Arterial blood gas interpretation: practice problems with step-by-step solutions

Also called: ABG · blood gas analysis · acid–base balance

Free Arterial blood gas interpretation practice problems with diagrams, hints and full worked solutions. New values every time, at three levels: acid–base classification, compensation and expected values, mixed disorders and gas exchange.

Free practice problemEasy
pH7.357.457.58PaCO₂mmHg354521HCO₃⁻mmol/L222619respiratorymetabolicacidosisalkalosis

A practice arterial blood gas shows pH 7.58, PaCO₂ 21 mmHg (2.8 kPa) and HCO₃⁻ 19 mmol/L. Which primary disorder is it? Click the cell in the grid.

pH 7.58PaCO₂ 21 mmHg (2.8 kPa)HCO₃⁻ 19 mmol/L
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What you practise

  1. Easyacid–base classification
  2. Mediumcompensation and expected values
  3. Hardmixed disorders and gas exchange

How to solve Arterial blood gas interpretation problems

  1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
  3. Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
  4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
  5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.

Common mistakes

  • Starting with PaCO₂ or HCO₃⁻ instead of the pH.
  • Calling a normal pH normal when both other values are abnormal.
  • Using Winter’s formula outside metabolic acidosis.
  • Including potassium in the anion gap when the formula leaves it out.
  • Using FiO₂ as a percentage in the P/F ratio.

Key formulas

Normal ranges
pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L
ROME
Respiratory: PaCO₂ opposite to pH; Metabolic: HCO₃⁻ equal to pH
Winter’s formula
PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2
Metabolic alkalosis
PaCO₂ ≈ 0.7·HCO₃⁻ + 21
Anion gap
AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12
Albumin-corrected gap
AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)
Delta ratio
(AG − 12)/(24 − HCO₃⁻)
P/F ratio
PaO₂/FiO₂
Alveolar gas equation
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8
Serum osmolality
2·Na⁺ + glucose + urea (2·Na⁺ + glucose/18 + BUN/2.8)

Learn Arterial blood gas interpretation from scratch

Before this: Nursing fundamentals

1What a blood gas tells you

An arterial blood gas measures how acidic the blood is (pH), the carbon dioxide pressure (PaCO₂), the bicarbonate (HCO₃⁻) and the oxygen pressure (PaO₂). Exam questions use these practice ranges:

  • pH 7.35–7.45.
  • PaCO₂ 35–45 mmHg (4.7–6.0 kPa). CO₂ behaves like an acid and is controlled by the lungs.
  • HCO₃⁻ 22–26 mmol/L. Bicarbonate is a base and is controlled by the kidneys.

A pH below 7.35 is acidaemia, above 7.45 alkalaemia. Laboratories use slightly different ranges; follow the ones your course gives.

Self-check Which value reflects the respiratory side of acid–base balance?
  1. HCO₃⁻
  2. PaCO₂
  3. Sodium
  4. pH

Answer: PaCO₂. The lungs control CO₂, so PaCO₂ is the respiratory value. Bicarbonate is the metabolic value, regulated by the kidneys, and the pH is the result of both.

2ROME: finding the primary disorder

Respiratory Opposite, Metabolic Equal:

  • In a respiratory disorder PaCO₂ moves opposite to the pH: high PaCO₂ with a low pH is respiratory acidosis.
  • In a metabolic disorder HCO₃⁻ moves in the same direction as the pH: low HCO₃⁻ with a low pH is metabolic acidosis.
  1. Look at the pH: acidosis or alkalosis?
  2. Find the value that explains this pH: PaCO₂ (respiratory) or HCO₃⁻ (metabolic).
  3. Look at the other value: it shows whether there is compensation.
pH7.357.457.29PaCO₂354558HCO₃⁻222627

In the figure the pH is low and PaCO₂ is high: a respiratory acidosis. HCO₃⁻ is above its range, so the kidneys are already compensating.

Self-check pH 7.54, PaCO₂ 40 mmHg, HCO₃⁻ 33 mmol/L. What is the primary disorder?
  1. Metabolic acidosis
  2. Metabolic alkalosis
  3. Respiratory acidosis
  4. Respiratory alkalosis

Answer: Metabolic alkalosis. The pH is high and HCO₃⁻ is high too: same direction, so metabolic. PaCO₂ is normal, so it is uncompensated.

3Compensation

The other system tries to bring the pH back: the kidneys change HCO₃⁻ in a respiratory disorder, and the lungs change PaCO₂ in a metabolic disorder. Compensation moves the other value in the same direction as the primary value.

