Clinical Medicine • Acid-Base Disorders

ABG Interpretation Explained

Arterial blood gas interpretation is an essential clinical skill for medical students, nursing students and junior doctors. ABGs help assess oxygenation, ventilation and acid-base status. A systematic approach prevents common mistakes and helps identify simple and mixed acid-base disorders. This article explains ABG interpretation step by step using pH, PaCO2, bicarbonate, compensation, anion gap, Winter's Formula and Delta Ratio.

Dr. Seneth Gajasinghe, MBBS, MD Published: 27 June 2026 Updated: 27 June 2026 30 min read Reviewed Content

ABG interpretation becomes much easier when every result is approached in the same order. The aim is not to memorise isolated patterns, but to build a repeatable clinical workflow.

This article is the hub for the acid-base cluster. It connects Acid-Base Disorders Explained, Metabolic Acidosis Explained, Anion Gap Explained, Winter's Formula Explained and Delta Ratio Explained.

Learning Objectives

  • Explain what an ABG measures
  • Use normal ABG values correctly
  • Identify the primary acid-base disorder
  • Assess compensation and mixed disorders
  • Calculate and correct the anion gap
  • Use Winter's Formula and Delta Ratio appropriately
  • Interpret common clinical ABG examples

What Is an ABG?

An arterial blood gas is a blood test taken from an artery. It provides rapid information about oxygenation, ventilation, acid-base balance and metabolic status.

ABG interpretation is especially useful in DKA, sepsis, shock, COPD exacerbation, asthma, respiratory failure, renal failure, poisoning, ICU care and emergency medicine.

Oxygenation↓Ventilation↓Acid-base balance↓Metabolic status
Arterial blood gas assessment of oxygenation ventilation and acid base status
Figure 1. An ABG helps assess oxygenation, ventilation and acid-base status.

What Does an ABG Measure?

An ABG contains several values, but pH, PaCO2 and bicarbonate are the core acid-base variables. PaO2 and SaO2 help assess oxygenation. Lactate and base excess add important metabolic information.

ComponentMeaning
pHAcidaemia or alkalaemia
PaCO2Respiratory component and ventilation
HCO3-Metabolic component
PaO2Oxygenation
SaO2Oxygen saturation
Base excessMetabolic acid-base burden
LactateTissue hypoperfusion or anaerobic metabolism
Key Teaching Point

PaCO2 reflects ventilation. HCO3- reflects the metabolic or kidney component. pH tells the overall direction.

ABG components including pH PaCO2 bicarbonate PaO2 saturation base excess and lactate
Figure 2. ABG components must be interpreted together rather than as isolated numbers.

Normal ABG Values

ParameterNormal value
pH7.35-7.45
PaCO235-45 mmHg
HCO3-22-26 mmol/L
PaO280-100 mmHg
SaO295-100%
LactateUsually < 2 mmol/L
Important

Normal ranges may vary slightly between laboratories. Always interpret ABG results with the clinical picture.

Normal ABG values for pH PaCO2 bicarbonate PaO2 oxygen saturation and lactate
Figure 3. Normal ABG values provide the starting point for interpretation.

Step-by-Step ABG Interpretation Algorithm

The safest way to read an ABG is to use the same order every time. This prevents common mistakes such as jumping to compensation before identifying the primary disorder.

  1. Check pH.
  2. Check PaCO2 and HCO3-.
  3. Identify the primary disorder.
  4. Assess compensation.
  5. Calculate anion gap if metabolic acidosis is present.
  6. Correct the anion gap for albumin.
  7. Use Winter's Formula if metabolic acidosis is present.
  8. Use Delta Ratio if high anion gap metabolic acidosis is present.
  9. Combine with clinical context.

You can practise the full sequence using the Blood Gas Analyser.

Stepwise ABG interpretation algorithm from pH to clinical context
Figure 4. A structured algorithm prevents common ABG interpretation errors.

Step 1: Check pH

First decide whether the blood is acidic, alkaline or within the normal pH range.

pHMeaning
< 7.35Acidaemia
> 7.45Alkalaemia
7.35-7.45Normal pH range

A normal pH does not exclude an acid-base disorder. A mixed disorder may bring pH back toward normal.

