ABG & Acid-Base Diagnostic Algorithm | Anion Gap

ABG & Acid-Base Diagnostic Algorithm v2.0 Primary Derangements, Compensation & Anion Gap Synthesis

📈 Diagnostic Engine: Evaluates oxygenation status, primary acid-base disturbances, and expected physiological compensations (including Corrected Anion Gap, Delta Ratio, and Winters’ Formula) to isolate the underlying clinical aetiology.

1 Core Gas Parameters

pH (7.35-7.45)

PCO₂ (mmHg)

PO₂ (mmHg)

HCO₃^– (mEq/L)

2 Patient Context

FiO₂ (%)

Respiratory Status

Patient Temp °C

3 Comprehensive Chemistry (Optional)

Sodium (Na⁺)

Potassium (K⁺)

Chloride (Cl^–)

Albumin (g/dL)

📚 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

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 the standard α-stat correction formulas.

⚠ Common Pre-Analytical Errors (Pitfalls)

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.

🧮 The Mathematical Architecture

Winters’ Formula: (1.5 × HCO₃^–) + 8 ± 2. Used to calculate the expected PCO₂ compensation in metabolic acidosis. If the actual PCO₂ falls outside this range, a secondary respiratory disorder exists.
Corrected Anion Gap: Measured AG + 2.5 × (4.5 – Albumin). Hypoalbuminaemia falsely lowers the anion gap. This correction unmasks hidden high-anion-gap metabolic acidosis (HAGMA).
Delta Ratio (Δ/Δ): (Measured AG – 12) / (24 – HCO₃^–). Used purely in the setting of a HAGMA to determine if a secondary normal-anion-gap metabolic acidosis (NAGMA) or metabolic alkalosis is simultaneously present.

💉 ABG Sampling & Modified Allen’s Test

Before puncturing the radial artery, collateral circulation from the ulnar artery must be verified:

Ask the patient to clench their fist tightly.
Simultaneously apply firm pressure over both the radial and ulnar arteries to occlude them.
Ask the patient to relax their hand (the palm should appear blanched).
Release pressure on the ulnar artery only, while maintaining pressure on the radial artery.
Positive Result: The hand flushes pink within 5–15 seconds. Safe to proceed.
Negative Result: The hand remains pale. Do not puncture this radial artery.

Abbreviations: ABG (Arterial Blood Gas) · AG (Anion Gap) · BE (Base Excess) · FiO₂ (Fraction of Inspired Oxygen) · HAGMA (High Anion Gap Metabolic Acidosis) · NAGMA (Normal Anion Gap Metabolic Acidosis)

⚠ Clinical Disclaimer: This computational engine assumes the input values are from an arterial sample, not venous. Temperature corrections are mathematical approximations; extreme physiological states require comprehensive critical care correlation.

ABG & Acid-Base Diagnostic Algorithm — Clinical decision support for educational purposes. Unauthorised reproduction, redistribution, or modification of this tool is prohibited. For permissions, contact mediscuss.org/contact.

Algorithm References & Evidence Base

Kraut JA, Madias NE. Approach to patients with acid-base disorders. Respir Care. 2012;57(1):18-34.
Seifter JL. Integration of acid-base and electrolyte disorders. N Engl J Med. 2014;371(19):1821-1831.
Bissonnette B, et al. Temperature correction of blood gases and pH. Anesthesiology. 1990;73(1):15-18.

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