Normal ABG values
| Value | Normal range | Low means | High means |
|---|---|---|---|
| pH | 7.35–7.45 | Acidosis | Alkalosis |
| PaCO₂ (respiratory) | 35–45 mmHg | Pushes towards alkalosis | Pushes towards acidosis |
| HCO₃ (metabolic) | 22–26 mEq/L | Pushes towards acidosis | Pushes towards alkalosis |
Remember the direction: carbon dioxide behaves like an acid, bicarbonate behaves like a base.
The four-step method
- Look at the pH. Below 7.35 is acidosis. Above 7.45 is alkalosis. Between the two, note whether it sits on the acid side (below 7.40) or the alkaline side (above 7.40).
- Look at the PaCO₂. Does it explain the pH? A high PaCO₂ explains acidosis; a low PaCO₂ explains alkalosis. If yes, the problem is respiratory.
- Look at the HCO₃. Does it explain the pH? A low HCO₃ explains acidosis; a high HCO₃ explains alkalosis. If yes, the problem is metabolic.
- Check compensation. If the other value is normal, the problem is uncompensated. If it is abnormal in the opposite direction and the pH is still abnormal, it is partially compensated. If the pH is back to normal, it is fully compensated.
ROME: Respiratory Opposite (pH and PaCO₂ move in opposite directions), Metabolic Equal (pH and HCO₃ move in the same direction). If both PaCO₂ and HCO₃ push the pH the same way, suspect a mixed disorder.
10 ABG practice questions with answers
Cover the answer, work through the four steps, then open it.
Question 1. pH 7.30, PaCO₂ 55 mmHg, HCO₃ 24 mEq/L
Show answer
Uncompensated respiratory acidosis. pH low = acidosis. PaCO₂ high explains it, so it is respiratory. HCO₃ is normal, so there is no compensation yet.
Question 2. pH 7.50, PaCO₂ 30 mmHg, HCO₃ 24 mEq/L
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Uncompensated respiratory alkalosis. pH high = alkalosis. PaCO₂ low explains it, so it is respiratory. HCO₃ is normal.
Question 3. pH 7.28, PaCO₂ 40 mmHg, HCO₃ 18 mEq/L
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Uncompensated metabolic acidosis. pH low = acidosis. PaCO₂ is normal. HCO₃ low explains the acidosis, so it is metabolic, with no respiratory compensation.
Question 4. pH 7.52, PaCO₂ 40 mmHg, HCO₃ 32 mEq/L
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Uncompensated metabolic alkalosis. pH high = alkalosis. HCO₃ high explains it. PaCO₂ is normal.
Question 5. pH 7.33, PaCO₂ 58 mmHg, HCO₃ 30 mEq/L
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Partially compensated respiratory acidosis. pH low = acidosis, explained by the high PaCO₂. HCO₃ has risen to compensate, but the pH is still below 7.35.
Question 6. pH 7.36, PaCO₂ 56 mmHg, HCO₃ 31 mEq/L
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Fully compensated respiratory acidosis. pH is normal but on the acid side of 7.40. The high PaCO₂ explains an acidosis, so it is the primary problem. The high HCO₃ is compensation, and it has brought the pH back into range.
Question 7. pH 7.32, PaCO₂ 30 mmHg, HCO₃ 16 mEq/L
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Partially compensated metabolic acidosis. pH low = acidosis, explained by the low HCO₃. The low PaCO₂ is the lungs compensating by blowing off CO₂, but the pH is still low.
Question 8. pH 7.44, PaCO₂ 28 mmHg, HCO₃ 19 mEq/L
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Fully compensated respiratory alkalosis. pH is normal but on the alkaline side of 7.40. The low PaCO₂ explains an alkalosis, so it is primary. The low HCO₃ is compensation.
Question 9. pH 7.48, PaCO₂ 48 mmHg, HCO₃ 34 mEq/L
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Partially compensated metabolic alkalosis. pH high = alkalosis, explained by the high HCO₃. The raised PaCO₂ is respiratory compensation, but the pH is still above 7.45.
Question 10. pH 7.25, PaCO₂ 60 mmHg, HCO₃ 18 mEq/L
Show answer
Combined (mixed) respiratory and metabolic acidosis. pH low. The high PaCO₂ and the low HCO₃ both push towards acidosis, so neither is compensating for the other. Both problems are present.
ECG Quiz: EKG Rhythms & ABG
Want more than ten? The ABG Quiz in ECG Quiz keeps generating new blood gases and makes you walk pH, PaCO₂, HCO₃ and compensation in order, every time.
