Blood gas testing is a critical diagnostic tool used to assess a patient’s oxygenation, ventilation, acid-base balance, and metabolic status. It plays an essential role in urgent clinical settings such as Emergency Departments (A&E), Intensive Care Units (ICU), Theatres, and neonatal units, where immediate decision-making is vital.

Traditional blood gas analysis in central laboratories often introduces delays due to specimen transport, queueing, and processing time. Point-of-Care Testing (POCT) overcomes these limitations by providing real-time, bedside measurements—improving response time and supporting early intervention in deteriorating patients.

In the NHS, POCT blood gas analysers are now an integral part of critical care pathways, supporting clinicians with fast, reliable data to guide oxygen therapy, ventilation strategies, and metabolic resuscitation.

🔗 Table of Contents

• 1. Introduction
• 2. Detailed Blood Gas Analytes
• 3. Stepwise Interpretation of Acid-Base Disorders
• 4. Diagnostic Algorithm
• 5. NHS-Used POCT Devices for Blood Gas
• 6. Clinical Use Cases in the NHS
• 7. Governance and Accreditation Requirements
• 8. Anion Gap Calculator
• 9. Clinical Case Examples
• 10. Explore More POCT Tests
• 11. Closing Section

Detailed Blood Gas Analytes

This table summarises key blood gas parameters frequently tested in POCT, including their clinical relevance and adult arterial reference ranges typically used across NHS trusts.

Stepwise Interpretation of Acid-Base Disorders

A structured framework helps clinicians and POCT users interpret ABG results with greater confidence. Follow this 4-step method to identify the primary disturbance and assess for compensation or mixed disorders:

1. Assess pH: Determine whether the blood is acidotic (pH < 7.35) or alkalotic (pH > 7.45).
2. Identify the Primary Disturbance: Is the pH change driven by respiratory (pCO₂) or metabolic (HCO₃⁻) abnormality?
3. Check for Compensation: Has the opposing system responded appropriately? Use expected compensation formulas (e.g. Winter’s formula for metabolic acidosis, or expected HCO₃⁻ rise in chronic respiratory acidosis).
4. Calculate Anion Gap (if metabolic acidosis): AG = Na⁺ – (Cl⁻ + HCO₃⁻). An elevated AG suggests unmeasured acids (e.g. lactate, ketones, toxins).

💡 Common Patterns and Examples:

• Acute Respiratory Acidosis: pH ↓, pCO₂ ↑, HCO₃⁻ normal — e.g. opiate overdose with hypoventilation
• Metabolic Acidosis with Compensation: pH ↓, HCO₃⁻ ↓, pCO₂ ↓ — e.g. diabetic ketoacidosis (with Kussmaul respiration)
• Mixed Acidosis: pH ↓↓↓, pCO₂ ↑, HCO₃⁻ ↓ — e.g. cardiac arrest, multi-organ failure
• Metabolic Alkalosis: pH ↑, HCO₃⁻ ↑, pCO₂ ↑ — e.g. post-vomiting with hypokalaemia

🚨 Red Flag Patterns (Require Urgent Review):

• Lactate > 4 mmol/L with pH < 7.2: Suggests severe lactic acidosis (e.g. sepsis, ischaemia)
• pCO₂ > 8 kPa with drowsiness: Hypercapnic respiratory failure — consider non-invasive or mechanical ventilation
• K⁺ > 6.5 mmol/L with ECG changes: Severe hyperkalaemia — immediate treatment needed
• COHb > 10%: Carbon monoxide poisoning — requires 100% O₂ or hyperbaric therapy
• MetHb > 5%: Methylene blue may be required for symptomatic methaemoglobinaemia

Note: Venous samples may be used in emergencies, but arterial sampling is preferred for accurate pO₂ and acid-base status. Always verify sample source when interpreting.