Checking pH first in ABG interpretation
Figure 5. The pH tells whether the overall blood gas direction is acidic, alkaline or near normal.

Step 2: Identify the Primary Disorder

Use pH direction, PaCO2 and HCO3- together. CO2 is respiratory. HCO3- is metabolic.

DisorderpHPaCO2HCO3-
Metabolic acidosisLowLow or normalLow
Metabolic alkalosisHighHigh or normalHigh
Respiratory acidosisLowHighHigh or normal
Respiratory alkalosisHighLowLow or normal
Memory Rule

CO2 = respiratory. HCO3- = metabolic.

Identifying primary acid base disorder using pH PaCO2 and bicarbonate
Figure 6. Primary disorder identification depends on pH direction, PaCO2 and bicarbonate.

Step 3: Assess Compensation

Compensation is the body's attempt to reduce the pH disturbance. Metabolic problems are compensated by the lungs. Respiratory problems are compensated by the kidneys.

Metabolic problem↓Lungs compensate↓PaCO2 changes
Respiratory problem↓Kidneys compensate↓HCO3- changes
Clinical Pearl

Compensation does not fully correct the disorder and should not overshoot. If it appears to overshoot, suspect a mixed disorder.

In metabolic acidosis, use Winter's Formula Explained to assess whether respiratory compensation is appropriate.

ABG compensation overview showing lungs compensating metabolic disorders and kidneys compensating respiratory disorders
Figure 7. Compensation moves pH toward normal but does not erase the primary disorder.

Step 4: Calculate the Anion Gap

Calculate the anion gap when metabolic acidosis is present. The anion gap helps separate high anion gap metabolic acidosis from normal anion gap metabolic acidosis.

Anion Gap = Na+ - (Cl- + HCO3-)

Use Anion Gap Explained for the full concept and the Anion Gap Calculator for calculations.

Anion gap calculation during ABG interpretation
Figure 8. The anion gap classifies metabolic acidosis into high gap and normal gap patterns.

Step 5: Correct the Anion Gap for Albumin

Low albumin lowers the anion gap and may hide high anion gap metabolic acidosis. This is especially important in sepsis, chronic illness, liver disease, malnutrition and critical care.

Corrected AG = Observed AG + 2.5 × (4 - Albumin)
Albumin is measured in g/dL.
Do Not Miss This

In critically ill patients, always consider albumin correction before deciding that the anion gap is normal.

Albumin correction during ABG and anion gap interpretation
Figure 9. Low albumin can hide a high anion gap metabolic acidosis.

Step 6: Use Winter's Formula

Winter's Formula is used only in metabolic acidosis. It predicts the expected PaCO2 for the degree of bicarbonate reduction.

Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2
Measured PaCO2Meaning
Within expected rangeAppropriate compensation
Higher than expectedAdditional respiratory acidosis
Lower than expectedAdditional respiratory alkalosis

For the full method, read Winter's Formula Explained.

Winter formula used in ABG interpretation for metabolic acidosis compensation
Figure 10. Winter's Formula checks whether respiratory compensation is appropriate in metabolic acidosis.

Step 7: Use Delta Ratio

Delta Ratio is used only in high anion gap metabolic acidosis. It helps determine whether another metabolic disorder is present.

Delta Ratio = (AG - 12) / (24 - HCO3-)
Delta RatioMeaning
< 0.4Pure NAGMA
0.4-0.8HAGMA + NAGMA
0.8-2.0Pure HAGMA
> 2.0HAGMA + metabolic alkalosis

For full interpretation, read Delta Ratio Explained.

Delta ratio used in ABG interpretation to detect mixed metabolic disorders
Figure 11. Delta Ratio helps detect mixed metabolic disorders in high anion gap metabolic acidosis.

Worked ABG Examples

Examples help convert the algorithm into pattern recognition. Always start with pH, then identify the primary disorder, then assess compensation and context.