  • Uncompensated: the pH is abnormal and the other value is still normal.
  • Partially compensated: both PaCO₂ and HCO₃⁻ are abnormal, but the pH is still outside 7.35–7.45.
  • Fully compensated: the pH is back within 7.35–7.45 while both values are abnormal. The side of 7.40 on which the pH lies shows the primary disorder. Careful: a normal pH with two abnormal values can also be a mixed disorder (two primary problems at once). The expected-compensation rules in the next part tell the two apart.

Compensation rarely brings the pH all the way back. Exams test the rule above; real reports are interpreted by the clinical team.

Self-check pH 7.36, PaCO₂ 58 mmHg, HCO₃⁻ 32 mmol/L. How is this classified?
  1. Fully compensated metabolic alkalosis
  2. Fully compensated respiratory acidosis
  3. Partially compensated respiratory acidosis
  4. Uncompensated respiratory acidosis

Answer: Fully compensated respiratory acidosis. The pH is normal but on the acid side of 7.40, both values are abnormal, and the high PaCO₂ explains acidosis: a fully compensated respiratory acidosis. A metabolic alkalosis would need a pH above 7.40.

4Expected compensation and the anion gap

Winter’s formula predicts the PaCO₂ in a metabolic acidosis:

expected PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg

A measured PaCO₂ above that range means an added respiratory acidosis; below it, an added respiratory alkalosis.

The anion gap separates two kinds of metabolic acidosis:

AG = Na⁺ − (Cl⁻ + HCO₃⁻), about 8–12 mmol/L

A high gap means unmeasured acids, as in ketoacidosis or lactic acidosis; a normal gap with a high chloride fits bicarbonate loss. Low albumin hides a high gap: add 2.5 for every g/dL below 4.0 (0.25 per g/L below 40).

The delta ratio (AG − 12)/(24 − HCO₃⁻) checks for a second disorder: below 1 an added normal-gap acidosis, 1 to 2 a pure high-gap acidosis, above 2 an added metabolic alkalosis.

Worked example

Practice values: pH 7.17, PaCO₂ 34 mmHg, HCO₃⁻ 12 mmol/L, Na⁺ 138 and Cl⁻ 100 mmol/L. Interpret them.

1

Primary disorder

The pH is low and HCO₃⁻ is low: same direction, so a metabolic acidosis.

2

Winter’s formula

1.5·12 + 8 = 26 mmHg, range 24–28 mmHg

The measured 34 mmHg is above the range: the lungs are not compensating enough, an added respiratory acidosis.

3

Anion gap

AG = 138 − (100 + 12) = 26 mmol/L

A high anion gap.

4

Delta ratio

(26 − 12)/(24 − 12) = 14/12 = 1.17

Between 1 and 2: no third disorder.

Answer

A high anion gap metabolic acidosis with an added respiratory acidosis.

5Oxygenation and other derived values

  • The P/F ratio is PaO₂ divided by FiO₂ as a fraction: 80 mmHg on 40% oxygen gives 80/0.4 = 200 mmHg. Values below about 300 mmHg show impaired oxygenation.
  • The alveolar oxygen pressure at sea level is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8, and the A–a gradient is PAO₂ − PaO₂.
  • Calculated serum osmolality is 2·Na⁺ + glucose + urea (all in mmol/L), or 2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL. Normal is about 275–295 mOsm/kg.
Self-check A practice gas shows PaO₂ 90 mmHg on 30% oxygen. What is the P/F ratio?
  1. 120 mmHg
  2. 27 mmHg
  3. 3 mmHg
  4. 300 mmHg

Answer: 300 mmHg. FiO₂ is used as a fraction: 90/0.30 = 300 mmHg. Dividing by 30 gives 3, and 27 is 90 times 0.3.

6Common mistakes

  • Starting with PaCO₂ or HCO₃⁻ instead of the pH.
  • Calling a normal pH “no disorder” when both PaCO₂ and HCO₃⁻ are abnormal: it is full compensation or a mixed disorder. Check the expected compensation, for example with Winter’s formula.
  • Using Winter’s formula for a metabolic alkalosis or a respiratory disorder.
  • Adding potassium to the anion gap when the course formula leaves it out.
  • Using FiO₂ as a percentage in the P/F ratio.

7How exam questions look

  • Easy: acid–base classification. You click the primary disorder in a grid, calculate the anion gap, or work out the P/F ratio.
  • Medium: compensation and expected values. You decide whether a gas is uncompensated, partially or fully compensated, apply Winter’s formula and the rules for respiratory compensation, and calculate serum osmolality.
  • Hard: mixed disorders and gas exchange. You compare a measured PaCO₂ with Winter’s formula, use the delta ratio, tell acute from chronic respiratory acidosis, correct the anion gap for albumin, and calculate the A–a gradient.

