ABG & Acid-Base Interpretation Pathway

Primary Derangements, Compensation & Anion Gap Synthesis · v1.9
How to use this tool: Enter the arterial blood gas values, with serum electrolytes and albumin if available. The tool reports oxygenation, identifies the primary acid-base disturbance, checks whether compensation is appropriate, and calculates the corrected anion gap and delta ratio to point to the underlying cause.
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 Parameters

2. Patient Context

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

3. Additional Chemistry (Optional)

Pathway Architecture & Clinical Pearls

The Golden Rule of ABG Analysis: Always look at the patient. If an ABG shows profound hypoxaemia but the patient is sitting up comfortably texting on their phone with a normal pulse oximetry, you likely have venous admixture or an air bubble. Do not intubate based on a spurious gas.

1. The Temperature Correction Mandate (α-stat)

Standard ABG analysers heat the blood sample to 37°C before measurement. In patients with significant hypothermia (e.g., targeted temperature management post-cardiac arrest) or severe hyperthermia, the uncorrected values will be dangerously inaccurate. As blood cools, gas solubility increases, meaning the true in vivo PCO₂ and PO₂ are lower than the machine reports, and the true pH is higher. This engine automatically applies standard α-stat correction formulas.

2. Practice Advisory: Intravenous Sodium Bicarbonate

Avoid Knee-Jerk Bicarbonate Administration
In Indian ICUs, there is a dangerous tendency to "chase" a normal pH by pushing IV Sodium Bicarbonate (NaHCO₃) in patients with mixed or respiratory acidosis. Pathophysiology: NaHCO₃ combines with H⁺ to form H₂CO₃, which immediately dissociates into H₂O and CO₂. If the patient cannot ventilate off this massive new CO₂ load, the CO₂ rapidly crosses cell membranes, worsening intracellular acidosis and depressing myocardial contractility, even if the blood pH temporarily looks better. Bicarbonate is generally reserved for severe non-anion gap metabolic acidosis (NAGMA) or specific toxicological emergencies.

3. Illness Scripts: The Anion Gap Mnemonics

Identifying the aetiology of metabolic acidosis requires splitting it by the Corrected Anion Gap.

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) · 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) · RTA (Renal Tubular Acidosis) · SpO₂ (Peripheral Capillary Oxygen Saturation) · TPN (Total Parenteral Nutrition)
Algorithm References & Evidence Base
  1. Association of Physicians of India (API). Evidence-Based Guidelines for Management of Acid-Base Disorders. J Assoc Physicians India.
  2. Indian Society of Critical Care Medicine (ISCCM). Guidelines for Mechanical Ventilation and Oxygen Therapy. Indian J Crit Care Med.
  3. Kraut JA, Madias NE. Approach to patients with acid-base disorders. Respir Care. 2012;57(1):18-34.
  4. Seifter JL. Integration of acid-base and electrolyte disorders. N Engl J Med. 2014;371(19):1821-1831.
How to Cite This Tool

AMA Style:
Umakanth S. ABG & Acid-Base Interpretation Pathway. MEDiscuss. Published 2026. Accessed .

Vancouver Style:
Umakanth S. ABG & Acid-Base Interpretation Pathway [Internet]. MEDiscuss.org; 2026 [cited ]. Available from:

Disclaimer and scope
Disclaimer: This decision-support system is intended for healthcare professionals. It does not substitute clinical judgment. These algorithms are evidence-based, but medical knowledge evolves continuously. The treating physician retains absolute responsibility for all diagnostic, dosing, and therapeutic decisions. Always verify outputs against current institutional protocols and individual patient contexts.
© 2026 MEDiscuss.org. All rights reserved. This CDSS Module was designed and developed by Dr. Shashikiran Umakanth using the available evidence base. Provided free of charge for clinical use. Unauthorised reproduction or redistribution is strictly prohibited.
Category Advanced DiagnosticsPathway
Specialties Critical Care, Internal Medicine
Written and maintained by Dr Shashikiran Umakanth.
Last revised: 29 July 2026
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