ExampleABGInterpretation
Normal ABGpH 7.40, PaCO2 40, HCO3- 24Normal acid-base status
Simple metabolic acidosispH 7.25, PaCO2 30, HCO3- 14Metabolic acidosis with respiratory compensation
DKApH 7.15, PaCO2 24, HCO3- 8, high AGHigh anion gap metabolic acidosis, likely DKA
DKA + respiratory acidosispH 7.05, PaCO2 40, HCO3- 10Expected PaCO2 is 23 ± 2, so measured PaCO2 is high: metabolic acidosis + respiratory acidosis
Sepsis / lactic acidosispH 7.28, PaCO2 20, HCO3- 10, high lactate, high AGHigh anion gap metabolic acidosis with additional respiratory alkalosis
COPD exacerbationpH 7.28, PaCO2 65, HCO3- 30Respiratory acidosis with metabolic compensation
VomitingpH 7.50, PaCO2 48, HCO3- 36Metabolic alkalosis with respiratory compensation
Panic attackpH 7.52, PaCO2 25, HCO3- 22Respiratory alkalosis
Salicylate toxicitypH 7.44, PaCO2 20, HCO3- 13, high AGMixed respiratory alkalosis + high anion gap metabolic acidosis
DKA + vomitingpH 7.30, PaCO2 28, HCO3- 14, AG 32, Delta Ratio > 2High AG metabolic acidosis + metabolic alkalosis
Worked ABG examples including DKA sepsis COPD vomiting panic attack salicylates and mixed disorders
Figure 12. Worked examples show how the ABG algorithm applies to common clinical scenarios.

Common Pitfalls

  • Looking at HCO3- before pH.
  • Ignoring PaCO2.
  • Calling compensation a primary disorder.
  • Forgetting the anion gap.
  • Not correcting the anion gap for albumin.
  • Using Winter's Formula in the wrong disorder.
  • Using Delta Ratio when the anion gap is normal.
  • Ignoring oxygenation.
  • Ignoring clinical context.
  • Assuming normal pH means normal ABG.
Common ABG interpretation pitfalls including ignoring pH PaCO2 anion gap oxygenation and clinical context
Figure 13. Common ABG mistakes usually come from skipping a structured sequence.

One Minute Revision

  • Check pH.
  • Check PaCO2 and HCO3-.
  • Identify the primary disorder.
  • Assess compensation.
  • If metabolic acidosis is present, calculate the anion gap.
  • Correct the anion gap for albumin.
  • Use Winter's Formula when appropriate.
  • Use Delta Ratio when appropriate.
  • Interpret everything with clinical context.
One minute ABG interpretation revision summary
Figure 14. ABG interpretation should follow the same structured order every time.

Frequently Asked Questions

What is ABG interpretation?
ABG interpretation is the systematic analysis of arterial blood gas values to assess oxygenation, ventilation and acid-base status.
What are the main ABG values?
The main ABG values are pH, PaCO2, HCO3, PaO2 and oxygen saturation. Lactate and base excess may also be reported.
How do you interpret ABG step by step?
First check pH, then PaCO2 and HCO3, identify the primary disorder, assess compensation, calculate the anion gap when appropriate and interpret everything with the clinical picture.
What does PaCO2 show?
PaCO2 reflects ventilation and the respiratory component of acid-base balance. High PaCO2 causes respiratory acidosis and low PaCO2 causes respiratory alkalosis.
What does HCO3 show?
HCO3 reflects the metabolic component of acid-base balance. Low HCO3 suggests metabolic acidosis and high HCO3 suggests metabolic alkalosis.
When should the anion gap be calculated?
The anion gap should be calculated when metabolic acidosis is present. It helps distinguish high anion gap from normal anion gap metabolic acidosis.
When should Winter's Formula be used?
Winter's Formula should be used in metabolic acidosis to assess whether respiratory compensation is appropriate.
When should Delta Ratio be used?
Delta Ratio should be used in high anion gap metabolic acidosis to look for additional metabolic disorders.
Can pH be normal in a mixed acid-base disorder?
Yes. A normal pH can occur when two opposing acid-base disorders coexist. Therefore pH should not be interpreted alone.
What is the most common mistake in ABG interpretation?
A common mistake is identifying one abnormal value without using a systematic approach. ABGs should always be interpreted step by step and with clinical context.
Medical Education Disclaimer

This article is intended for medical education only. ABG interpretation must be combined with clinical assessment, oxygen delivery settings, local laboratory ranges, medication history and senior or specialist input when patients are acutely unwell.