Normal ABG values
Normal arterial blood gas values for an adult: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3− 22 to 26 mEq/L and PaO2 75 to 100 mmHg. [MedlinePlus]
The ranges
| Measurement | Normal | What it reflects |
|---|---|---|
| pH | 7.35 to 7.45 | Acid-base balance |
| PaCO2 | 35 to 45 mmHg | The respiratory component |
| HCO3− | 22 to 26 mEq/L | The metabolic component |
| PaO2 | 75 to 100 mmHg | Oxygenation |
| O2 saturation | 95 to 100% | Oxygenation |
The ranges are MedlinePlus’s. [MedlinePlus] It adds that your results list their own normal ranges, so on a real report use the ones printed there. The pH, PaCO2 and HCO3− ranges are the ones our practice cases and app use.
The respiratory and metabolic lines in the “what it reflects” column are the Merck Manual’s: changes in PaCO2 reflect the respiratory component, and changes in HCO3− reflect the metabolic component. [Merck Manual: acid-base disorders]
Acidemia and alkalemia
A pH below 7.35 is acidemia, and a pH above 7.45 is alkalemia. [Merck Manual: acid-base disorders] Acidosis and alkalosis are the processes behind them: acid accumulating or base being lost, and the reverse. [Merck Manual: acid-base disorders]
Where the pH sits
The Merck Manual describes the body keeping blood hydrogen ion concentration within a narrow range, with an ideal pH of 7.40. [Merck Manual: acid-base regulation] Nursing courses use 7.40 as the midpoint when they ask which side of normal a pH is on. [RegisteredNurseRN: ROME] See how to interpret an ABG.
The three numbers belong together
On an ABG the HCO3− is usually calculated, not measured: the machine measures pH and PaCO2 and uses the Henderson-Hasselbalch equation for the rest. [Merck Manual: acid-base disorders] That equation, as the Merck Manual’s calculator prints it, is pH = 6.1 + log(HCO3 / (0.03 × PaCO2)). [Merck Manual: Henderson-Hasselbach] So the three values on a real report are tied together by that equation, and a set that does not fit it is worth checking for an error. Our app flags such a set for re-checking.
This is also why acid-base balance depends on the ratio of PaCO2 to HCO3−, not on either number alone. [Merck Manual: acid-base regulation]
Sources
- MedlinePlus (National Library of Medicine), Arterial Blood Gas (ABG) Test, last updated June 26, 2025
- Merck Manual Professional Edition, Acid-Base Disorders (Lewis; peer reviewed by Braunstein), last updated April 2025
- Merck Manual Professional Edition, Acid-Base Regulation (Lewis; peer reviewed by Braunstein), last updated March 2025
- Merck Manual, Henderson-Hasselbach Equation calculator (EBMcalc)
- RegisteredNurseRN, ROME Method ABG Interpretation: Uncompensated vs. Compensated (nursing education)
Every fact on this page was checked against these sources on . noshu is not affiliated with Merck, the National Library of Medicine or any source named here, and this is a study aid, not clinical guidance: your program, your institution’s protocols and a qualified clinician decide what is done for a patient. If something here is wrong, tell us at a@noshu.net.