Common causes to link with each result
| Disorder | Common causes taught in class |
|---|---|
| Respiratory acidosis | Hypoventilation: COPD exacerbation, opioid or sedative overdose, severe asthma, neuromuscular weakness |
| Respiratory alkalosis | Hyperventilation: anxiety, pain, fever, early pulmonary embolism, high altitude |
| Metabolic acidosis | Diabetic ketoacidosis, lactic acidosis, kidney failure, severe diarrhoea |
| Metabolic alkalosis | Vomiting, nasogastric suction, diuretics, excess antacid or bicarbonate intake |
The tic-tac-toe method in 30 seconds
Draw a three by three grid with the columns Acidosis, Normal and Alkalosis. Place pH, PaCO₂ and HCO₃ in the column each value falls into, remembering that a high PaCO₂ goes under acidosis and a high HCO₃ goes under alkalosis. Whichever value shares a column with the pH is the cause. If the third value sits in the opposite column, compensation is happening. It is the same logic as the four steps, just drawn out.
How much compensation should you expect?
Naming the disorder is step one. Exams then ask whether the compensation is appropriate. If the body has not compensated as much as it should, a second disorder is hiding underneath. These are the numbers most courses expect you to know.
| Primary problem | Expected response |
|---|---|
| Metabolic acidosis | Winter’s formula: expected PaCO₂ = (1.5 × HCO₃) + 8, plus or minus 2 |
| Metabolic alkalosis | PaCO₂ rises about 0.7 mmHg for every 1 mEq/L rise in HCO₃ |
| Acute respiratory acidosis | HCO₃ rises about 1 mEq/L for every 10 mmHg rise in PaCO₂ |
| Chronic respiratory acidosis | HCO₃ rises about 4 mEq/L for every 10 mmHg rise in PaCO₂ |
| Acute respiratory alkalosis | HCO₃ falls about 2 mEq/L for every 10 mmHg fall in PaCO₂ |
| Chronic respiratory alkalosis | HCO₃ falls about 4 mEq/L for every 10 mmHg fall in PaCO₂ |
Worked example. pH 7.20, PaCO₂ 34, HCO₃ 13. The low HCO₃ explains the acidosis, so it is metabolic. Winter’s formula gives (1.5 × 13) + 8 = 27.5, so you expected a PaCO₂ between 25.5 and 29.5. The real PaCO₂ is 34, which is higher than it should be. The lungs are not blowing off enough CO₂, so there is a second, respiratory acidosis on top of the metabolic one.
The acute versus chronic split is what separates a COPD patient who lives at a PaCO₂ of 60 from someone who has just stopped breathing. Chronic retainers have had days to pull bicarbonate back, so their HCO₃ is much higher for the same PaCO₂.
The anion gap, and why it changes the answer
Once you have called a metabolic acidosis, the next question is always which kind. That is what the anion gap tells you.
Anion gap = Na⁺ − (Cl⁻ + HCO₃)
Normal is roughly 8 to 12 mEq/L, though labs differ and some use 3 to 11. If albumin is low, add about 2.5 to the gap for every 1 g/dL the albumin sits below 4.
High anion gap: something acidic has been added
- Lactic acidosis — sepsis, shock, bowel ischaemia, seizures
- Ketoacidosis — diabetic, alcoholic or starvation
- Kidney failure — retained sulphate and phosphate
- Toxins — methanol, ethylene glycol, salicylate, paraldehyde
Normal anion gap: bicarbonate has been lost
- Severe diarrhoea — bicarbonate straight out of the gut
- Renal tubular acidosis
- Large volumes of normal saline — the chloride load pushes bicarbonate down
- Ureteric diversion, acetazolamide
In a normal gap acidosis the chloride rises to fill the space the bicarbonate left, which is why it is also called a hyperchloraemic acidosis.
Oxygenation: the half of the ABG people skip
pH, PaCO₂ and HCO₃ answer the acid–base question. PaO₂ and SaO₂ answer a completely separate one: is this patient getting enough oxygen?
| Value | Normal on room air | What it means |
|---|---|---|
| PaO₂ | 80–100 mmHg | Oxygen dissolved in arterial blood. Below 60 is respiratory failure. |
| SaO₂ | 95–100% | How much haemoglobin is carrying oxygen. |
| P/F ratio | above 400 | PaO₂ divided by FiO₂. Under 300 is mild, under 200 moderate and 100 or less severe ARDS. |
A PaO₂ of 90 sounds reassuring until you notice the patient is on 60% oxygen. That is a P/F ratio of 150, which is moderate to severe lung injury. Always read the PaO₂ next to the FiO₂, never on its own.
10 clinical ABG questions
The first ten questions were pure numbers. These ten come with a patient, which is how they appear in exams and on the ward. Work the four steps, check the compensation, then check the gap.
Question 11. A 19-year-old with type 1 diabetes, vomiting, deep sighing breaths, glucose 500 mg/dL. pH 7.13, PaCO₂ 22, HCO₃ 7, Na 134, Cl 96.
