Arterial Blood Gas (ABG): Nursing Guide
An arterial blood gas (ABG) shows how well the lungs move oxygen in and carbon dioxide out, and whether acid-base balance is safe — but nurses must interpret every value with respiratory status, oxygen delivery, mental status, and trend. A single ABG cannot diagnose alone, and arterial sampling carries bleeding and interpretation risks that require immediate, documented escalation when hypoxemia or acidemia accompanies clinical deterioration.
Contents
Quick Facts
Key Takeaway
The main nursing priority with an ABG is rapid recognition of oxygenation, ventilation, and acid-base problems.
Specimen & Collection Details
Nurse quick-reference for collection prep that affects result quality.
Dedicated blood gas specimen (institutional)
Institutional blood gas collection container — not a standard venous tube; pre-analytic handling per laboratory and Performance procedure guide
Arterial blood (not venous — required for accurate oxygenation and ventilation data)
Small-volume arterial sample — exact volume not specified in reviewed standard clinical references; follow local analyzer requirements
When ordered for acute respiratory change, ventilator adjustment, or acid-base evaluation; if on oxygen, keep delivery constant for about 20 minutes before draw when clinically safe per provider direction
No routine fasting for ABG per reviewed standard clinical references — follow ordering clinician and local policy
Analyze immediately on point-of-care analyzer or transport without delay on ice per laboratory policy; exclude air bubbles and hemolyzed samples
Point-of-care results may be available within minutes; central laboratory timing not specified in reviewed references — follow local policy
Blood gas / respiratory laboratory or point-of-care analyzer
What is Arterial Blood Gas (ABG)?
Arterial Blood Gas (ABG) measures oxygen and carbon dioxide levels in a sample of arterial blood and usually reports pH, partial pressure of oxygen (PaO₂), partial pressure of carbon dioxide (PaCO₂), oxygen saturation, and often bicarbonate (HCO₃⁻). Because arterial blood reflects gas exchange at the lungs more accurately than venous blood, clinicians order ABG to assess oxygenation, ventilation, and acid-base balance — not routine venous chemistry.
Overview
Nurses request, assist with, or respond to ABG results when patients have shortness of breath, increasing work of breathing, altered mental status, or known COPD or asthma exacerbation. ABGs guide whether oxygen therapy, non-invasive ventilation, or mechanical ventilation is effective and whether metabolic conditions such as diabetic ketoacidosis contribute to acid-base disturbance.
On this Tests & Diagnostics page, nursing focus is interpretation and escalation — not step-by-step collection technique (see the Performance procedure guide). Every ABG must be read with documented FiO₂ and device settings at draw time. standard clinical references emphasize that an ABG alone usually cannot provide a diagnosis — nurses correlate results with pulse oximetry, respiratory rate, and trend, and compare with lactate or electrolytes when sepsis or shock is suspected.
Before result review: confirm indication, documented oxygen device and FiO₂ at draw time, and sample validity (no air bubble, timely analysis). After result: compare pH, PaCO₂, PaO₂, and HCO₃⁻ with symptoms and prior values; notify prescriber and respiratory therapy when hypoxemia, hypercapnia, or acidemia conflicts with the bedside picture.
ABG Interpretation and Escalation Safety
ABG results can trigger life-saving changes in oxygen and ventilation — but only when paired with accurate FiO₂ documentation, valid specimens, and bedside assessment. A single normal value never overrides a deteriorating patient. Collection technique and post-collection monitoring belong in the Performance procedure guide.
- Severe hypoxemia or acidemia with confusion, fatigue, or hemodynamic instability
- Rising PaCO₂ with somnolence in COPD or neuromuscular weakness — SpO₂ may mislead
- Uncontrolled bleeding, hematoma, or limb pallor after arterial sample collection
- Air bubble or mislabeled sample driving incorrect ventilator changes
Document: oxygen/ventilator settings at draw, sample validity, all result components with reference range, notifications, trend comparison, and repeat testing plan.
