Arterial blood gas: worked cases for the station

What it is

In the OSCE and on the ward a blood gas is read in the same order every time, so nothing is missed. First the context: the patient, the inspired oxygen (FiO2) and whether the sample is truly arterial. The pH then shows acidaemia or alkalaemia. The carbon dioxide tension (PaCO2, the respiratory component) and the bicarbonate (the metabolic component) show which change explains the pH: the primary process. Each primary process brings a predictable compensation by the other system; when the measured value falls outside the expected range, a second disorder is present. In a metabolic acidosis the anion gap separates added acids (ketones, lactate, kidney failure, toxins) from loss of bicarbonate (diarrhoea, renal tubular acidosis), and the delta ratio uncovers a hidden extra disorder. Oxygenation is judged separately, against the FiO2, with the alveolar–arterial (A–a) gradient or the PaO2/FiO2 ratio. Compensation does not bring the pH back to normal in a simple disorder, so a normal pH with an abnormal PaCO2 and bicarbonate means a mixed disorder. The cases in the table follow the method so it can be practised aloud.

Reference values and concepts

• Usual arterial ranges: pH 7.35–7.45, PaCO2 35–45 mmHg, bicarbonate 22–26 mmol/L, base excess −2 to +2 mmol/L.
• PaO2 on room air about 80–100 mmHg at sea level in young adults, lower with age; lactate below 2 mmol/L.
• Metabolic alkalosis: expected PaCO2 ≈ 0.7 × HCO3 + 21 ± 2; the respiratory response is limited by hypoxaemia.
• Respiratory alkalosis: bicarbonate falls about 2 mmol/L per 10 mmHg fall in PaCO2 when acute, about 4 to 5 when chronic.
• Alveolar gas equation on room air at sea level: PAO2 ≈ 150 − PaCO2 ÷ 0.8; A–a gradient = PAO2 − PaO2, normally about age ÷ 4 + 4 mmHg.
• PaO2/FiO2 ratio: below 300 mmHg is impaired oxygenation; in ARDS, 200–300 is mild, 100–200 moderate and 100 or less severe.
• Osmolar gap = measured − calculated osmolality, normally under about 10 mOsm/kg; high with a high anion gap, think of methanol or ethylene glycol.

Reading it step by step

1. Read the context and FiO2 and check the sample is arterial — the same PaO2 means different things on room air and on oxygen.
2. Look at the pH — below 7.35 is acidaemia, above 7.45 alkalaemia; it shows which process dominates.
3. Find the primary process from PaCO2 and bicarbonate — the one moving in the direction that explains the pH is primary.
4. Check the compensation with the formula for that process — a value outside the expected range means a second disorder.
5. In any metabolic acidosis calculate the anion gap, corrected for albumin — a high gap means added acid, a normal gap bicarbonate loss.
6. If the gap is high, work out the delta ratio — it reveals a hidden normal-gap acidosis or a metabolic alkalosis.
7. If a high gap is unexplained, check lactate, ketones and the osmolar gap — toxic alcohols and salicylate are missed otherwise.
8. Judge oxygenation against FiO2 with the A–a gradient or PaO2/FiO2 — a normal gradient with a high PaCO2 means pure hypoventilation.
9. State the result in one line and link it to the patient — e.g. high anion gap metabolic acidosis, appropriately compensated, likely ketoacidosis.

Worked cases (room air; Na and Cl in mmol/L)

Patterns and common mistakes

• Reading PaO2 without the FiO2: a PaO2 of 100 mmHg on high-flow oxygen is severe impairment, not a normal result.
• Calling a mixed disorder 'compensated': a normal pH with an abnormal PaCO2 and bicarbonate is two processes.
• Forgetting to correct the anion gap for a low albumin hides an acidosis in critically ill, malnourished or cirrhotic patients.
• Salicylate poisoning classically gives a respiratory alkalosis together with a high anion gap metabolic acidosis.
• A venous gas is acceptable for pH, bicarbonate, lactate and potassium in many emergencies, but not for judging oxygenation.
• At altitude, as in Bogotá, the lower barometric pressure lowers the normal PaO2 and PaCO2: read results against local reference values.

Next step

• Life-threatening results (pH below about 7.1, rising PaCO2 with drowsiness, severe hypoxaemia, very high potassium): call a senior and treat while you reassess.
• Hypercapnic respiratory acidosis in COPD: titrate oxygen to the target saturation, consider non-invasive ventilation, and repeat the gas after each change.
• High anion gap acidosis: look for the cause at once (glucose and ketones, lactate, creatinine, osmolar gap, salicylate and paracetamol levels).
• Repeat the gas to follow the response to treatment, and consider an arterial line if many samples will be needed.
• Record the interpretation with the FiO2 and time: a one-line diagnosis plus whether the compensation is appropriate.

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Elsewhere in DurgiPedia

• Reading an arterial blood gas step by step
• Metabolic acidosis, the anion gap and compensation
• Respiratory acidosis and alkalosis
• Mixed acid–base disorders
• The alveolar gas equation and the A–a gradient
• Anion gap and delta ratio
• Serum osmolality and osmolar gap

Connections

← All clinical skills

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility (StatPearls (NCBI Bookshelf), 2026); Anion Gap and Non-Anion Gap Metabolic Acidosis (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.