- An arterial blood gas (ABG) measures oxygen, carbon dioxide, and the acid-base balance (pH) of your blood.
- It's most often done in acute or hospital settings, such as the ER or ICU, to assess breathing and metabolic status.
- The sample comes from an artery (often the wrist), which differs from a routine vein draw.
- Reference ranges and units vary by lab and altitude, and are affected by any supplemental oxygen; your report's range applies.
- ABG results are interpreted together by the care team in real time, alongside your clinical condition.
An arterial blood gas (ABG) test measures how well your lungs are moving oxygen into the blood and carbon dioxide out of it, along with the blood’s acid-base balance. Unlike most blood tests, an ABG usually happens in an acute or hospital setting — such as an emergency department, intensive care unit, or during a serious breathing problem. This guide explains what the test measures and how results are generally read. It is educational only and is not a substitute for the care team looking after you.
What the test measures
According to sources such as MedlinePlus and the Mayo Clinic, an ABG typically reports several values from a small sample of arterial blood:
- pH — how acidic or basic the blood is;
- PaO2 — the partial pressure of oxygen, reflecting how much oxygen is dissolved in the blood;
- PaCO2 — the partial pressure of carbon dioxide, reflecting how effectively CO2 is being cleared;
- Bicarbonate (HCO3-) — a measure related to the metabolic (kidney) side of acid-base balance;
- Oxygen saturation — the percentage of hemoglobin carrying oxygen.
Together these values help the care team judge both respiratory function (oxygen and CO2) and metabolic balance (pH and bicarbonate).
Why it may be ordered
An ABG may be used when a clinician needs a precise, real-time picture of breathing and acid-base status, for example:
- Severe shortness of breath or a suspected lung problem;
- Monitoring someone on a ventilator or supplemental oxygen;
- Assessing a metabolic disturbance, such as in serious illness;
- Guiding urgent treatment decisions in the ER or ICU.
Because the information is often time-sensitive, ABGs are generally performed in monitored settings rather than as a routine outpatient screen.
How the sample is taken
An ABG sample comes from an artery — commonly the radial artery at the wrist — rather than a vein. This can feel different from a standard blood draw and is performed by trained staff. In some settings a small catheter (arterial line) provides repeated samples. If you have questions about the procedure, the clinicians drawing the sample can walk you through it.
Typical ranges (yours may differ)
ABG values have commonly cited ranges, but these are influenced by altitude, whether you are receiving supplemental oxygen, body temperature, and the specific analyzer used. Units and reference ranges vary between laboratories.
| Value | Commonly cited range (varies by lab and setting) |
|---|---|
| pH | about 7.35–7.45 |
| PaCO2 | about 35–45 mmHg |
| PaO2 | about 75–100 mmHg (on room air, at sea level) |
| Bicarbonate (HCO3-) | about 22–26 mEq/L |
Important: these figures are general reference points only. The range printed on your own report, and the amount of oxygen you were receiving at the time, are what determine how a value is read. Do not interpret an ABG on your own.
What abnormal results may suggest
Abnormal ABG values can point in many directions, and their meaning depends entirely on context. A low PaO2 may suggest the blood is not picking up enough oxygen; an abnormal PaCO2 may reflect how the lungs are clearing carbon dioxide; and pH together with bicarbonate helps distinguish respiratory from metabolic causes of an acid-base imbalance. Crucially, these values are read together, not one at a time, and always alongside your symptoms, vital signs, and other tests. Because ABGs are used in urgent situations, interpretation and any treatment are handled by the care team in real time — this is not a test to interpret from home. For unrelated everyday lab tests, you may find our other lab-test explainers helpful background reading.
Frequently asked questions
Is an ABG the same as the oxygen reading on my finger?
No. A fingertip pulse oximeter estimates oxygen saturation non-invasively, while an ABG measures oxygen, CO2, and pH directly from arterial blood and gives more detailed information.
Does an ABG hurt more than a regular blood test?
Because it samples an artery, some people find it more uncomfortable than a routine vein draw. Trained staff take steps to minimize this.
Why is it usually done in a hospital?
ABGs are typically ordered when precise, real-time breathing and acid-base information is needed to guide urgent care, which is why they’re common in ERs and ICUs.
Does supplemental oxygen change the results?
Yes. The amount of oxygen you are receiving affects the oxygen reading, so the care team records it alongside the result.
Can I get an ABG for a routine check-up?
It is generally not used for routine screening. Your clinician decides when the detailed information it provides is needed.
How quickly are results available?
ABGs are usually analyzed rapidly so the care team can act on them promptly.
Making sense of an ABG report
ABG reports can look intimidating because they pack several linked values onto one page. The key idea is that the numbers are read as a system, not one at a time. The care team first looks at pH to see whether the blood is too acidic or too basic, then checks PaCO2 (the respiratory side) and bicarbonate (the metabolic side) to work out what is driving the imbalance and whether the body is compensating. The oxygen values are interpreted in light of how much supplemental oxygen you were receiving, which is why that detail is always recorded. Because this reasoning happens quickly and depends on your live condition, an ABG is not something to decode from a printout at home. If you’re reviewing a past result with your clinician, helpful questions include what the pattern showed, what was driving it, and how it fit with your symptoms at the time.
This article is for general education and is not medical advice. Lab tests and their reference ranges vary by laboratory and must be interpreted by a qualified clinician in the context of your health. Do not use this page to diagnose a condition or to start, stop, or change any treatment. Talk to your doctor or a licensed clinician about your results.
Sources
- MedlinePlus — Blood gases
- Mayo Clinic — blood gas and respiratory testing overviews
- NIH / National Heart, Lung, and Blood Institute — lung health information
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