What Arterial Blood Gas Can and Cannot Tell You
This test can help identify:
- Hypoxemia, hypercapnia, and acid-base disturbances affecting oxygen delivery and ventilation
- Response to oxygen therapy, BiPAP, or mechanical ventilation when trended with clinical status
- Contributions of respiratory versus metabolic processes to pH changes
- Need for urgent escalation when values conflict with work of breathing or mental status
This test cannot:
- Diagnose COPD, pneumonia, or renal failure alone — requires full clinical picture
- Replace continuous SpO₂ and respiratory assessment at the bedside
- Define universal critical thresholds — institution-specific policies apply
- Remain valid without FiO₂/device context or timely, integrity-checked analysis
Pre-analytic Checks Before ABG
Verify
Clarify before acting on results when:
- FiO₂ or ventilator settings changed within minutes without repeat gas ordered
- Only venous access available but arterial sample ordered — clarify with prescriber/RT
- Prior sample hemolyzed, clotted, or contained air bubbles
- Result markedly abnormal but patient looks stable — verify sample source and therapy context
- Order conflicts with unstable patient who cannot tolerate brief oxygen change per prescriber
- Gas drawn before current BiPAP or ventilator settings — trend may mislead
- Patient on therapeutic anticoagulation with new bleeding or neurologic symptoms after collection
Reading ABG With SpO₂, Ventilation, and Trends
Integrate pH, PaCO₂, PaO₂, and HCO₃⁻ with respiratory rate, effort, device settings, mental status, and serial trend. Compare with pulse oximetry and lactate when perfusion or sepsis is in question.
| Clinical context | Pair with ABG | Nursing focus |
|---|---|---|
| COPD exacerbation on NIV | SpO₂, work of breathing, mental status, PaCO₂ trend | Escalate somnolence with rising PaCO₂; document BiPAP settings with each gas |
| Pneumonia / ARDS | FiO₂, PaO₂, imaging, fever curve | Assess oxygenation response; prepare for RT/prescriber adjustment per protocol |
| DKA / metabolic acidosis | Anion gap, glucose, ketones, K⁺ | Track pH correction with treatment; monitor K⁺ shifts during therapy |
| Invalid or delayed sample | Draw time, transport delay, air bubble | Do not file erroneous values; recollect and compare with bedside picture |
Named acid-base patterns on ABG
Use disorder labels with pH, PaCO₂, and HCO₃⁻ together — compensation develops over hours to days and never replaces bedside assessment.
| Disorder | Primary ABG change | Typical compensation | Nursing cue |
|---|---|---|---|
| Respiratory acidosis | ↑ PaCO₂ with ↓ pH | Rising HCO₃⁻ over time (renal compensation) | Somnolence, weak respiratory effort, inadequate NIV or ventilator support |
| Respiratory alkalosis | ↓ PaCO₂ with ↑ pH | Falling HCO₃⁻ over time | Tachypnea, pain, anxiety, sepsis early course, or over-ventilation |
| Metabolic acidosis | ↓ HCO₃⁻ with ↓ pH | ↓ PaCO₂ (hyperventilation) | DKA, lactate elevation, renal failure — pair with anion gap and perfusion data |
| Metabolic alkalosis | ↑ HCO₃⁻ with ↑ pH | Modest ↑ PaCO₂ | Vomiting, NG suction, diuretics, or excessive bicarbonate therapy |
Reference ranges and critical values may vary by laboratory, institution, analyzer, age, sex, pregnancy status, and clinical context. Always interpret results using the reporting laboratory’s reference range and local escalation policy.
Pre-analytic and Interpretation Traps at the Bedside
| Bedside point | Nursing note |
|---|---|
| FiO₂ trap | PaO₂ is meaningless without device, flow, and mode documented at draw time |
| SpO₂ vs PaCO₂ | Acceptable saturation does not exclude CO₂ retention — watch mental status |
| Air bubble | Discard sample and recollect — do not file erroneous values |
| Therapy change | Gas drawn before ventilator or BiPAP adjustment may not reflect current support |
| Trend beats snapshot | Serial gases after ventilator changes show whether therapy is working |
ABG Workflow in Respiratory Deterioration
Diagnostic safety badge: Critical-result test — hypoxemia and acidemia may require immediate RT and prescriber response.
