How to Read an ABG: A Step-by-Step Guide for Nurses

To read an arterial blood gas, work in order: check the pH, decide whether the cause looks respiratory or metabolic, check whether the body’s compensation fits, and calculate the anion gap if there is a metabolic acidosis. The steps below use the order described by the American Thoracic Society. This is education only, not a substitute for your facility’s policies or a provider’s judgment.

The steps

A blood gas reports several values. MedlinePlus lists these normal ranges:

Labs can differ, so compare against the range on your own report. Then work through the steps:

1. Acidemia or alkalemia? A pH below 7.35 is acidemia. A pH above 7.45 is alkalemia. A disorder can be present even when the pH is normal (ATS).
2. Respiratory or metabolic? In a primary respiratory disorder, pH and PaCO2 move in opposite directions. In a primary metabolic disorder, they move in the same direction (ATS).
3. Is compensation appropriate? The lungs and kidneys respond to an acid-base change. For example, MedlinePlus explains that in chronic respiratory acidosis the kidneys raise bicarbonate to help restore balance. The Merck Manual adds that compensation cannot return pH completely to normal and never overshoots. If the measured compensation does not match the expected amount, another disorder may be present (ATS).
4. Anion gap, if metabolic acidosis. Anion gap = sodium - (chloride + bicarbonate). ATS uses about 12 mEq/L as the reference point.

For metabolic acidosis, ATS gives the expected PaCO2 as (1.5 x HCO3) + 8, plus or minus 2 mmHg.

Using it in NurseTools

The ABG Analyzer is in clinical review and is not available yet. The app is coming soon to the App Store, and nothing is downloadable now. The app shows a release-gates timeline: source and rights audit, clinical editorial review, independent testing, and regulatory and App Store review. There is also a “Notify me when available” switch. Nothing unlocks a tool that has not passed review. Only BMI and MAP are planned for launch.

As planned, every tool will show its sources, version and review date. The app is designed to need no account, never ask for patient data, and work offline.

Worked example

This is a made-up practice example, not a patient. Suppose a practice gas reads:

• pH 7.28
• PaCO2 30 mmHg
• HCO3 14 mEq/L
• Sodium 140 mEq/L, chloride 108 mEq/L

Step 1. The pH of 7.28 is below 7.35, so this is acidemia.

Step 2. The pH is low and the PaCO2 is low. They move in the same direction, which points to a primary metabolic disorder. The HCO3 of 14 is below the 22 to 26 range, which fits a metabolic acidosis.

Step 3. Expected PaCO2 = (1.5 x 14) + 8 = 21 + 8 = 29 mmHg. With the plus or minus 2 margin, the expected range is 27 to 31 mmHg. The measured 30 mmHg falls inside it, so the respiratory compensation looks appropriate and there is no sign of a second disorder from this check.

Step 4. Anion gap = 140 - (108 + 14) = 140 - 122 = 18 mEq/L. That is above the reference point of about 12, so this practice gas shows a metabolic acidosis with an elevated anion gap.

In words: metabolic acidosis with appropriate respiratory compensation and a high anion gap. What the cause is, and what to do about it, belongs to the provider and your facility’s protocols.

Limits and common pitfalls

• A normal pH does not rule out a disorder. Two opposing disorders can leave the pH near normal (ATS).
• Reference ranges and cutoffs differ between sources and labs. The Merck Manual uses slightly different thresholds than MedlinePlus, so use the ranges your lab reports.
• Acute and chronic changes compensate differently. MedlinePlus notes that in acute respiratory acidosis, CO2 rises faster than the kidneys can compensate.
• The anion gap formula depends on the reference point in use. ATS notes the gap falls about 2.5 mEq/L for each 1 g/dL drop in albumin, so albumin matters when you interpret it.
• A gas is a snapshot. Interpret it with the clinical picture, the oxygen being delivered, and your facility’s policy.