Diagnostic Algorithm for Blood Gas Interpretation

This stepwise algorithm helps clinicians identify and interpret acid-base disorders based on ABG results:

1. Step 1: Check the pH
   
   • pH < 7.35 → Acidaemia
   • pH > 7.45 → Alkalaemia
   • pH 7.35–7.45 → May still be mixed disorder if pCO₂/HCO₃⁻ are abnormal
2. Step 2: Determine the Primary Disorder
   
   • Respiratory: pCO₂ moves opposite to pH
   • Metabolic: HCO₃⁻ moves in same direction as pH
3. Step 3: Assess Compensation
   
   • Is the opposing system compensating appropriately?
   • Check expected compensation values (e.g. Winter’s formula for metabolic acidosis)
4. Step 4: Calculate the Anion Gap (if acidosis)
   
   • AG = Na⁺ – (Cl⁻ + HCO₃⁻)
   • Normal AG: 8–16 mmol/L
   • High AG → Unmeasured acids (lactate, ketones, toxins)
5. Step 5: Look for Mixed Disorders
   
   • Normal pH with abnormal pCO₂ and HCO₃⁻ suggests mixed
   • Metabolic acidosis + respiratory acidosis often seen in critical illness

Quick Reference:

• Metabolic Acidosis: ↓ pH, ↓ HCO₃⁻
• Respiratory Acidosis: ↓ pH, ↑ pCO₂
• Metabolic Alkalosis: ↑ pH, ↑ HCO₃⁻
• Respiratory Alkalosis: ↑ pH, ↓ pCO₂

Use caution when interpreting venous samples — arterial is preferred for accurate oxygenation assessment.

NHS-Used POCT Devices for Blood Gas

These are the main blood gas platforms deployed across NHS hospitals for rapid, reliable testing in ICU, A&E, theatres, and neonatal units. Each analyser is validated for near-patient use, has UK support, and meets ISO 15189:2022 traceability requirements.

• Radiometer ABL90 FLEX PLUS: Bench-top analyser delivering 19 parameters in ~35 seconds from a 65 μL sample. Features onboard AQM (Automatic Quality Management) for continuous QC and calibration. Widely adopted in major NHS trusts. Connects via AQURE middleware.
• Abbott i-STAT Alinity: Handheld, cartridge-based device ideal for A&E, NICU, and field use. Delivers rapid results (≤120 seconds) from a 95 μL capillary or arterial sample. Auto-calibrates per cartridge. Integration via AlinIQ or Telcor.
• Siemens epoc® Blood Analysis System: Portable, wireless BGEM analyser using credit-card-sized test cards. Results in under 1 minute. Cards stable at room temperature. Integrates with RAPIDComm middleware or LIS systems.
• Werfen GEM Premier 5000: Used in high-throughput or specialist critical care units. Sample volume ~150 μL. Results in ~45 seconds. Intelligent Quality Management (iQM2) ensures auto error detection and lockout. Integrates with GEMweb Plus or Telcor.

🔌 Connectivity & Data Management

All devices support secure NHS network integration with middleware platforms (AQURE, Telcor, RAPIDComm) to ensure:

• Real-time result upload to EPR/LIMS
• User traceability via barcode

  Clinical Use Cases in the NHS

  Blood gas testing is widely used across the NHS in a variety of urgent care pathways. Point-of-Care systems enable clinicians to make faster, more informed decisions—often within minutes of sample collection.

  • Intensive Care Units (ICU): Continuous assessment of oxygenation and acid-base status for ventilated patients, patients with multi-organ failure, and those undergoing weaning.
  • Accident & Emergency (A&E): Rapid triage in acutely unwell patients—e.g. sepsis (lactate), DKA (pH, HCO₃⁻), COPD exacerbation (pCO₂, SaO₂).
  • Theatres & Recovery: Intraoperative monitoring during complex surgeries (e.g. cardiac, transplant), helping maintain ventilation and fluid balance.
  • Neonatal Intensive Care (NICU/SCBU): Low-volume sampling for fragile neonates. Blood gases help guide respiratory support and detect metabolic instability.
  • Ambulance & Pre-Hospital Teams: Paramedics and air ambulance staff increasingly use handheld POCT analyzers (i-STAT, epoc) to identify shock, acidosis, or respiratory failure in the field.

