Acid Base Calculator for ABG Analysis and Anion Gap

Provides an interpretation of arterial blood gases (ABG) to discover metabolic conditions and determines the anion gap based on sodium, chloride and bicarbonate.

Refer to the text below the tool for more information about ABG analysis and AG interpretation.

Arterial blood gas (ABG) analysis is a crucial part of diagnosing and managing a patient’s oxygenation status and acid–base balance. Disorders of acid–base balance can lead to severe complications in many disease states and occasionally become a life-threatening risk.

The Anion Gap offers information about the plasma and serum ions and is a measure of the anions that cannot be directly determined. To avoid the underestimation occurring in hypoalbuminemia, a corrected anion gap calculation is also provided.

Arterial blood gases normal ranges:

  • pH: 7.35-7.45
  • PCO2: 35-45 mmHg
  • HCO3-: 22-26 mEq/L

Anion gap formulas:

  • AG, mEq/L = Sodium, mEq/L - (Chloride, mEq/L+ Bicarbonate, mEq/L)
  • Albumin corrected anion gap, mEq/L = Anion gap + [0.25 × (40 - Albumin, g/L)]

Bicarbonate (HCO3-)
Sodium (Na+)
Chloride (Cl-)
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1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


ABG Analysis

Arterial blood gas interpretation can provide a near-immediate reflection of the physiology of the patient. The table below summarizes the main interpretations of the relationship between the arterial blood gases.

pH PCO2 HCO3- ABG Interpretation
<7.35 <35 <22 Partly Compensated Metabolic Acidosis
35-45 <22 Acute Metabolic Acidosis
>45 <22 Mixed Respiratory and Metabolic Acidosis
>45 22-26 Acute Respiratory Acidosis
>45 >26 Partly Compensated Respiratory Acidosis
7.35-7.45 <35 <22 Compensated Respiratory Alkalosis and/or Compensated Metabolic Acidosis
>45 >26 Compensated Respiratory Acidosis and/or Compensated Metabolic Alkalosis
>7.45 <35 <22 Partly Compensated Respiratory Alkalosis
<35 22-26 Acute Respiratory Alkalosis
<35 >26 Mixed Respiratory and Metabolic Alkalosis
35-45 >26 Acute Metabolic Alkalosis
>45 >26 Partly Compensated Metabolic Alkalosis

Disorders of acid–base balance can lead to severe complications in many disease states and occasionally become a life-threatening risk.

The table below summarizes the main causes of the main metabolic conditions:

Metabolic condition Causes
Respiratory acidosis Chronic obstructive pulmonary disease (COPD)
Respiratory depression
Iatrogenic (incorrect mechanical ventilation settings)
Respiratory alkalosis Hypoxia: resulting in increased alveolar ventilation in an attempt to compensate.
Pulmonary embolism
Anxiety (i.e. panic attack)
Pain: causing an increased respiratory rate.
Iatrogenic (e.g. excessive mechanical ventilation)
Metabolic alkalosis Gastrointestinal loss of H+ ions
Renal loss of H+ ions (e.g. loop and thiazide diuretics, heart failure, nephrotic syndrome, cirrhosis)
Iatrogenic (e.g. addition of excess alkali)
Metabolic acidosis See table from the Anion Gap section below.
Mixed respiratory and metabolic alkalosis Excessive ventilation in COPD
Liver cirrhosis in addition to diuretic use
Hyperemesis gravidarum
Mixed respiratory and metabolic acidosis Cardiac arrest
Multi-organ failure

Anion Gap and Albumin Correction

The Anion Gap offers information about the plasma and serum ions and is a measure of the anions that cannot be directly determined.

  • Anion gap, mEq/L = Sodium, mEq/L - (Chloride, mEq/L+ Bicarbonate, mEq/L)

AG normal values are between 8 – 16 mEq/L. The anion gap determination has several clinical uses:

  • Primarily used to check for the presence of metabolic acidosis;
  • Differentiation between the causes of metabolic acidosis;
  • Severity assessment of the acidosis;

Anion gap may be underestimated in hypoalbuminaemia (which is common in critical illness), because if albumin decreases by 1 g/L then the anion gap decreases by 0.25 mmol. Therefore, a correction of AG may be applied:

  • Albumin corrected anion gap, mEq/L = anion gap + [0.25 × (40 - albumin, g/L)]

The table below summarizes the three types of metabolic acidosis (depending on AG):

  Elevated Anion Gap Normal Anion Gap Low Anion Gap
Metabolic Acidosis Mnemonic: MUDPILES Methanol.
Diabetic ketoacidosis.
Paraldehyde, phenformin.
Iron, isoniazid, inhalants.
Lactic acidosis.
Ethylene glycol, ethanol (alcoholic ketoacidosis).
Salicylates, solvents, starvation.
Gastrointestinal bicarbonate loss: diarrhea, Pancreatic fistula or uretero-intestinal diversion. Hyperalimentation. Posthypocapnia. Renal tubular acidosis.
Drugs: ammonium chloride, hydrochloric acid.
Hypoalbuminemia, Hypercalcemia, Hyperkalemia, Hypermagnesemia, Lithium toxicity, Multiple myeloma.

Please note that the results from this tool should not substitute clinical judgment and context. Other laboratory tests to consider include lactate, creatinine, urea, glucose, urinary ketones etc.



Baillie JK. Simple, easily memorised "rules of thumb" for the rapid assessment of physiological compensation for respiratory acid-base disorders. Thorax. 2008; 63(3):289-90.

Larkin BG, Zimmanck RJ. Interpreting Arterial Blood Gases Successfully. AORN J. 2015; 102(4):343-54; quiz 355-7.

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.

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

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.

Specialty: Endocrinology

Article By: Denise Nedea

Published On: October 24, 2020

Last Checked: October 24, 2020

Next Review: October 24, 2025