Interpretation workflow
- Confirm identity, indication, and oxygen/ventilator snapshot at draw time
- Coordinate collection per Performance procedure guide when sample is needed
- Reject invalid samples; ensure timely analysis
- Interpret pH, PaCO₂, PaO₂, and HCO₃⁻ with SpO₂, mental status, and trend
- Notify prescriber and RT for critical or discordant values; document read-back
Critical teach-back questions
- “Can you tell me why we are checking an arterial blood gas today?”
- “What worsening breathing or confusion should you report immediately?”
- “Why do we record your oxygen or BiPAP settings when the blood is drawn?”
Care coordination: prescriber, respiratory therapy, laboratory/point-of-care team, and rapid response per institutional protocol.
ABG Quick Clinical Checklist
- Did I document FiO₂/device at the exact time of the draw?
- Is the patient more somnolent even if SpO₂ looks acceptable?
- Was the sample valid and analyzed without undue delay?
- Does this result match the bedside picture — or do I need a repeat gas?
- Who must be notified now for hypoxemia, acidemia, or post-collection complications?
Why Arterial Blood Gas (ABG) is Ordered
An ABG is ordered when clinicians need objective data on oxygenation, ventilation, and acid-base status — usually with clinical assessment and often serial trends, not as a standalone diagnostic.
| Clinical Indication | What the Test Answers | Nursing Rationale |
|---|---|---|
| Acute respiratory distress or failure evaluation | Is the patient hypoxemic, hypercapnic, or acidotic despite visible effort to breathe? | Supports assessment of serious lung and breathing problems when symptoms, SpO₂, or work of breathing suggest inadequate gas exchange. |
| Monitoring response to oxygen or ventilation therapy | Is oxygen therapy, BiPAP, or mechanical ventilation improving gas exchange? | Serial ABGs may evaluate effectiveness of breathing support and guide ventilator or oxygen adjustments under prescriber and RT direction. |
| Suspected acid-base disorder | Does pH disturbance reflect respiratory, metabolic, or mixed pathology? | Helps distinguish respiratory versus metabolic contributions when symptoms include rapid breathing, confusion, nausea, or arrhythmia per reviewed references. |
| Renal or metabolic disease monitoring | Are kidneys or metabolism failing to maintain normal acid-base balance? | Kidneys help regulate bicarbonate and CO₂ clearance; ABG may support evaluation alongside renal function and electrolytes when acid-base imbalance is suspected. |
Contraindications and Precautions
There is no absolute contraindication to ABG when clinically indicated, but collection may be deferred or modified when bleeding risk, infection at the access site, or hemodynamic instability makes sampling unsafe until clarified with prescriber — technique decisions belong in the Performance procedure guide.
- Severe hypoxemia or acidemia on ABG with altered mental status, fatigue, or hemodynamic instability — escalate per facility protocol.
- Uncontrolled bleeding, expanding hematoma, pallor, or numbness at the arterial access site after sample collection — escalate immediately per facility policy.
- ABG result conflicts with bedside picture (e.g., acceptable PaO₂ but patient in severe distress) — reassess device, sample quality, and trend.
- Unrecorded FiO₂ or ventilator changes make PaO₂ and PaCO₂ uninterpretable — always document delivery device, flow, and mode at draw time.
- Air bubbles or delayed analysis may falsely alter oxygen and CO₂ values — reject and recollect per protocol when sample integrity is compromised.
- Certain medicines may affect acid-base balance; anticoagulants increase post-puncture bleeding risk — review with prescriber before draw.
- ABG showing severe hypoxemia or acidemia with respiratory distress, cyanosis, confusion, or falling blood pressure.
- Rising PaCO₂ with somnolence in COPD or neuromuscular patient — risk of ventilatory failure despite prior “acceptable” SpO₂.
- Post-collection bleeding that does not respond to initial first aid per policy, neurologic deficit in the limb, or signs of vascular compromise at the access site.
