An arterial blood gas (ABG) reports the values a clinician uses to assess acid-base status and oxygenation. This guide explains how to read an arterial blood gas in a simple, systematic order, and lists the common normal reference ranges for pH, PaCO2, HCO3, and PaO2. It is educational only and helps with recognition and interpretation, not with making a diagnosis.
This article is educational and does not replace clinical training or advice for a specific patient. All clinical decisions rest with the treating clinician.
What Is an Arterial Blood Gas?
An arterial blood gas is a report of several values that describe a patient’s acid-base balance and oxygenation. The core numbers you will read are the pH, the PaCO2 (the respiratory component), the bicarbonate or HCO3 (the metabolic component), and the PaO2 (a measure of oxygenation).
Reading an ABG well is mostly about order. Rather than scanning every number at once, you move through the values in a fixed sequence so that each one adds meaning to the last. The systematic approach is to look at the pH first to see whether there is acidemia or alkalemia, then decide whether the PaCO2 (respiratory) or the HCO3 (metabolic) explains that change (StatPearls).
Reference ranges vary by lab, so always compare a result against the ranges printed on the report you are reading.
How to Read an ABG Step by Step
Step 1: Look at the pH first
Start with the pH. The normal pH range is 7.35 to 7.45. A value under 7.35 is acidemia and a value over 7.45 is alkalemia (StatPearls). This single number tells you the direction of the acid-base disturbance before you look at anything else.
Step 2: Check the respiratory component (PaCO2)
Next, read the PaCO2, which reflects the respiratory side of acid-base balance. The PaCO2 normal range is 35 to 45 mmHg (StatPearls). After you know the pH direction, ask whether the PaCO2 could explain it (StatPearls).
Step 3: Check the metabolic component (HCO3)
Then read the bicarbonate (HCO3), which reflects the metabolic side. The HCO3 normal range is 22 to 26 mEq/L (StatPearls). Ask whether the HCO3, rather than the PaCO2, better explains the pH change (StatPearls).
Step 4: Check oxygenation (PaO2)
Finally, read the PaO2, which describes oxygenation. The PaO2 normal range is 80 to 100 mmHg (StatPearls). This value is read alongside the acid-base picture, not in place of it.
Normal ABG Reference Values
| Parameter | Normal reference range | What it reflects |
|---|---|---|
| pH | 7.35 to 7.45 | Overall acid-base balance |
| PaCO2 | 35 to 45 mmHg | Respiratory component |
| HCO3 (bicarbonate) | 22 to 26 mEq/L | Metabolic component |
| PaO2 | 80 to 100 mmHg | Oxygenation |
All four reference ranges above are from StatPearls (StatPearls). Reference ranges vary by lab, so read each result against the ranges on the report in front of you.
Common Mistakes When Reading an ABG
- Skipping the pH and jumping straight to the PaCO2 or HCO3. The recommended order starts with the pH, then moves to the respiratory or metabolic component (StatPearls).
- Confusing the respiratory driver (PaCO2) with the metabolic driver (HCO3). These describe two different components of acid-base balance (StatPearls).
- Reading a single value in isolation instead of relating it back to the pH.
- Assuming the same cutoffs apply everywhere. Reference ranges vary by lab.
When to See a Clinician
An arterial blood gas is ordered and interpreted by a clinician within the full clinical context. If an ABG shows a value outside its normal range, that result should be reviewed and explained by the treating clinician, who decides what it means for the individual patient. This article supports recognition and interpretation only, and a diagnosis is made by a clinician.
How Phở Fits In
Phở Labs publishes cited explainers like this one, where each fact links back to the source it came from so you can check it yourself. That is the whole point of this page: the normal ranges here are attributed to their reference, and where a value depends on the lab or on clinical judgment, we say so rather than inventing a number. If a claim is not backed by a cited source, we leave it out.
References
- Arterial Blood Gas. StatPearls. (StatPearls)