ABG interpretation, made simple
ABG interpretation from three numbers: pH, PaCO₂ and HCO₃⁻. Learn the normal ranges, the 20 : 1 ratio behind pH, and a three-step method for naming the primary disorder and its compensation.
Chapters (14)
Given an arterial pH, PaCO₂ and HCO₃⁻, name the primary acid–base disorder (one of four) and identify the compensatory change, using Henderson–Hasselbalch ratio logic.
The short version
- pH depends on the ratio of bicarbonate to dissolved CO₂, about 20 : 1 when normal, not on either number alone.
- Normal ranges: pH 7.35–7.45, PaCO₂ 35–45 mm Hg, HCO₃⁻ 22–26 mmol/L; below 7.35 is acidemia, above 7.45 is alkalemia.
- The number that explains the pH is the primary disorder; the other number moves the same way to compensate.
- In metabolic disorders pH and PaCO₂ move in the same direction; in respiratory disorders they move in opposite directions.
- Compensation usually stops short of a normal pH, so a normal pH with two abnormal numbers suggests a mixed disorder.
Two patients, the same pH
ABG interpretation starts with a puzzle. Two patients have arterial blood gases with exactly the same pH, 7.29. Patient one has a PaCO₂ of 30 mm Hg and an HCO₃⁻ of 14 mmol/L. Patient two has a PaCO₂ of 60 mm Hg and an HCO₃⁻ of 28 mmol/L.
Same pH, completely different problems. By the end of this lesson you can name the primary acid–base disorder from just those three numbers: pH, PaCO₂ and bicarbonate.
Normal ranges for ABG interpretation: pH, PaCO₂ and HCO₃⁻
| Value | Normal range | Meaning |
|---|---|---|
| pH | 7.35–7.45 | below 7.35 = acidemia; above 7.45 = alkalemia |
| PaCO₂ | 35–45 mm Hg | the acid side (respiratory), set by the lungs |
| HCO₃⁻ (bicarbonate) | 22–26 mmol/L | the base side (metabolic), set by the kidneys |
A pH below 7.35 is acidemia and a pH above 7.45 is alkalemia.[1][2]
Carbon dioxide forms carbonic acid in blood, so think of PaCO₂ as the acid side, set by the lungs. Bicarbonate is the base side, set by the kidneys.[3][2]
The Henderson–Hasselbalch equation and the 20 : 1 ratio
pH does not depend on either number alone. It depends on the ratio between bicarbonate and carbon dioxide.[3][4] That relationship is the Henderson–Hasselbalch equation.[1][3]
6.1 = apparent pKa of the bicarbonate–CO₂ buffer system at body temperature; 0.03 (mmol/L per mm Hg) converts PaCO₂ into dissolved CO₂. HCO₃⁻ in mmol/L.
- Typical values: HCO₃⁻ 24 mmol/L, PaCO₂ 40 mm Hg
- 0.03 × 40 = 1.2
- 24 / 1.2 = 20, so the ratio is 20 : 1
- log 20 ≈ 1.3
- 6.1 + 1.3 = 7.4
The direction follows from the equation. Raise bicarbonate, or lower carbon dioxide, and pH rises. Do the reverse, and pH falls.[1][3]
Because pH follows the ratio, a change in one number can be offset by a change in the other in the same direction. That is the idea behind compensation, and it is why two very different gases can share one pH.
The four primary acid–base disorders and compensation
Those two levers give four primary disorders. The body then fights back by moving the other number in the same direction, pulling the ratio back toward normal.[2][4]
| Disorder | Primary change | Compensation | Who compensates |
|---|---|---|---|
| Metabolic acidosis | HCO₃⁻ ↓ | PaCO₂ ↓ | Lungs |
| Metabolic alkalosis | HCO₃⁻ ↑ | PaCO₂ ↑ | Lungs |
| Respiratory acidosis | PaCO₂ ↑ (breathing too little) | HCO₃⁻ ↑ | Kidneys |
| Respiratory alkalosis | PaCO₂ ↓ (breathing too much) | HCO₃⁻ ↓ | Kidneys |
In metabolic disorders the lungs compensate, so PaCO₂ follows bicarbonate down or up. In respiratory disorders the kidneys compensate, so bicarbonate follows PaCO₂.[2]
The two systems work on different clocks: the lungs respond in minutes to hours, the kidneys over hours to days.[3]
ABG interpretation in 3 steps
- Is the pH low or high?Below 7.35 is acidemia; above 7.45 is alkalemia.
- Which number explains that pH?That number is the primary disorder: bicarbonate for a metabolic disorder, PaCO₂ for a respiratory one.
- Look at the other numberIf it moves the same way, that is compensation. If it moves the opposite way, suspect a second (mixed) disorder.
A quick check for step two: in metabolic disorders, pH and PaCO₂ move in the same direction; in respiratory disorders, they move in opposite directions.[4]
Run the practice question below through the same three steps: a high pH, a high bicarbonate that explains it, and a PaCO₂ that has moved the same way.
