ABG Interpretation for UK Student Nurses: A Calm Way to Read the Results
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An arterial blood gas can look like a tiny sheet of numbers designed to ruin your shift. It is not. Read it in the same order every time and it becomes easier to interpret.
The important bit is that an ABG never sits on its own. The result has to match the patient in front of you, their oxygen prescription, the sample type and the clinical reason it was taken.
Before you start: look at the patient
If the patient looks acutely unwell, use an Airway, Breathing, Circulation, Disability and Exposure assessment and call for help. Do not wait until you have solved every number.
Then check:
- Was this definitely an arterial sample?
- What oxygen device and flow rate or fraction of inspired oxygen was the patient receiving?
- Was the sample analysed promptly and in line with local procedure?
- What has changed in the patient's respiratory rate, oxygen saturation, conscious level, circulation and work of breathing?
Write down the oxygen details with the result. A partial pressure of oxygen without that context tells only part of the story.
The numbers you will usually meet
Always use the reference interval printed by your laboratory. Analysers and local laboratories differ, and oxygen values also vary with age and clinical context. The ranges below are common adult examples, not universal cut-offs.
| Measure | Common adult reference range | What it helps you assess |
|---|---|---|
| pH | 7.35 to 7.45 | Whether the blood is acidotic or alkalotic |
| PaCO2 | About 4.3 to 6.4 kPa | The respiratory contribution |
| HCO3− | About 22 to 29 mmol/L | The metabolic contribution |
| PaO2 | Often about 10 to 14 kPa on room air | Oxygenation, interpreted with age and oxygen therapy |
| Base excess | Often around −2 to +2 mmol/L | Another clue to the metabolic component |
| Lactate | Often below 2 mmol/L | A marker that may rise with impaired perfusion or other illness |
Your local result may use a slightly different range. That is normal. Use the range on the report and the local escalation policy.
A calm order for reading an ABG
1. Start with the pH
- Below 7.35 suggests acidaemia.
- Above 7.45 suggests alkalaemia.
- A pH within range does not prove the ABG is normal. Compensation or a mixed disorder may have pulled it back towards the middle.
2. Compare PaCO2 and bicarbonate
Carbon dioxide behaves like an acid. A raised PaCO2 pushes the pH down, while a low PaCO2 pushes it up.
Bicarbonate behaves like a base. A low bicarbonate pushes the pH down, while a high bicarbonate pushes it up.
Ask which value moves in the direction that best explains the pH:
- Low pH with raised PaCO2: a respiratory acidosis pattern.
- Low pH with low bicarbonate: a metabolic acidosis pattern.
- High pH with low PaCO2: a respiratory alkalosis pattern.
- High pH with raised bicarbonate: a metabolic alkalosis pattern.
These are patterns, not complete diagnoses. The cause still comes from the clinical picture.
3. Look for compensation, then stay alert for mixed problems
The lungs and kidneys try to limit changes in pH. In a respiratory problem, bicarbonate may shift over time. In a metabolic problem, ventilation may change PaCO2.
Compensation usually moves the pH towards normal. It does not make the underlying problem disappear. If PaCO2 and bicarbonate are both abnormal, or the numbers do not fit neatly, ask a registered clinician to review the result. Do not force it into a single label.
4. Check oxygenation and lactate
Interpret PaO2 alongside the patient's oxygen device, target saturation and clinical state. For many acutely ill adults, the British Thoracic Society recommends a target oxygen saturation of 94% to 98%. A target of 88% to 92% is commonly used for people at risk of hypercapnic respiratory failure while blood gas results are awaited. Follow the patient's prescribed target and local policy.
A raised lactate needs clinical attention, but it is not a diagnosis by itself. Look at the trend, perfusion, medicines, seizures, liver function and the wider assessment. Escalate unexpected or rising results.
Three quick examples
Example one: respiratory acidosis pattern
pH 7.28, PaCO2 8.1 kPa, bicarbonate 25 mmol/L.
The pH is low and the raised carbon dioxide points towards a respiratory cause. Now look at the patient: reduced ventilation, fatigue, airway obstruction, medicines and chronic respiratory disease are some possibilities for the clinical team to consider.
Example two: metabolic acidosis pattern
pH 7.24, PaCO2 3.5 kPa, bicarbonate 15 mmol/L.
The low bicarbonate fits the low pH. The low PaCO2 may reflect respiratory compensation. The cause could include several urgent conditions, so this needs prompt clinical review rather than a guess from the numbers alone.
Example three: a normal pH that deserves attention
pH 7.39, PaCO2 7.2 kPa, bicarbonate 32 mmol/L.
The pH is in range, but PaCO2 and bicarbonate are not. This could reflect compensation, a mixed picture or a chronic process. Normal pH is not permission to ignore the rest of the result.
What should make you escalate promptly?
- A patient who is tiring, confused, drowsy or working hard to breathe
- A pH that is markedly abnormal or moving in the wrong direction
- Unexpected carbon dioxide retention
- Low oxygenation despite oxygen therapy
- A raised or rising lactate with clinical deterioration
- A result that does not fit the patient's condition or may be affected by a sampling problem
As a student nurse, your job is not to independently diagnose and manage an acid-base disorder. Your job is to recognise concern, link the result to the patient, communicate clearly and stay within your competence.
A simple handover sentence
Try: “The ABG was taken at 14:10 while the patient was on 2 litres per minute via nasal cannula. The pH is 7.29, PaCO2 is 7.4 kPa and they are becoming drowsier with a respiratory rate of 10. I am concerned about respiratory acidosis and need an urgent review.”
That gives the team the result, the oxygen context, the trend and the reason you are worried.
Keep this in your pocket
- Look at the patient.
- Confirm the sample and oxygen details.
- Read pH first.
- Compare PaCO2 with bicarbonate.
- Check oxygenation, lactate and the trend.
- Escalate the patient, not just the numbers.
Sources
- NHS: Electrolyte test and arterial pH range
- Royal United Hospitals Bath: blood gas reference ranges and sample handling
- British Thoracic Society: oxygen use in healthcare and emergency settings
- Resuscitation Council UK: Quick Reference Handbook
Clinical practice and reference intervals vary. Follow the patient's plan, your local policy and the direction of the registered clinician supervising you.