ARTERIAL BLOOD GAS ANALYSIS

✓ Medically ReviewedINVESTIGATIONID: MDBT0295🚨 TRIAGE: ROUTINEICD-10: R79.81

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Definition

CLINICAL DEFINITION

Arterial blood gas (ABG) analysis is a diagnostic investigation that measures the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2), pH, bicarbonate (HCO3-), base excess (BE), and derived parameters such as arterial oxygen saturation (SaO2) in an arterial blood sample. It provides a simultaneous assessment of oxygenation, ventilation, and acid-base status, and is used to evaluate respiratory and metabolic disorders, guide oxygen therapy, and monitor disease progression or response to treatment.

IN PLAIN ENGLISH

An arterial blood gas analysis is a blood test taken from an artery that shows how well your lungs are moving oxygen into your blood and removing carbon dioxide, and whether your blood's acid level is balanced. It helps doctors understand how well your breathing and circulation are working.

COMMON MISCONCEPTION / WHAT IT DOES NOT MEAN

Arterial blood gas analysis is NOT the same as a venous blood gas (VBG), which is easier to obtain but less reliable for assessing oxygenation and ventilation. ABG values reflect arterial blood; VBG values differ significantly, particularly for PaO2 and PaCO2, and should not be substituted when accurate oxygenation assessment is required.

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Word Breakdown

arteri- (artery) + blood + gas

=arteri- from Greek 'arteria' (windpipe, artery); blood from Old English 'blōd'; gas from Greek 'khaos' (chaos, empty space), coined by van Helmont

-analysis

=analysis from Greek 'analysis' (a breaking up, a loosening)

Literal Meaning: Analysis of gases in arterial blood

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Abbreviations

ABG=Arterial Blood Gas — the test and the sample type

PaO2=Partial pressure of oxygen in arterial blood — measure of oxygenation

PaCO2=Partial pressure of carbon dioxide in arterial blood — measure of ventilation

HCO3-=Bicarbonate — the principal metabolic buffer, reflecting the metabolic component

BE=Base Excess — calculated estimate of the metabolic acid-base component

SaO2=Arterial oxygen saturation — percentage of haemoglobin binding sites occupied by oxygen

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Synonyms & Variants

Arterial Blood Gas Test›ABG Analysis›

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Category-Specific Context

As an Investigation, arterial blood gas analysis is a diagnostic test that measures specific physiological parameters in arterial blood. It sits alongside other bedside investigations such as pulse oximetry and capnography, but uniquely provides direct measurement of PaO2, PaCO2, and acid-base status. Results are classified by the primary acid-base disturbance (respiratory or metabolic, acidosis or alkalosis) and by the adequacy of compensation; mixed disorders are common in critically ill patients.

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Clinical Context & Usage

CLINICAL SETTINGS

Emergency DepartmentIntensive Care UnitRespiratory WardOperating TheatreGeneral Medicine WardNeonatal UnitPulmonary Function Laboratory

USAGE & DOCUMENTATION NOTES

ABG results are documented in notes as a set of values, typically written as 'pH 7.35 / PaCO2 45 / PaO2 80 / HCO3 24 / BE -1 on room air' or with the FiO2 specified. In ward rounds, clinicians refer to 'an arterial gas' or 'an ABG' and describe the primary disorder (e.g., 'metabolic acidosis with respiratory compensation'). In referral letters, the key values and the interpretation (e.g., 'type 2 respiratory failure') are summarised alongside the FiO2 and the patient's clinical trajectory. Serial ABGs are used to track response to treatment, such as non-invasive ventilation or fluid resuscitation.

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Primary Causes & Pathophysiology

PATHOPHYSIOLOGY

Arterial blood gas analysis directly measures the partial pressures exerted by dissolved oxygen and carbon dioxide in arterial plasma, the hydrogen ion concentration (expressed as pH), and the bicarbonate concentration. Oxygenation is determined by the interplay of alveolar oxygen tension, diffusion across the alveolar-capillary membrane, ventilation-perfusion matching, and the haemoglobin concentration and saturation. Ventilation is reflected by PaCO2, which is inversely proportional to alveolar ventilation for a given metabolic rate. Acid-base status is described by the Henderson-Hasselbalch relationship, where pH is determined by the ratio of bicarbonate to dissolved carbon dioxide; the respiratory component (PaCO2) and the metabolic component (HCO3-, BE) are assessed together to identify the primary disturbance and the degree of physiological compensation. The test therefore provides a snapshot of gas exchange and acid-base homeostasis at the time of sampling.

PRIMARY CAUSES

Suspected respiratory failure (type 1 or type 2)

ABG directly measures PaO2 and PaCO2, which are required to classify hypoxaemic versus hypercapnic respiratory failure and to guide oxygen therapy or ventilatory support.