Back to the two patients
Both patients are acidemic, with a pH of 7.29.
In patient one, the low bicarbonate (14 mmol/L) explains it: primary metabolic acidosis, with the lungs compensating (PaCO₂ 30 mm Hg).
In patient two, the high PaCO₂ (60 mm Hg) explains it: primary respiratory acidosis, with the kidneys compensating (HCO₃⁻ 28 mmol/L).
The direction check agrees. In patient one, pH and PaCO₂ are both low: same direction, so metabolic. In patient two, pH is low and PaCO₂ is high: opposite directions, so respiratory.
- Patient one: 0.03 × 30 = 0.9; 14 / 0.9 ≈ 15.6
- Patient two: 0.03 × 60 = 1.8; 28 / 1.8 ≈ 15.6
- Both ratios are about 15.6 : 1, below the normal 20 : 1, so both give the same pH of 7.29
Practice question
Original · USMLE styleA 26-year-old woman comes to the emergency department because of persistent vomiting for 3 days. She has been unable to keep down food or fluids. Pulse is 104/min and blood pressure is 102/64 mm Hg. Mucous membranes are dry. Arterial blood gas analysis on room air shows:
| pH | 7.51 |
|---|---|
| PaCO₂ | 47 mm Hg |
| HCO₃⁻ | 36 mmol/L |
Common questions
What are the normal ABG values?
This lesson uses pH 7.35–7.45, PaCO₂ 35–45 mm Hg and HCO₃⁻ 22–26 mmol/L. Ranges vary slightly between sources and laboratories, so use your own lab's ranges.
Why is a normal blood pH 7.4?
With HCO₃⁻ 24 mmol/L and PaCO₂ 40 mm Hg, the Henderson–Hasselbalch equation gives 24 / (0.03 × 40) = 20. The log of 20 is about 1.3, and 6.1 + 1.3 = 7.4.
How do you tell a metabolic from a respiratory disorder on an ABG?
Find the number that explains the pH. A quick check: in metabolic disorders pH and PaCO₂ move in the same direction; in respiratory disorders they move in opposite directions.
Does compensation bring the pH back to normal?
Usually not. Compensation pulls pH toward normal, but usually not all the way, and it does not overshoot. A normal pH with two abnormal values should make you suspect a mixed disorder.
How fast do the lungs and kidneys compensate?
The lungs respond in minutes to hours. The kidneys take hours to days.
Transcript
Read the full narration, by chapter662 words
0:00Two patients, same pH
Two patients, two arterial blood gases, and exactly the same pH: 7.29. Patient one has a PaCO2 of 30 and an HCO3 of 14. Patient two has a PaCO2 of 60 and an HCO3 of 28. Same pH, completely different problems. By the end of this lesson, you'll name the primary acid–base disorder from just those three numbers.
0:26Normal ranges: pH, PaCO2, HCO3
First, the normal ranges: pH runs from 7.35 to 7.45. Below 7.35 is acidemia, and above 7.45 is alkalemia. Arterial PaCO2 runs from 35 to 45 mm Hg. And bicarbonate, HCO3, runs from 22 to 26 mmol/L.
0:51Acid side vs base side
Carbon dioxide forms carbonic acid in blood, so think of it as the acid side, set by the lungs. Bicarbonate is the base side, set by the kidneys. And pH doesn't depend on either number alone. It depends on the ratio between them.
1:08The Henderson–Hasselbalch equation
That's the Henderson–Hasselbalch equation: pH equals 6.1, plus the log of bicarbonate over 0.03 times PaCO2. 6.1 is the pKa of carbonic acid, and 0.03 converts PaCO2 into dissolved carbon dioxide.
1:27Worked example: why normal pH is 7.40 (20 : 1), and which way pH moves
Plug in typical values: bicarbonate 24, PaCO2 40. 0.03 times 40 is 1.2, and 24 over 1.2 is 20. The log of 20 is about 1.3, and 6.1 plus 1.3 gives 7.4. Raise bicarbonate, or lower carbon dioxide, and pH rises. The reverse, and pH falls.
1:56The four primary disorders
That gives four primary disorders. Metabolic acidosis is a primary fall in bicarbonate. Metabolic alkalosis is a primary rise. Respiratory acidosis is a primary rise in PaCO2, from breathing too little. Respiratory alkalosis is a primary fall in PaCO2, from breathing too much.
2:18Compensation: lungs vs kidneys
But the body fights back by moving the other number the same way, pulling the ratio back toward normal. In metabolic disorders, the lungs compensate, so PaCO2 follows bicarbonate down, or up. In respiratory disorders, the kidneys compensate, so bicarbonate follows PaCO2. Lungs respond in minutes to hours; kidneys take hours to days. And compensation pulls pH toward normal, but usually not all the way, and it doesn't overshoot.
