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How to read an arterial blood gas

Three numbers are measured and the rest are calculated. Knowing which is which is most of the skill.

William Owens, MD, Attending Intensivist Physician · Updated 2026-10-05

What the machine actually measures

A blood gas analyzer measures three things: pH, PaCO2 and PaO2. Bicarbonate, base excess, oxygen saturation and the anion gap are all computed from those three and from the electrolytes. This matters at the bedside, because a calculated bicarbonate that disagrees with the one on the chemistry panel is usually the chemistry panel being older, not the gas being wrong.

The three sit in a fixed relationship, so you cannot change one in your head without the others moving.

Step 1. Look at the pH

Acidemia below 7.35, alkalemia above 7.45. Below 7.20 or above 7.55 is an immediate problem regardless of what is causing it.

A normal pH does not mean a normal gas. It frequently means two disorders that happen to cancel.

Step 2. Name the primary disorder

Whichever one moved the pH in the direction it actually went is the primary.

Step 3. Check whether the compensation fits

If the observed compensation falls outside the expected window, there is a second primary disorder. This is the step that finds the thing you were not looking for.

Compensation never fully corrects the pH. A textbook-normal pH in a patient with an obvious disorder is a mixed picture by definition, not impressive physiology.

Step 4. Calculate the anion gap, always

Calculate it even when the pH and bicarbonate are normal. A raised gap hidden by a coexisting alkalosis is a common and consequential miss.

Above 12 is a high anion gap metabolic acidosis. The usual causes are lactate, ketones, uremia, and toxic alcohols, with salicylates and a handful of drugs behind them.

Step 5. The delta ratio, if the gap is raised

Then look at the oxygen

The acid-base work says nothing about oxygenation. Take the PaO2 against the FiO2 the patient was actually on: the P/F ratio puts ARDS at 300 or below, moderate at 200 or below, severe at 100 or below.

A widened alveolar-arterial gradient points at shunt or V/Q mismatch. A low PaO2 with a normal gradient points at hypoventilation or a low inspired oxygen, which are different problems with different fixes.

Expect mixed pictures

Pure single disorders are a teaching device. Most patients in an intensive care unit have at least two at once: a septic patient with kidney injury has a gap acidosis and an early respiratory alkalosis, and the same patient tiring hours later has a gap acidosis and a respiratory acidosis with a pH that falls off a cliff.

The five steps are worth doing in order precisely because they surface the second disorder rather than stopping at the first.