ABG and Acid-Base Interpretation

The primary disorder, whether the compensation fits, and the anion gap · v1.9

Enter the arterial gas values, with the sodium, chloride and albumin if you have them.
Set the respiratory status and the temperature before you read anything. The respiratory status decides which compensation the gas is measured against; the temperature corrects the values back to the patient.
You get the oxygenation, the primary acid-base disturbance by name, the compensation checked against what that disturbance should produce, and the corrected anion gap with the delta ratio.
Compensation that does not fit is the finding worth waiting for. It means a second disturbance is sitting underneath the first, and the single diagnosis has been hiding it.

Children and adolescents under 18. The compensation formulas are the adult ones.
The cause of the disturbance. Naming a high anion gap acidosis narrows the field to a list; the patient, the lactate, the ketones, the glucose and the drug history decide which of them you are looking at.
Whether to intubate, whether to give bicarbonate, and how much fluid to run.
The osmolar gap, which is what would exclude methanol or ethylene glycol. No module in this catalogue calculates it yet.
Venous and capillary samples, and anything that went wrong between the artery and the analyser. Every number it prints assumes an arterial sample run promptly.

A high anion gap acidosis in someone who drinks has three distinct answers. Alcoholic ketoacidosis: recent heavy drinking then stopping because of vomiting and abdominal pain, ketones present, glucose normal or low. It responds to thiamine followed by glucose-containing saline and does not need insulin. Methanol or ethylene glycol: country liquor or hooch, visual disturbance, a severe acidosis and a raised osmolar gap. That is a toxicological emergency needing fomepizole or ethanol and early haemodialysis, and no module in this catalogue yet calculates the osmolar gap or holds antidote dosing. Lactic acidosis from sepsis or thiamine deficiency. If the patient is also withdrawing, see the Alcohol Withdrawal: CIWA-Ar and Thiamine Protocol.

1. Core Gas Values

pH (7.35-7.45)

PCO₂ (mmHg)

PO₂ (mmHg)

HCO₃⁻ (mEq/L)

2. Patient Context

FiO₂ (%)

On a Venturi mask? The connector and flow chart gives the delivered FiO₂.

Respiratory Status The kidney takes days to answer a respiratory disturbance, so the bicarbonate expected of an acute one is not the bicarbonate expected of a chronic one. This choice decides which expectation the gas is measured against, and getting it wrong will invent a second disturbance that is not there.

Patient Temp

The analyser warms every sample to 37°C (98.6°F) before it measures anything. Enter the patient's actual core temperature, in whichever unit the chart is written in, and the values are corrected back to it.

3. Electrolytes and Albumin (Optional)

Without the sodium and the chloride there is no anion gap, and half of what this tool can tell you goes with it. Without the albumin the gap is left uncorrected, and in a hypoalbuminaemic patient an uncorrected gap reads normal when it is not.

Sodium (Na⁺) ?

Potassium (K⁺)

Chloride (Cl⁻) ?

Albumin (g/dL, optional)

Look at the patient before you look at the gas: If the report shows severe hypoxaemia and the patient is sitting up comfortably, texting, with a normal pulse oximetry reading, the likelier explanations are venous admixture or an air bubble in the syringe. Repeat the sample. Nobody should be intubated on the strength of a gas that does not match the person in front of you.

1. Temperature Correction, and Why It Is Not α-stat

Every standard analyser warms the sample to 37°C before it measures anything, which is fine until the patient is not at 37°C. As blood cools, gas solubility rises, so the true in vivo PCO₂ and PO₂ are lower than the machine reports and the true pH is higher. In targeted temperature management after cardiac arrest, or in severe hyperthermia, the uncorrected numbers can be wrong enough to change a decision.

Convention	What it manages	Where it is conventional
α-stat	The uncorrected 37°C values, with no correction at all. Holds the ionisation state of the imidazole α-histidine constant	Adult cardiopulmonary bypass
pH-stat	The values corrected to the patient's actual temperature. Raises cerebral blood flow	Preferred by many in paediatric bypass and in deep hypothermic circulatory arrest

This tool corrects the measured values to the temperature you enter and reports the corrected figures, which is the pH-stat convention. The two names are the wrong way round in a great deal of writing and this page carried the error until 24 August 2026. The arithmetic is unchanged: only the name of the convention it follows was put right.

2. Practice Advisory: Intravenous Sodium Bicarbonate

Bicarbonate given to correct a number
A low pH on a chart invites treatment of the chart. Pushing IV Sodium Bicarbonate (NaHCO₃) at a mixed or respiratory acidosis is common in Indian ICUs, and the blood pH does improve for a while, which is the part that makes the practice durable. Pathophysiology: NaHCO₃ combines with H⁺ to form H₂CO₃, which dissociates at once into H₂O and CO₂. That CO₂ has to leave through the lungs. A patient who could not ventilate off the CO₂ they were already making cannot ventilate off this load either. CO₂ crosses cell membranes far more readily than bicarbonate does, so the acidosis inside the cell deepens while the blood gas looks better, and myocardial contractility falls with it. Bicarbonate is generally reserved for severe non-anion gap metabolic acidosis (NAGMA) or specific toxicological emergencies.