2:51Reading a gas in 3 steps
So, to read a gas, step one: is pH low or high? Step two: which number explains that pH? That's the primary disorder. A quick check: in metabolic disorders, pH and PaCO2 move in the same direction, and in respiratory disorders, opposite directions. Step three: the other number, moving the same way, is compensation.
3:17Back to the two patients
Back to our patients. Both are acidemic. In patient one, the low bicarbonate explains it: primary metabolic acidosis, with the lungs compensating. In patient two, the high PaCO2 explains it: primary respiratory acidosis, with the kidneys compensating. And both work out to a ratio of about 15.5 to 1, which is why their pH is identical.
3:43The trap: normal pH with two abnormal values
One trap: a normal pH with an abnormal PaCO2 and an abnormal bicarbonate should make you suspect a mixed disorder.
3:53Practice question
Your turn. Pause if you want to work it out. A 26-year-old woman has had persistent vomiting for 3 days. Her arterial blood gas shows a pH of 7.51, a PaCO2 of 47, and an HCO3 of 36. Which best describes her acid–base status?
4:16Answer and explanation
The answer is B: primary metabolic alkalosis, with compensatory hypoventilation. She's alkalemic, and the high bicarbonate explains it. Vomiting loses stomach acid. Her high PaCO2 is the lungs compensating, not the cause. C is wrong, because a primary respiratory acidosis would push pH down, not up. A, metabolic acidosis, needs a low pH and a low bicarbonate. D, respiratory alkalosis, needs a low PaCO2. And E, an extra respiratory acidosis on top, doesn't fit, because 47 sits inside the range expected from compensation.
5:00Recap
Quick recap. One: pH depends on the ratio of bicarbonate to dissolved CO2, about 20 to 1 when normal. Two: the number that explains the pH is primary, and the other moves the same way to compensate. Three: compensation usually stops short of a normal pH, so a normal pH with two abnormal numbers suggests a mixed disorder.
5:26Next: the anion gap
Next, we take metabolic acidosis apart with the anion gap, and see why it splits the causes into two groups.
Sources
Numbers in the text, e.g. [1], point here- Habib T, Nair A, Murphy S, Saeed H, Ishaya N. Mastering blood gas interpretation: a practical guide for primary care providers. S Afr Fam Pract. 2025;67(1):6058. doi:10.4102/safp.v67i1.6058 pmc.ncbi.nlm.nih.gov/articles/PMC12067511/
- Lewis JL III. Acid-Base Disorders. MSD Manual Professional Version (reviewed Mar 2025, updated Apr 2025). msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-disorders
- Lewis JL III. Acid-Base Regulation. MSD Manual Professional Version (Mar 2025). msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-regulation
- Kaufman DA. Interpretation of Arterial Blood Gases (ABGs). American Thoracic Society. thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php
- Lewis JL III. Metabolic Alkalosis. MSD Manual Professional Version (Mar 2025). msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-alkalosis
Normal ranges vary slightly between sources and laboratories; this lesson uses pH 7.35–7.45, PaCO2 35–45 mm Hg, HCO3 22–26 mmol/L.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations (lungs, kidney, red blood cell, alveoli, stomach): Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (cropped, upscaled, recoloured). All other graphics are original.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-001.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01ABG interpretation, made simple5:35
- EP 02Anion gap explained: high vs normal gap metabolic acidosis5:25
- EP 03Winters' formula: the expected PaCO₂ in metabolic acidosis5:10
- EP 04Delta ratio: find the hidden second acid–base disorder5:09
- EP 05New bicarbonate: how the kidney excretes acid with ammonium5:23
- EP 06High anion gap metabolic acidosis causes, sorted by the anion5:54
- EP 07Osmolar gap: how toxic alcohols hide from the anion gap5:31
- EP 08Urine anion gap: telling diarrhea from RTA5:33
- EP 09Renal tubular acidosis: tell RTA types 1, 2 and 4 apart5:24
- EP 10Metabolic alkalosis: generation, maintenance and urine chloride5:30
- EP 11Vomiting metabolic alkalosis: why the urine turns acid5:36
- EP 12Bartter vs Gitelman: NKCC2, NCC and urine calcium5:21
- EP 13Respiratory acidosis compensation: acute vs chronic5:22
- EP 14Respiratory alkalosis compensation: acute vs chronic5:33
- EP 15Metabolic alkalosis compensation: the expected PaCO₂5:12
- EP 16Salicylate poisoning acid-base: a classic mixed disorder5:31
- EP 17Type A vs type B lactic acidosis: split it by mechanism5:33
- EP 18DKA anion gap explained: from high gap to hyperchloraemic acidosis5:42
- EP 19Acidosis and hyperkalemia: how pH shifts potassium5:43
- EP 20ABG interpretation in 6 steps: the acid-base algorithm5:32
- EP 21Acid-base practice cases: three ABG vignettes, one algorithm5:44