Assessment of acid-base disorders

pH, PaCO2, HCO3-, and BE allow identification of metabolic or respiratory acidosis/alkalosis and evaluation of compensation.

Monitoring severity and response in critical illness (e.g., sepsis, diabetic ketoacidosis, COPD exacerbation)

Serial ABGs track the trajectory of acid-base and oxygenation parameters in response to treatment.

Evaluation of oxygenation when pulse oximetry is unreliable or insufficient

ABG provides direct PaO2 and SaO2 measurement, which may be needed in poor perfusion, dyshaemoglobinaemia, or when precise oxygen titration is required.

Intraoperative and intensive care monitoring

ABG informs ventilator adjustment and detects acute changes in gas exchange or acid-base status during anaesthesia and critical care.

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Epidemiology

Prevalence:Arterial blood gas analysis is one of the most frequently performed investigations in acute care settings; it is routinely used in emergency departments, intensive care units, and respiratory wards worldwide. Precise utilisation rates vary by setting and case mix.

Age Group:All age groups, from neonates (umbilical or capillary samples in newborns) to older adults; interpretation requires age-appropriate reference ranges, particularly for PaO2 in infants and for base excess in neonates.

Sex Predilection:Equal

Red Flags & Triage ★

TRIAGE LEVEL

🚨 ROUTINE

DO NOT MISS — LIFE-THREATENING PRESENTATIONS

Severe acidemia (pH <7.20) or severe alkalemia (pH >7.60) — requires urgent clinical review and identification of the underlying cause.
Severe hypoxaemia (PaO2 <60 mmHg on room air) — consider immediate oxygen therapy and evaluation for respiratory failure.
Hypercapnia with acidaemia (PaCO2 >50 mmHg and pH <7.35) — suggests ventilatory failure and may require non-invasive or invasive ventilation.
A normal pH does not exclude a mixed acid-base disorder; always calculate compensation and consider the anion gap.

URGENT EVALUATION TRIGGERS

pH <7.20 or >7.60
PaO2 <60 mmHg on room air
PaCO2 >50 mmHg with pH <7.35
Rapidly deteriorating respiratory status or oxygen saturation
New confusion or reduced consciousness in a patient with abnormal ABG

Contraindications & Precautions

CONTRAINDICATIONS

Absolute: Abnormal modified Allen test indicating inadequate collateral circulation to the hand (for radial artery puncture).
Absolute: Local infection
cellulitis
or distorted anatomy at the intended puncture site.
Absolute: Severe peripheral vascular disease or Raynaud's syndrome affecting the selected limb.
Absolute: Puncture distal to a surgical shunt (e.g.
arteriovenous fistula for dialysis).
Relative: Coagulopathy or therapeutic anticoagulation — weigh bleeding risk against diagnostic need; prefer a compressible site and monitor closely.
Relative: Severe thrombocytopenia or bleeding diathesis — relative contraindication; may still be performed if the result is essential and precautions are taken.

SPECIAL PRECAUTIONS

Perform a modified Allen test before radial artery puncture to assess collateral circulation; if abnormal
select an alternative site.
Use aseptic technique and appropriate barrier precautions.
Expel air bubbles immediately and cap the syringe; air bubbles can falsely alter PaO2 and PaCO2.
Analyse the sample promptly: within 15 minutes at room temperature or within 1 hour if stored on ice
depending on local protocol.
Avoid excessive heparin in liquid-heparin syringes
which can dilute the sample and lower PaCO2.
In patients on anticoagulants or with coagulopathy
apply prolonged pressure after puncture and monitor for bleeding.

Complications

DOWNSTREAM CLINICAL COMPLICATIONS

Haematoma or bleeding at the puncture site — most common complication
especially in patients with coagulopathy or on anticoagulants
Arterial injury or thrombosis — rare but can compromise distal perfusion
particularly with repeated punctures or indwelling catheters
Pain and vasovagal response — needling of the artery can cause discomfort and
occasionally
syncope
Air or clot embolism — rare; associated with improper syringe handling or inadvertent injection of air
Pseudoaneurysm — uncommon
related to arterial wall trauma
Infection at the puncture site — uncommon with aseptic technique

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Reference Ranges

NORMAL RANGETypical adult arterial values on room air at sea level: pH 7.35–7.45; PaCO2 35–45 mmHg; PaO2 80–100 mmHg (age-dependent, lower at extremes of age); HCO3- 22–26 mmol/L; base excess −2 to +2 mmol/L; SaO2 ≥95%. Reference ranges vary by laboratory and clinical context.

CRITICAL (PANIC) VALUEpH <7.20 or >7.60; PaO2 <60 mmHg (on room air, suggesting severe hypoxaemia); PaCO2 >70 mmHg (with acidaemia) — these values require immediate clinical review and may prompt emergency intervention. Critical thresholds may vary by institution.