Patient Preparation
Review medicines and supplements with the provider; do not stop blood thinners including aspirin unless instructed. If the patient receives oxygen in hospital, concentration may need to stay constant for about 20 minutes before the test when they can breathe without supplemental oxygen — only per provider direction.
Pre-test checksReview anticoagulants, antiplatelet agents including aspirin, sedatives, and medicines that may affect acid-base balance. Never stop prescription medicines without prescriber authorization. Document recent bronchodilator, diuretic, or sodium bicarbonate therapy when relevant to interpretation.
Performance — nursing procedure guide
This page is a Tests & Diagnostics guide for Arterial Blood Gas (ABG). It emphasizes why the test is ordered, how to interpret results, when to escalate, and preparation factors that affect validity — not step-by-step performance technique (those live under Nursing Procedures when available).
Step-by-step technique, supplies, infection prevention, and immediate post-procedure monitoring for this test are covered in:
Use the Patient preparation, Results and interpretation, and Nursing responsibilities sections on this page for order verification, pre-analytic checks, result follow-up, critical-value escalation, and documentation.
Result follow-up at a glance
Nursing workflow on this page — from order to safe action on results:
Results and Interpretation
ABG components are reported with institution-specific reference intervals. standard clinical references lists commonly cited adult ranges: O₂ saturation 95–100%, PaO₂ 75–100 mmHg, PaCO₂ 35–45 mmHg, pH 7.35–7.45, HCO₃⁻ 22–26 mEq/L. Abnormal results may reflect lung, kidney, or metabolic disorders but require clinical correlation and often repeat or serial testing.
Reference ranges, critical values, and protocols may vary by laboratory, institution, patient population, and testing method. Always follow local policy and the reporting laboratory’s reference range.
| Result | Range / Finding | Clinical Meaning | Nursing Action |
|---|---|---|---|
| Within reference interval | Within reporting laboratory reference interval (see standard clinical references typical adult ranges above) | Oxygenation, ventilation, and acid-base balance within laboratory reference for context | Continue monitoring respiratory status and trend; does not exclude clinical deterioration if symptoms worsen — reassess SpO₂ and work of breathing |
| Borderline / near reference limit | Near limit of reference interval or compensating values | May represent early compensation or evolving disorder — trend and symptoms matter | Notify prescriber when paired with dyspnea, fatigue, or changing ventilator needs; repeat per orders |
| High / above reference interval | High PaCO₂, low pH (acidemia), or low PaO₂ depending on component | May indicate hypoventilation, respiratory failure, V/Q mismatch, or mixed disorder — interpret component pattern with prescriber/RT | Escalate hypoxemia or acidemia with symptoms per protocol; prepare for oxygen or ventilation adjustment as ordered |
| Low / below reference interval | Low PaCO₂, high pH (alkalemia), or values below reference | May reflect hyperventilation, metabolic alkalosis, or over-correction of acidosis | Assess for tachypnea, vomiting, diuretic use, or over-ventilation; notify prescriber with trend |
Critical ABG Results and Escalation
Institution-specific critical ABG thresholds vary. Any ABG showing severe hypoxemia, dangerous acidemia, or clinical conflict with the patient’s condition requires urgent response regardless of numeric flagging — follow local critical-value and rapid response policy.
| Critical Finding | Threshold / Value | Immediate Action |
|---|---|---|
| Severe hypoxemia with respiratory distress | Low PaO₂ and/or O₂ saturation with dyspnea, confusion, or cyanosis | Ensure airway and oxygen delivery; notify prescriber and respiratory therapy immediately per protocol |
| Acidemia with ventilatory failure pattern | Low pH with rising PaCO₂ and somnolence or fatigue | Escalate for ventilatory support evaluation; continuous monitoring; repeat ABG per orders |
| Post-arterial draw hemorrhage or ischemia | Non-stop bleeding, expanding hematoma, or hand pallor/numbness | Escalate to prescriber and activate emergency pathway per facility policy for vascular compromise — follow Performance procedure guide for site monitoring. |
Stop routine workflow and escalate according to facility policy when the patient has severe hypoxemia, dangerous acidemia, rapid neurologic decline, uncontrolled post-puncture bleeding, or ABG results that do not match life-threatening bedside findings.