  🧪 Pre-Analytical Considerations

  Accuracy depends heavily on sample handling. NHS best practice includes:

  • Use of pre-heparinised syringes (avoid dry heparin capillaries)
  • Expelling all air bubbles immediately to avoid oxygen diffusion errors
  • Mixing the sample gently (10–15 inversions) before analysis
  • Testing within 5–10 minutes or storing on ice briefly if delayed
  • Clearly labelling arterial vs venous samples to avoid misinterpretation

  🧬 Haemolysis in POCT Blood Gas Samples

  Haemolysis can significantly distort results on blood gas analysers and must be considered before acting on abnormal findings.

  • What causes haemolysis?
    – Excessive syringe suction during collection
    – Using small gauge needles or forcing blood into the analyser
    – Poor handling (e.g. vigorous shaking, delay in testing)
  • How to identify haemolysis?
    – Elevated potassium (K⁺ > 6.5 mmol/L) without clinical correlation
    – Low calcium (Ca²⁺) or falsely low haemoglobin
    – Discrepancy between central lab and POCT results
    – Some devices may trigger “haemolysis suspected” flags based on conductivity or interference curves
  • Impact:
    Haemolysed samples can mislead clinical management—especially when acting on hyperkalaemia, acidosis, or haemoglobin-related decisions. If suspected, repeat sampling is advised before treatment.

  Governance and Accreditation Requirements

  POCT blood gas services must operate under a robust governance framework to ensure accuracy, traceability, and patient safety. NHS Trusts are expected to meet the requirements set out in ISO 15189:2022 and the UKAS POCT Accreditation Scheme.

  • Internal Quality Control (IQC): All devices must run IQC at defined intervals. Results should be auto-flagged for review, and failed QC must prevent patient testing (via lockout if available).
  • External Quality Assessment (EQA): Participation in recognised EQA schemes is essential. EQA failures should trigger Root Cause Analysis (RCA), corrective action, and documentation.
  • Training and Competency: All users (e.g. nurses, doctors, ODPs) must receive formal training. Competency should be reassessed periodically and logged electronically, with expiry tracking enabled via middleware.
  • Access Control and Lockouts: Operator ID login (often via barcode or smartcard) ensures accountability. Devices should prevent use if staff are uncertified or if QC is overdue.
  • Traceability: Logs must capture who performed the test, when, on which device, using which batch of reagents and consumables. This is a core requirement under ISO 15189 clause 5.8 and 7.1.
  • Connectivity and Data Integrity: Blood gas devices should be connected to the Laboratory Information System (LIS) via secure middleware (e.g. Aqure, RAPIDComm, Telcor). This ensures result integrity, audit trails, and exception reporting.
  • Exception Handling: All critical errors (e.g. sample clotting, temperature alerts, QC failures) must be captured and reviewed regularly. This forms part of routine POCT clinical audit.

  Anion Gap Calculator (with Interpretation)

  The anion gap (AG) is a useful tool for evaluating metabolic acidosis by identifying unmeasured anions such as lactate, ketones, or toxins. Use the calculator below to determine the AG:

  Formula: AG = Na⁺ − (Cl⁻ + HCO₃⁻)

  Reference range (normal anion gap): 8–16 mmol/L (without correction for albumin)

  Na⁺ (mmol/L): Cl⁻ (mmol/L): HCO₃⁻ (mmol/L):
  
  Calculate Anion Gap

  Note: This calculator does not apply albumin correction. Always interpret within full clinical context.

  Frequently Asked Questions (FAQs)

  What’s the difference between arterial and venous blood gas (ABG vs VBG)?

