How to interpret an ABG, step by step

Step 1: the pH

A pH below 7.35 is acidemia. A pH above 7.45 is alkalemia. [Merck Manual: acid-base disorders] A pH from 7.35 to 7.45 is the normal range. [MedlinePlus]

Acidemia and acidosis are not the same word. Acidosis is the process that piles up acid or loses base; acidemia is the blood pH. [Merck Manual: acid-base disorders] Compensation moves the pH toward the normal range, so a near-normal pH can still sit on top of a disorder. [Merck Manual: acid-base disorders]

Step 2: PaCO₂ and HCO₃⁻

Changes in PaCO₂ reflect the respiratory component, and changes in HCO₃⁻ reflect the metabolic component. [Merck Manual: acid-base disorders] Normal PaCO₂ is 35 to 45 mmHg and normal HCO₃⁻ is 22 to 26 mEq/L. [MedlinePlus] Mark each one high, normal or low.

Step 3: which one is the cause (ROME)

ROME stands for Respiratory Opposite, Metabolic Equal. [RegisteredNurseRN: ROME]

• Respiratory, opposite. If PaCO₂ and pH move in opposite directions (CO₂ up and pH down, or CO₂ down and pH up), the disorder is respiratory. [RegisteredNurseRN: ROME]
• Metabolic, equal. If HCO₃⁻ and pH move the same way, both up or both down, the disorder is metabolic. [RegisteredNurseRN: ROME]

That gives four patterns: respiratory acidosis (PaCO₂ high, pH low), respiratory alkalosis (PaCO₂ low, pH high), metabolic acidosis (HCO₃⁻ low, pH low) and metabolic alkalosis (HCO₃⁻ high, pH high). [RegisteredNurseRN: ROME]

When the pH is inside the normal range, nursing courses ask which side of 7.40 it is on: below 7.40 is the acid side, above it the alkaline side, and that side tells you the primary disorder. [RegisteredNurseRN: ROME] That is a teaching convention for exam questions, and it is what our practice cases use.

Step 4: compensation

Metabolic disorders are compensated by the lungs, which change PaCO₂. Respiratory disorders are compensated by the kidneys, which change HCO₃⁻. [Merck Manual: acid-base disorders]

The ROME labels for how far that has gone:

• Uncompensated. The pH is abnormal and the other value is normal, so it is not compensating yet. [RegisteredNurseRN: ROME]
• Partially compensated. The pH is abnormal and the other value is abnormal in the opposite direction, so it is trying to compensate. [RegisteredNurseRN: ROME]
• Fully compensated. The pH is back in the normal range and the other value is abnormal in the opposite direction. [RegisteredNurseRN: ROME]

Step 5: is the compensation the right size?

The ROME labels say where the numbers sit, not whether the compensation is the size the primary disorder should produce. The Merck Manual says to check that PaCO₂ and HCO₃⁻ show the expected compensation, and if they do not, to suspect a second primary process. [Merck Manual: acid-base disorders] It also warns that in mixed disorders the values “may be deceptively normal.” [Merck Manual: acid-base disorders] The expected sizes are on compensation rules, and the most used one is Winter’s formula.

PaCO₂ and HCO₃⁻ can also both be off in the acid direction at once, PaCO₂ high and HCO₃⁻ low, or both in the alkaline direction. That is two primary disturbances, which the Merck Manual calls a mixed disorder. [Merck Manual: acid-base disorders] Our practice cases include both patterns.

What this does not cover

Oxygenation (PaO₂), the anion gap and the cause of a disorder are separate questions. The Merck Manual says to calculate the anion gap for every acid-base evaluation. [Merck Manual: acid-base disorders] Interpretation also has to consider the clinical picture. [Merck Manual: acid-base disorders]