Factors Affecting Results
ABG interpretation depends on pre-analytic quality and therapy context. Document factors that may distort or limit interpretation.
- Air bubble in sample — may falsely lower PaO₂ and alter CO₂
- Sample from venous line mislabeled as arterial — higher CO₂, lower O₂
- Delayed analysis — values may drift from true in vivo state
- Normal PaO₂ while patient is on high FiO₂ — may mask shunt; assess PaO₂/FiO₂ context
- Acceptable SpO₂ with rising PaCO₂ in COPD — hypercapnia may still threaten consciousness
- Single normal ABG while work of breathing is increasing — trend and serial gases matter
- Unstable oxygen or ventilator settings during or before draw
- Recent suctioning, nebulizer, or position change without documentation
- Medicines affecting acid-base (diuretics, bicarbonate, sedatives)
An ABG test alone usually cannot provide a diagnosis. Pulse oximetry may suffice when only oxygen saturation is needed. Venous blood gas may be used in some settings but does not replace arterial assessment when precise oxygenation and ventilation data are required. Reference ranges and critical values vary by laboratory, age, and altitude.
Nursing Responsibilities
Nursing responsibilities on this page emphasize accurate FiO₂ documentation, valid pre-analytic context, clinical correlation, and escalation when gas exchange or acid-base status threatens the patient. Collection technique is covered in the Performance procedure guide.
Before the TestDocumentation
Documentation must link the result to respiratory therapy and bedside findings.
“ABG 1430 on BiPAP IPAP 14 EPAP 5 FiO₂ 0.40. Result pH 7.31, PaCO₂ 58, PaO₂ 62, HCO₃⁻ 29 — Dr. Lee and RT notified via SBAR for hypercapnia with somnolence; repeat ABG ordered post-intervention. Patient drowsy but arousable; SpO₂ 90% on device; trend compared with gas drawn 4 hours ago on FiO₂ 0.35.”
- Time of draw, oxygen/ventilator settings, indication, and pre-analytic context
- Sample validity concerns (air bubble, hemolysis, delay) and recollection if needed
- Sample quality (air bubble, hemolysis) and transport time
- All ABG components with laboratory reference interval
- Prescriber/RT notification and read-back for critical values
- Patient response after intervention and repeat testing plan
Patient and Family Education
Use plain language: the test measures oxygen and carbon dioxide in the blood and checks whether the body’s acid balance is safe.
Arterial Blood Gas (ABG) NCLEX practice questions
Practice NCLEX-style clinical judgment focused on Arterial Blood Gas (ABG) safety and nursing judgment. Use the case tabs (orders, results, assessment, nursing notes), then answer eight Next Gen–style items (including an ordered workflow step) and evaluate outcomes with the answer key.
Select a tab to view orders, results, assessment, and nursing note details for this case.
- Order: ABG — STAT after BiPAP adjustment
- Indication: COPD exacerbation with rising work of breathing; evaluate ventilation and oxygenation
- Timing: Draw now; FiO₂ 0.40 on BiPAP — last ABG 4 hours ago pre-NIV
- Related orders: BiPAP, bronchodilators, BMP, lactate; antibiotics for pneumonia per protocol
- Result: Pending repeat ABG; prior pH 7.34, PaCO₂ 52, PaO₂ 58 on FiO₂ 0.35
- Trend / prior value: PaCO₂ rising; patient more sleepy over 2 hours despite SpO₂ 91%
- Pending tests: Repeat ABG in process; lactate 1.8 mmol/L; K⁺ 4.2
- Vital signs: HR 104/min, BP 138/76, RR 26, SpO₂ 91% on BiPAP, temp 37.8°C
- Symptoms: Accessory muscle use, diminished breath sounds R base, productive cough, drowsy but arousable
- Focused assessment: Known COPD; pneumonia on CXR; on apixaban — prescriber aware of ABG order
- Preparation notes: BiPAP settings documented; FiO₂ 0.40 recorded for pre-analytic context
- Collection events: First sample rejected for air bubble — repeat collection coordinated with RT
- Teaching gaps / safety concerns: Rising PaCO₂ with somnolence; invalid first sample; anticoagulant increases post-draw bleeding risk
Answer key & rationale
Frequently Asked Questions
FAQ
Why is an ABG taken from an artery instead of a vein?
