Anion Gap Calculator

Variables and formula

The above calculator determines the anion gap, the delta gap and the delta ratio based on the measured serum ions (sodium, chloride and bicarbonate) in order to help with diagnosis of metabolic acidosis and other metabolic and renal conditions.

The three variables can be input in two different measurement units:

- milliequivalents/liter abbreviated as mEq/L;

- millimole/liter abbreviated as mmol/L.

The three formulas used are:

Anion gap (mEq/L) = [Na] – ([Cl] + [HCO3])

Delta gap (mEq/L) = Anion Gap – Baseline gap (here 12)

Delta ratio = Delta gap / (24 – [HCO3])

The normal values for the serum electrolytes can be found in the table below:

Anion gap explained

AG is an indicator that offers information about the undetermined ions in plasma or serum (for urine ions, there is a separate Urine Anion Gap), because it is the difference between primary measured cations and anions in serum.

What results, the AG, is a measure of the anions that cannot be directly determined. The complete equation is the following:

AG = (Na + K) - (Cl + HCO3)

However, the simplified version (used in the above calculator) is most often preferred.

The reference range for AG is 3-11 mEq/L. It is used in several clinical determinations:

■ Checks for the presence of metabolic acidosis;

■ Differentiation between the causes of metabolic acidosis;

■ Severity assessment of the acidosis;

■ Monitoring the response to treatment.

Since albumin is the most revelant of the unmeasured anions, determined by AG, values below 6 mEq/L are suggestive of hypoalbuminemia.

Normal values don’t necessarily mean the serum ionic balance is right and when accompanied by other symptoms may indicate:

■ Loss of bicarbonate;

■ Recovery from diabetic ketoacidosis;

■ Renal tubular acidosis.

When AG is above normal values (greater than 11 mEq/L), the following conditions may be suspected:

■ Uremia;

■ Diabetic ketoacidosis;

■ Lactic acidosis;

■ Renal failure.

About the delta gap and ratio

The delta ratio is defined as the increase in anion gap divided by the decrease in bicarbonate. There are four categories of results:

When the delta ratio is between 0.4 - 0.8 and 1 - 2, this is an indication of mixed acid-base disorder.

Low ratios occur when the HCl levels are accompanied by the decrease in bicarbonate. High levels occur when the bicarbonate values are elevated, often at the onset of metabolic acidosis.

References

1. Gabow PA, Kaehny WD, Fennessey PV, Goodman SI, Gross PA, Schrier RW. Diagnostic importance of an increased serum anion gap. N Engl J Med. 1980; 303(15):854-8.

2. Oh MS, Carroll HJ. The anion gap. N Engl J Med. 1977; 297(15):814-7.

3. 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.