  Arterial blood gas (ABG) gives the most accurate values for oxygenation and ventilation (pO₂ and pCO₂), essential in respiratory failure. Venous blood gas (VBG) is less invasive and often acceptable for acid-base status (pH, HCO₃⁻) but not for assessing oxygenation. VBGs are commonly used in A&E or ICU as a screening tool.

  How quickly should a blood gas be analysed after collection?

  Ideally within 5–10 minutes. Delays can cause changes in pH and gases due to ongoing cellular metabolism. Always expel air bubbles, mix well, and use heparinised syringes. If delayed, refrigerate the sample and clearly note time collected.

  What causes errors or rejected samples in blood gas testing?

  • Clotted samples (due to improper mixing or lack of heparin)
  • Air bubbles present in the syringe
  • Delay to analysis exceeding 15–20 minutes
  • Wrong sample type (e.g. non-heparinised syringe)
  • Insufficient sample volume

  Why might potassium results differ between blood gas and lab serum values?

  Blood gas analysers measure potassium in whole blood immediately, whereas serum K⁺ from central lab involves clotting and centrifugation. Haemolysis, sample type, or delays may cause differences. Always correlate with clinical context and other labs.

  How do I know if a blood gas sample is haemolysed?

  Visual signs (pink-red discolouration) may be absent. Some analysers (e.g. ABL90) alert for haemolysis index. If K⁺ is unexpectedly high (>6.5 mmol/L) without clinical cause, haemolysis is likely. Always compare with other clinical data.

  Can lactate levels be used to monitor sepsis?

  Yes — lactate is a critical early marker for tissue hypoxia and poor perfusion. A level >2.0 mmol/L is abnormal; >4.0 mmol/L suggests severe sepsis/shock and may warrant ICU referral. Repeat testing is recommended to monitor trends after fluid resuscitation or intervention.

  Clinical Case Examples

  🩺 Case 1: Acute Exacerbation of COPD

  ABG Findings: pH 7.28, pCO₂ 8.2 kPa, HCO₃⁻ 29 mmol/L

  Interpretation: Respiratory acidosis with metabolic compensation.

  Action: Administer controlled oxygen via Venturi mask (target SpO₂ 88–92%), initiate non-invasive ventilation (NIV) if persistent hypercapnia with acidosis.

  Guideline Reference: British Thoracic Society (BTS) COPD Guidelines

  🩸 Case 2: Diabetic Ketoacidosis (DKA)

  ABG Findings: pH 7.10, HCO₃⁻ 9 mmol/L, pCO₂ 3.2 kPa

  Interpretation: Metabolic acidosis with respiratory compensation (Kussmaul breathing).

  Action: Begin intravenous insulin and fluid resuscitation, correct potassium, monitor blood gases 2–4 hourly.

  Guideline Reference: Joint British Diabetes Societies (JBDS) DKA Management Guidelines (2021)

  🌡️ Case 3: Septic Shock with Lactic Acidosis

  ABG Findings: pH 7.19, HCO₃⁻ 13 mmol/L, pCO₂ 3.8 kPa, Lactate 6.1 mmol/L

  Interpretation: High anion gap metabolic acidosis with partial respiratory compensation.

  Action: Initiate Sepsis 6 bundle (oxygen, cultures, antibiotics, IV fluids, lactate monitoring, urine output).

  Guideline Reference: NHS England Sepsis Action Plan and UK Sepsis Trust Clinical Toolkits

  💊 Case 4: Salicylate Overdose

  ABG Findings: pH 7.42, pCO₂ 2.7 kPa, HCO₃⁻ 15 mmol/L

  Interpretation: Mixed acid-base disturbance (respiratory alkalosis + metabolic acidosis).

  Action: Admit for toxicology monitoring, give activated charcoal, monitor salicylate levels and renal function; consider haemodialysis if level >700 mg/L or with acidosis.

  Guideline Reference: TOXBASE Guidance (NHS National Poisons Information Service) and BNF – Aspirin Overdose Management

  

  Diagnostics. Supported.

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