3. Illness Scripts: The Anion Gap Mnemonics

Split a metabolic acidosis by the corrected anion gap before reaching for a cause. A raised gap means an acid has been added and its anion is sitting in the plasma unmeasured. A normal gap means bicarbonate has been lost, from the gut or from the kidney, and chloride has moved in to take its place. The two lists of causes barely overlap.

High Anion Gap (HAGMA) → MUDPILES	Normal Anion Gap (NAGMA) → HARDUP
M - Methanol / Metformin U - Uraemia (Renal Failure) D - Diabetic Ketoacidosis (DKA) P - Paracetamol (Acetaminophen) / Propylene Glycol I - Infection / Iron / Isoniazid L - Lactic Acidosis (Sepsis, Hypoperfusion) E - Ethylene Glycol / Ethanol S - Salicylates (Aspirin)	H - Hyperalimentation (TPN) A - Acetazolamide / Addison's Disease R - Renal Tubular Acidosis (RTA) D - Diarrhoea (Loss of HCO₃⁻) U - Ureteroenterostomy P - Pancreatic fistula

Common Pre-Analytical Errors (Points to Note)

Error Source	Physiological Impact
Air Bubbles in Syringe	Falsely increases PO₂, falsely decreases PCO₂. Equilibration occurs rapidly ex vivo.
Excess Heparin	Dilutional effect. Falsely decreases PCO₂ and HCO₃⁻. Alters measured pH.
Venous Admixture	Inadvertent venous sampling lowers PO₂ and slightly raises PCO₂.
Delayed Analysis	Cellular metabolism continues ex vivo. Results in falsely decreased PO₂ and pH, and increased PCO₂.
Hyperleukocytosis	Extreme white cell counts "steal" oxygen rapidly ex vivo, causing pseudohypoxaemia. Immediate icing required.

Abbreviations

A-a (Alveolar-Arterial) · ABG (Arterial Blood Gas) · AG (Anion Gap) · BD (Base Deficit) · BE (Base Excess) · CIWA-Ar (Clinical Institute Withdrawal Assessment for Alcohol, revised) · Cl⁻ (Chloride) · CO₂ (Carbon Dioxide) · COPD (Chronic Obstructive Pulmonary Disease) · DKA (Diabetic Ketoacidosis) · FiO₂ (Fraction of Inspired Oxygen) · HAGMA (High Anion Gap Metabolic Acidosis) · HARDUP (Hyperalimentation, Acetazolamide, Renal Tubular Acidosis, Diarrhoea, Ureteroenterostomy, Pancreatic Fistula) · HCO₃⁻ (Bicarbonate) · ICU (Intensive Care Unit) · IV (Intravenous) · K⁺ (Potassium) · Met (Metabolic) · MUDPILES (Methanol, Uraemia, Diabetic Ketoacidosis, Paracetamol, Infection, Lactic Acidosis, Ethylene Glycol, Salicylates) · Na⁺ (Sodium) · NAGMA (Normal Anion Gap Metabolic Acidosis) · NaHCO₃ (Sodium Bicarbonate) · PaO₂ (Partial Pressure of Arterial Oxygen) · PCO₂ (Partial Pressure of Carbon Dioxide) · P/F (PaO₂ to FiO₂ Ratio) · PO₂ (Partial Pressure of Oxygen) · Resp (Respiratory) · RTA (Renal Tubular Acidosis) · SpO₂ (Peripheral Capillary Oxygen Saturation) · TPN (Total Parenteral Nutrition)

References

Association of Physicians of India (API). Evidence-Based Guidelines for Management of Acid-Base Disorders. J Assoc Physicians India. [Citation incomplete: no year, volume or pages, and the document could not be traced on 28 August 2026.]
Indian Society of Critical Care Medicine (ISCCM). Guidelines for Mechanical Ventilation and Oxygen Therapy. Indian J Crit Care Med. [Citation incomplete: no year, volume or pages, and the document could not be traced on 28 August 2026.]
Kraut JA, Madias NE. Approach to patients with acid-base disorders. Respir Care. 2001;46(4):392-403.
Seifter JL. Integration of acid-base and electrolyte disorders. N Engl J Med. 2014;371(19):1821-1831.
Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322.
Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore). 1980;59(3):161-187.
Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-1445.
Berend K. Diagnostic use of base excess in acid-base disorders. N Engl J Med. 2018;378(15):1419-1428.
Adrogué HJ, Madias NE. Management of life-threatening acid-base disorders. First of two parts. N Engl J Med. 1998;338(1):26-34.
Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807-1810.
The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308.

How to Cite This Tool

DOIhttps://doi.org/10.5281/zenodo.22401522

AMA Style:Umakanth S. ABG and Acid-Base Interpretation. Version 1.9. MEDiscuss Clinical Decision Support System. Published 2026. Accessed . https://mediscuss.org/cdss/abg-diagnostic. doi:10.5281/zenodo.22401522

Vancouver Style:Umakanth S. ABG and Acid-Base Interpretation [Internet]. Version 1.9. MEDiscuss.org; 2026 [cited ]. Available from: https://mediscuss.org/cdss/abg-diagnostic. doi:10.5281/zenodo.22401522

Category Advanced DiagnosticsPathway

Specialties Critical Care, Internal Medicine

Written and maintained by

Dr Shashikiran Umakanth

Last revised 24 August 2026

How these tools are written and reviewed