UNIT OF MEASUREMENTpH (units); PaO2, PaCO2 (mmHg or kPa); HCO3-, BE (mmol/L or mEq/L); SaO2 (%)

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Confused With (LASA & DDx) ★

LEXICAL (LASA) — SIMILAR NAMES

Venous Blood Gas (VBG)

VBG is often confused with ABG; VBG is easier to obtain but has different reference ranges and is less reliable for assessing oxygenation and ventilation.

Capillary Blood Gas

Capillary samples may be used in neonates and children but reflect a mix of arterial and venous blood and are not a substitute for arterial sampling when accurate PaO2 is required.

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Clinical Pearls

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Always interpret the ABG in the context of the FiO2; a PaO2 of 80 mmHg on 100% oxygen represents severe gas exchange failure, whereas the same PaO2 on room air is normal.

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Check internal consistency using the Henderson-Hasselbalch equation; discrepancies suggest pre-analytical error, such as air bubbles, dilution, or delayed analysis.

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The modified Allen test is a screening tool for collateral circulation before radial puncture; an abnormal test is a relative contraindication to radial puncture, and an alternative site should be considered.

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In metabolic acidosis, use Winter's formula to calculate the expected PaCO2: expected PaCO2 = (1.5 × HCO3-) + 8 ± 2. A measured PaCO2 outside this range suggests an additional respiratory disorder.

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Base excess reflects the metabolic component independently of the respiratory contribution; a negative base excess indicates metabolic acidosis, while a positive base excess indicates metabolic alkalosis.

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Venous blood gas can be a useful alternative for pH and bicarbonate in selected contexts, but it cannot replace arterial sampling for accurate assessment of oxygenation and ventilation.

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Example Usage & SOAP

CLINICAL SENTENCE

""Arterial blood gas on 2 L/min nasal cannula: pH 7.28, PaCO2 58 mmHg, PaO2 62 mmHg, HCO3- 26 mmol/L, BE -1; consistent with acute-on-chronic type 2 respiratory failure.""

IN A SOAP NOTE

S: 68-year-old man with known COPD presents with worsening breathlessness and purulent sputum for 3 days. O: RR 24/min, SpO2 88% on room air, using accessory muscles. ABG on room air: pH 7.26, PaCO2 62 mmHg, PaO2 54 mmHg, HCO3- 27 mmol/L, BE +1. A: Acute exacerbation of COPD with type 2 respiratory failure and respiratory acidosis. P: Controlled oxygen to target SpO2 88–92%, nebulised bronchodilators, systemic corticosteroids, antibiotics, and consideration of non-invasive ventilation if no improvement.

DIFFERENTIAL DIAGNOSIS CONTEXT

Differential diagnosis of hypoxaemia (low PaO2): (1) Hypoventilation — suggested by elevated PaCO2 with normal alveolar-arterial gradient. (2) Ventilation-perfusion mismatch — common in pneumonia, pulmonary embolism, or asthma; widened A-a gradient. (3) Shunt — e.g., ARDS or intracardiac shunt; hypoxaemia refractory to oxygen. (4) Diffusion impairment — e.g., interstitial lung disease. ABG, together with the A-a gradient and clinical context, helps narrow the differential.

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Prognosis

SHORT-TERM OUTCOME

The procedure itself is brief and usually well tolerated; most complications are minor and self-limiting. The short-term clinical value depends on the prompt availability and accurate interpretation of results.

LONG-TERM PROGNOSIS

No long-term sequelae are expected from an uncomplicated arterial puncture. Rare serious complications (e.g., arterial occlusion, pseudoaneurysm) may require further intervention but are uncommon.

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References & Coding

ICD-10 DIAGNOSTIC CODE

R79.81

LITERATURE CITATIONS & GUIDELINES

[1]Núñez Sanagustín M, Osredkar J. Blood gas analysis: Clinical applications, interpretation and future directions (Review). Med Int (Lond). 2025;5(1):1-19. doi:10.3892/mi.2025.291

[2]Habib T, Nair A, Murphy S, Saeed H, Ishaya N. Mastering blood gas interpretation: A practical guide for primary care providers. S Afr Fam Pract. 2025;67(1):a6058. doi:10.4102/safp.v67i1.6058

[3]Arterial blood gas analysis. In: Knowledge @ AMBOSS. Last updated February 2, 2026. Accessed 2026.

[4]Higgins V, Nichols M, et al. Defining blood gas analysis stability limits across five sample types. Clin Biochem. 2023;115:107-111. doi:10.1016/j.clinbiochem.2022.09.006

[5]Cadogan M. Arterial Blood Gas Sample. Life in the Fast Lane (LITFL). Accessed 2026.

MEDICAL DISCLAIMER — This content is intended for educational purposes and qualified healthcare professionals only. Always refer to current clinical diagnostic criteria, institutional protocols, and official guidelines. Medobo is not responsible for clinical decisions made based on this content.