How do you interpret an ABG step by step?

Look at pH, then PaCO₂, then bicarbonate to identify the primary disturbance; assess whether compensation is present and adequate; then read oxygenation separately against the inspired oxygen concentration.

What are normal ABG values?

pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg on room air, and SaO₂ 95–100%.

How do you tell respiratory from metabolic acidosis?

In a respiratory acidosis the PaCO₂ is raised and is the cause of the low pH. In a metabolic acidosis the bicarbonate is low, and the PaCO₂ usually falls as the patient compensates by breathing more.

Can compensation return the pH fully to normal?

Compensation moves the pH toward the normal range but characteristically does not fully correct it or overshoot. A pH on the opposite side of 7.40 from the primary disturbance suggests a mixed disorder.

Clinical Tool

ABG Interpreter

Acid-base analysis · P/F ratio · A-a gradient

Acid-Base Values

pH 7.35–7.45 · PaCO₂ 35–45 · HCO₃⁻ 22–26

pHnormal 7.35–7.45

PaCO₂mmHg · normal 35–45

HCO₃⁻mEq/L · normal 22–26

Oxygenation — optional

Required for P/F ratio and A–a gradient

PaO₂mmHg · normal 80–100

FiO₂fraction · e.g. 0.40

About the ABG interpreter

An arterial blood gas answers two separate questions: what the acid–base status is, and how well the lung is oxygenating. Reading them in a fixed order stops the obvious abnormality from hiding the second one.

A repeatable order

Start with the pH: acidaemic, alkalaemic, or normal. Then look at PaCO₂ to decide whether the lung explains it, and at bicarbonate to decide whether the kidney does. Whichever moves in the direction that accounts for the pH is the primary disturbance.

Then ask whether compensation is present and whether it is adequate. Compensation moves the pH toward normal but does not overshoot it, so a pH that has crossed to the other side of 7.40 indicates a second disorder rather than vigorous compensation.

Acute or chronic

Respiratory compensation is quick; renal compensation takes a day or two. A markedly raised bicarbonate alongside a raised PaCO₂ suggests a long-standing respiratory acidosis, whereas an acute rise in PaCO₂ with a near-normal bicarbonate points at something that has just happened.

The rule of thumb is that pH falls about 0.08 for every 10 mmHg rise in PaCO₂ acutely, and only about 0.03 once renal compensation has had time to work.

Oxygenation is a separate reading

PaO₂ has to be interpreted against the inspired oxygen the patient was receiving. A PaO₂ of 90 is reassuring on room air and poor on 60% oxygen, which is what the P/F ratio and the A-a gradient exist to express.

Common questions

How do you interpret an ABG step by step?: Look at pH, then PaCO₂, then bicarbonate to identify the primary disturbance; assess whether compensation is present and adequate; then read oxygenation separately against the inspired oxygen concentration.
What are normal ABG values?: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg on room air, and SaO₂ 95–100%.
How do you tell respiratory from metabolic acidosis?: In a respiratory acidosis the PaCO₂ is raised and is the cause of the low pH. In a metabolic acidosis the bicarbonate is low, and the PaCO₂ usually falls as the patient compensates by breathing more.
Can compensation return the pH fully to normal?: Compensation moves the pH toward the normal range but characteristically does not fully correct it or overshoot. A pH on the opposite side of 7.40 from the primary disturbance suggests a mixed disorder.

Educational reference for respiratory therapy students and clinicians preparing for the NBRC examinations. It supports clinical reasoning and does not replace institutional protocol, a device's instructions for use, or the judgement of the team caring for the patient.