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Clinical Confidence • Student Nurse Guide

Common ABG Interpretation Mistakes for Student Nurses

Learn the common errors that can make arterial blood gas interpretation confusing and build a safer, more systematic approach to ABG results.

Key principle: most ABG mistakes happen when the result is interpreted as isolated numbers rather than as a physiological pattern connected to the patient.
Your safety routine

Keep the interpretation sequence simple

A consistent sequence reduces cognitive overload and makes it much less likely that you will miss an important abnormality.

1

Patient

Know why the ABG was taken and assess the patient first.

2

pH

Identify the overall acidic or alkaline direction.

3

PaCO₂

Assess the respiratory component and ventilation.

4

HCO₃⁻

Assess the metabolic component.

5

PaO₂

Assess oxygenation together with oxygen therapy and SpO₂.

6

Patient again

Check whether the ABG pattern fits the clinical picture.

Patient → pH → PaCO₂ → HCO₃⁻ → PaO₂ → patient

Use the same order each time until the process becomes automatic.

Mistake 1

Starting with PaCO₂ instead of pH

“The PaCO₂ is high, so this must be respiratory acidosis.”

A raised PaCO₂ can contribute to respiratory acidosis, but the pH tells you whether the overall blood gas is actually acidic, alkaline or closer to the reference range.

Raised PaCO₂ may also occur in a compensated respiratory disturbance or as part of compensation for metabolic alkalosis.

Better approach: check pH first, then ask whether PaCO₂ explains its direction.
Mistake 2

Assuming a normal pH means a normal ABG

“The pH is normal, so there is no acid-base problem.”

Compensation can move pH towards the reference range while PaCO₂ and bicarbonate remain significantly abnormal.

Opposing mixed disorders can also produce a pH that appears less abnormal than expected.

Better approach: always inspect PaCO₂ and bicarbonate even when pH is within the laboratory reference range.
Mistake 3

Confusing oxygenation with ventilation

Oxygenation

Think PaO₂ and SpO₂

These provide information about oxygen in arterial blood and haemoglobin saturation.

Ventilation

Think PaCO₂

PaCO₂ helps show whether carbon dioxide is being removed effectively through ventilation.

Clinical assessment

Think breathing

Respiratory rate, depth, effort, consciousness and oxygen therapy remain essential.

A patient can have an important ventilation problem even when their oxygen saturation appears acceptable, particularly if supplemental oxygen is being given.
Mistake 4

Ignoring the oxygen being delivered

“The PaO₂ looks reasonable, so oxygenation is fine.”

The significance of PaO₂ depends partly on how much supplemental oxygen the patient is receiving.

A patient requiring progressively more oxygen to maintain an appropriate saturation may be deteriorating even if an individual PaO₂ result appears reassuring.

Better approach: always report the oxygen device or prescribed oxygen therapy alongside PaO₂ and SpO₂.
Mistake 5

Calling compensation a second disorder

Once a primary acid-base disturbance occurs, the other physiological system may respond in a predictable direction.

Metabolic acidosis

PaCO₂ may fall

Increased ventilation can reduce PaCO₂ and partially compensate for the metabolic acidosis.

Respiratory acidosis

HCO₃⁻ may rise

Over time, renal compensation may increase bicarbonate.

Key question

Does the change oppose the pH disturbance?

If so, compensation may be more likely than a second primary disorder.

Better approach: identify the primary disturbance first, then decide whether the second value moves in an appropriate compensatory direction.
Mistake 6

Calling a mixed disorder “compensation”

“Both PaCO₂ and bicarbonate are abnormal, so one must be compensating.”

Not necessarily. If both abnormalities are pushing pH in the same direction, they may represent two primary acid-base disorders.

For example, raised PaCO₂ and reduced bicarbonate both push pH towards acidity.

Better approach: ask whether the second abnormality reduces the primary pH disturbance or makes it worse.
Mistake 7

Ignoring the patient's clinical condition

An ABG does not replace bedside assessment. The urgency of an abnormal result depends heavily on the patient's clinical presentation.