Learn it step by step in the appLearn it step by step in the app

Worked examples

Example 1Easy
pH7.357.457.27PaCO₂mmHg354525HCO₃⁻mmol/L222611respiratorymetabolicacidosisalkalosis

A practice arterial blood gas shows pH 7.27, PaCO₂ 25 mmHg (3.3 kPa) and HCO₃⁻ 11 mmol/L. Which primary disorder is it? Click the cell in the grid.

pH 7.27PaCO₂ 25 mmHg (3.3 kPa)HCO₃⁻ 11 mmol/L
  1. 1respiratory acidosis
  2. 2metabolic acidosis
  3. 3respiratory alkalosis
  4. 4metabolic alkalosis
Show hints
  1. Start with the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Then ask which value explains that pH: PaCO₂ acts like an acid, HCO₃⁻ like a base.
  3. HCO₃⁻ is low and moves with the pH: a metabolic cause.
Show the answer and explanation

Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.

ROME, value by value

pH 7.27: acidaemia (below 7.35)
PaCO₂ 25 mmHg: low (35–45)
HCO₃⁻ 11 mmol/L: low (22–26)

Primary disorder

metabolic acidosis

Answer

metabolic acidosis

The other value is already moving to compensate, but the pH is still outside 7.35–7.45: partially compensated.

Example 2Medium

Practice values: Na⁺ 151 mmol/L, glucose 17.6 mmol/L and urea 22.7 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.

Na⁺ 151 mmol/Lglucose 17.6 mmol/Lurea 22.7 mmol/L

Find Osm (mOsm/kg)

Show hints
  1. Sodium and its anions make most of the osmolality: 2·Na⁺.
  2. Add glucose and urea as they are; all values are in mmol/L.
  3. 2·151 = 302.
Show the answer and the step-by-step solution

Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.

Calculated osmolality

Osm = 2·151 + 17.6 + 22.7 = 342.3 ≈ 342 mOsm/kg

Answer

Osm = 342 mOsm/kg
Example 3Hard

Practice values: Na⁺ 138, Cl⁻ 106, HCO₃⁻ 18 mmol/L and albumin 20 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).

Na⁺ 138 mmol/LCl⁻ 106 mmol/LHCO₃⁻ 18 mmol/Lalbumin 20 g/L

Find AGc (mmol/L)

Show hints
  1. AG = 138 − (106 + 18) = 14.
  2. Albumin is 20 g/L below 40.
  3. 0.25·20 = 5.
Show the answer and the step-by-step solution

Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.

Anion gap

AG = 138 − (106 + 18) = 14

Correction for albumin

14 + 0.25·(40 − 20) = 19 mmol/L

Answer

AGc = 19 mmol/L

Above about 12 after correction: a high anion gap.

Questions and answers

What is Arterial blood gas interpretation?

An arterial blood gas measures how acidic the blood is (pH), the carbon dioxide pressure (PaCO₂), the bicarbonate (HCO₃⁻) and the oxygen pressure (PaO₂). Exam questions use these practice ranges:

A pH below 7.35 is acidaemia, above 7.45 alkalaemia. Laboratories use slightly different ranges; follow the ones your course gives.

How do I solve Arterial blood gas interpretation problems step by step?
  1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
  3. Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
  4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
  5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.
What are common mistakes in Arterial blood gas interpretation?
  • Starting with PaCO₂ or HCO₃⁻ instead of the pH.
  • Calling a normal pH normal when both other values are abnormal.
  • Using Winter’s formula outside metabolic acidosis.
  • Including potassium in the anion gap when the formula leaves it out.
  • Using FiO₂ as a percentage in the P/F ratio.
What do the Arterial blood gas interpretation exercises cover?
  • Easy: acid–base classification
  • Medium: compensation and expected values
  • Hard: mixed disorders and gas exchange

Every problem has new values, so you learn the method instead of an answer. Stuck? Open a small hint first, and the full explanation only when you need it.

How practising works

  • Every problem has new values, so you learn the method instead of an answer.
  • Stuck? Open a small hint first, and the full explanation only when you need it.
  • Your progress, mistakes and saved problems are kept, so you can review before the exam.

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Oxygen therapy & respiratory calculations

Calculators

Anion gap calculator (albumin-corrected, delta ratio)

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Printable worksheet

Arterial blood gas interpretation worksheet with answers
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