassium, and chloride for a different purpose and is not interchangeable with serum anion gap.
References
- Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore). 1977;56(1):38–54. PMID 401925.
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clinical Journal of the American Society of Nephrology. 2007;2(1):162–174. PMID 17699401.
- 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? Clinical Journal of the American Society of Nephrology. 2013;8(11):2018–2024. PMCID PMC3817910.
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Critical Care Medicine. 1998;26(11):1807–1810. PMID 9824071. doi:10.1097/00003246-199811000-00019.
- Nasir NM, Sthaneshwar P, Yunus PJM, Yap SF. Comparing measured total carbon dioxide and calculated bicarbonate. Malaysian Journal of Pathology. 2010;32(1):21–26. PMID 20614722.
- Chionh CY, et al. Serum anion gap revisited: a verified reference interval for contemporary use. Internal Medicine Journal. 2022;52(9):1531–1537. PMID 34028972. doi:10.1111/imj.15396.
- Alter DN, Zha L. The anion gap as a predictive tool for morbidity and mortality. Critical Reviews in Clinical Laboratory Sciences. 2025;62(8):568–584. PMID 40679254. doi:10.1080/10408363.2025.2526346.
- Haber LA, Dhaliwal G, Lo L, Rizzuto G. Evaluating a low anion gap: a practical approach. Cleveland Clinic Journal of Medicine. 2023;90(10):619–623. PMID 37783490. PMCID PMC11924111. doi:10.3949/ccjm.90a.23035.
Related Calculators
Adjusted AG
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.
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.
Urine Anion Gap
Calculate the one-decimal signed urine anion gap from sodium, potassium, and chloride in the same urine specimen using mmol/L or mEq/L, without treating UAG as direct urine ammonium or a universal positive/negative diagnosis.
Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.