  • Is the respiratory rate changing?
  • Is work of breathing increasing or is the patient tiring?
  • Is the oxygen requirement rising?
  • Is blood pressure falling?
  • Is peripheral perfusion worsening?
  • Is consciousness changing?
  • Is urine output falling?
  • Are there signs of infection, dehydration or another acute illness?
The patient outranks the puzzle. If someone is clearly deteriorating, assessment and escalation should not be delayed while you try to produce a perfect ABG interpretation.
Mistake 8

Ignoring trends and previous ABGs

Single result

A snapshot

One ABG tells you what was happening at a particular moment.

Previous result

Provides context

Comparing results can show whether pH, PaCO₂, bicarbonate or PaO₂ are improving or worsening.

Trend

Shows direction

A changing ABG alongside changing observations may provide strong evidence of deterioration.

Better approach: whenever possible, compare the current blood gas with previous results and the patient's clinical trajectory.
Mistake 9

Over-relying on memorised numbers

“I know the normal values, so I can interpret the ABG.”

Reference values are important, but interpretation also requires understanding the relationship between pH, PaCO₂, bicarbonate, oxygenation and the patient's condition.

Laboratory ranges can also vary slightly, so actual clinical results should be interpreted using the appropriate local reference range.

Better approach: learn the physiological directions and interpretation process, not just isolated numbers.
Mistake 10

Trying to diagnose beyond your level of competence

Student nurses should develop confidence recognising abnormal patterns and communicating concerns, but complex acid-base diagnosis requires appropriate clinical supervision and expertise.

Your role

Recognise

Identify abnormal results and recognise patterns that may indicate deterioration.

Your role

Assess

Connect the ABG with ABCDE, observations, symptoms and clinical trends.

Your role

Communicate & escalate

Clearly report abnormal findings and seek appropriate registered or medical review.

Clinical scenario

The danger of interpreting the number instead of the patient

Example

A patient with an acute respiratory illness has an oxygen saturation within their prescribed target while receiving supplemental oxygen.

They are becoming increasingly drowsy and their breathing is now shallower after a period of marked respiratory effort.

Their ABG shows a worsening respiratory acidosis with increasing PaCO₂.

Focusing only on the apparently acceptable oxygen saturation could provide false reassurance.

The important pattern is changing consciousness + respiratory fatigue + abnormal ventilation + worsening ABG.

Final checklist

Before you finish interpreting an ABG, ask yourself...

  • Have I looked at the patient first?
  • Have I checked pH before deciding the primary disorder?
  • Have I separated oxygenation from ventilation?
  • Do I know what oxygen therapy the patient is receiving?
  • Does PaCO₂ explain the pH direction?
  • Does bicarbonate explain the pH direction?
  • Could the other abnormal value represent compensation?
  • Could more than one primary disorder be present?
  • Have I compared with previous ABGs and observations?
  • Does my interpretation fit the patient's clinical condition?
Clinical Confidence Routine

Recognise → assess → communicate → escalate

Recognise

Spot the abnormal pattern

Work through the ABG systematically rather than reacting to one isolated number.

Assess

Connect it to the patient

Use ABCDE, oxygen therapy, observations and clinical trends to interpret significance.

Communicate & escalate

Do not wait for perfection

Clearly report concerning results and deterioration even if you cannot fully classify a complex ABG.

Educational resource: this NurseNet guide supports student learning and does not replace formal ABG training, laboratory reference ranges, individual clinical assessment, NEWS2 or ABCDE, prescribed oxygen targets, specialist interpretation, clinical supervision, local emergency procedures or professional judgement.
ABG Cluster

Your ABG interpretation foundation is now complete

You have progressed from individual blood-gas values through the four main acid-base patterns, compensation, mixed disorders, worked examples and common interpretation errors.

Continue Clinical Confidence →