Arterial blood has higher oxygen levels than venous blood, so sampling from an artery better reflects oxygenation and ventilation at the lungs. most tests use veins, but ABG requires arterial blood for accurate gas exchange data.
Does the patient need to fast before an ABG?
standard clinical references does not list routine fasting for ABG. Follow prescriber and local policy; focus preparation on medicines, oxygen stability, and FiO₂ documentation.
Can an ABG diagnose COPD or pneumonia by itself?
No. an ABG alone usually cannot provide a diagnosis. Results support assessment alongside history, examination, imaging, and other tests.
What should happen with oxygen therapy before an ABG?
If the patient receives oxygen in hospital, it may be turned off for about 20 minutes before the test only when they can breathe without it — per provider direction. Otherwise keep delivery constant for about 20 minutes before draw and document FiO₂.
What are risks of arterial blood gas sampling?
standard clinical references lists bleeding, bruising, soreness at the site, and very rarely nerve or artery injury. Step-by-step technique and post-collection monitoring are covered in the Arterial Blood Gas (ABG) Sampling nursing procedure guide; on this page, escalate uncontrolled bleeding or neurologic symptoms per facility policy.
When should nurses escalate ABG results?
Escalate severe hypoxemia, dangerous acidemia, rising PaCO₂ with altered mental status, or any result that conflicts with the bedside picture — according to facility critical-value and rapid response protocols.
How is ABG different from pulse oximetry?
Pulse oximetry estimates oxygen saturation noninvasively. it may suffice when oxygen levels alone are the concern, but ABG adds PaCO₂, pH, and bicarbonate for ventilation and acid-base assessment.
References
References
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MedlinePlus. Arterial Blood Gas (ABG) Test. U.S. National Library of Medicine.https://medlineplus.gov/lab-tests/arterial-blood-gas-abg-test/
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MedlinePlus Medical Encyclopedia. Blood gases. U.S. National Library of Medicine.https://medlineplus.gov/ency/article/003855.htm
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Merck & Co., Inc. Arterial Blood Gas (ABG) Analysis and Pulse Oximetry. Merck Manual Consumer Version.https://www.merckmanuals.com/home/lung-and-airway-disorders/diagnosis-of-and-procedures-for-lung-disorders/arterial-blood-gas-abg-analysis-and-pulse-oximetry
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Merck & Co., Inc. Overview of Acid-Base Balance. Merck Manual Consumer Version.https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/acid-base-balance/overview-of-acid-base-balance
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National Heart, Lung, and Blood Institute. How the Lungs Work. U.S. Department of Health and Human Services.https://www.nhlbi.nih.gov/health/lungs
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American Lung Association. How Lungs Work. American Lung Association.https://www.lung.org/lung-health-diseases/how-lungs-work
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Hansen JE, Aboussouan LS. Arterial Blood Gases. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.https://www.ncbi.nlm.nih.gov/books/NBK526028/
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Global Initiative for Chronic Obstructive Lung Disease. Global Strategy for the Diagnosis, Management, and Prevention of Chronic Obstructive Pulmonary Disease. GOLD.https://goldcopd.org/
Editorial Standards & Medical Review
About the author: Sid A. Abdala Balal, RN, writes evidence-based nursing education focused on diagnostic safety, clinical interpretation, and bedside nursing judgment.
Medical review: This guide is reviewed by Dr. Adam Sayedi, MD, for clinical accuracy, diagnostic safety, and alignment with current standards for Arterial Blood Gas (ABG).
Policies: Medical Review Process · Editorial Policy · Correction Policy