Show answer
High anion gap metabolic acidosis with respiratory compensation — diabetic ketoacidosis. pH low, HCO₃ low, so metabolic. Anion gap = 134 − (96 + 7) = 31, which is very high. Winter’s formula expects a PaCO₂ of (1.5 × 7) + 8 = 18.5, so 16.5 to 20.5. The actual 22 is a little above that, which is worth watching: it can mean the patient is starting to tire.
Question 12. A 68-year-old with COPD, drowsy but rousable, at home on long-term oxygen. pH 7.32, PaCO₂ 68, HCO₃ 34.
Show answer
Partially compensated chronic respiratory acidosis. The high PaCO₂ explains the acidosis. For a chronic retainer you expect HCO₃ to climb about 4 for every 10 mmHg above 40, which is 24 + 11 = 35. The measured 34 fits chronic retention rather than an acute event.
Question 13. A 24-year-old in the emergency department after a panic attack, tingling in the fingers and around the mouth. pH 7.51, PaCO₂ 27, HCO₃ 21.
Show answer
Acute respiratory alkalosis. The low PaCO₂ explains the alkalosis. For an acute drop you expect HCO₃ to fall about 2 for every 10 mmHg, which is 24 − 2.6 = 21.4. The measured 21 is exactly the acute buffering response, not true renal compensation.
Question 14. A 55-year-old vomiting for three days with gastric outlet obstruction. pH 7.53, PaCO₂ 47, HCO₃ 38, Cl 88, K 3.0.
Show answer
Partially compensated metabolic alkalosis. The high HCO₃ explains the alkalosis and the raised PaCO₂ is the lungs compensating. The low chloride and low potassium are the classic fingerprint of vomiting, and the alkalosis will not correct until both are replaced.
Question 15. A 40-year-old with four days of severe diarrhoea. pH 7.32, PaCO₂ 32, HCO₃ 16, Na 138, Cl 112.
Show answer
Normal anion gap metabolic acidosis with appropriate respiratory compensation. Anion gap = 138 − (112 + 16) = 10, which is normal. Bicarbonate was lost through the gut and chloride rose to fill the gap. Winter’s formula expects a PaCO₂ of 32, which is what you have.
Question 16. Found unresponsive beside empty opioid packets, respiratory rate 6. pH 7.20, PaCO₂ 75, HCO₃ 28.
Show answer
Acute respiratory acidosis. The very high PaCO₂ explains the acidosis. HCO₃ has risen only about 4 above baseline, which is the acute buffering response — the kidneys have had no time to act. That is what tells you this is sudden, not chronic.
Question 17. A teenager brought in after taking a bottle of aspirin, ringing in the ears, breathing fast. pH 7.46, PaCO₂ 22, HCO₃ 15, Na 140, Cl 100.
Show answer
Mixed respiratory alkalosis and high anion gap metabolic acidosis — the classic salicylate picture. Anion gap = 140 − (100 + 15) = 25, so there is a metabolic acidosis. But the pH is alkalotic, which a metabolic acidosis cannot cause. Salicylate stimulates the respiratory centre directly, producing both disorders at once.
Question 18. A 72-year-old in septic shock, mottled, lactate 8 mmol/L. pH 7.27, PaCO₂ 25, HCO₃ 11, Na 140, Cl 103.
Show answer
High anion gap metabolic acidosis with appropriate respiratory compensation — lactic acidosis. Anion gap = 140 − (103 + 11) = 26. Winter’s formula expects a PaCO₂ of (1.5 × 11) + 8 = 24.5, and the measured 25 sits inside that range, so the compensation is doing what it should.
Question 19. A 70-year-old on high-dose furosemide for heart failure. pH 7.50, PaCO₂ 44, HCO₃ 33.
Show answer
Partially compensated metabolic alkalosis. Loop diuretics drive off chloride, potassium and water, which lifts bicarbonate. The PaCO₂ has risen slightly as compensation. Expect the potassium and chloride to be low as well.
Question 20. A post-operative patient on a ventilator, set rate 20, otherwise well. pH 7.52, PaCO₂ 28, HCO₃ 22.
Show answer
Acute respiratory alkalosis from over-ventilation. Nothing is wrong with the patient’s kidneys or metabolism — the ventilator is blowing off too much CO₂. The fix is on the machine: reduce the rate or the tidal volume, then repeat the gas.
Answering an ABG question under time pressure
- Say the four values out loud in order. pH, PaCO₂, HCO₃, then PaO₂ with the FiO₂. Most wrong answers come from reading them out of order.
- Commit to acidosis or alkalosis before anything else. Even a normal pH picks a side once you compare it with 7.40.
- Pick the culprit. Whichever of PaCO₂ or HCO₃ explains the pH is the primary problem. If both do, it is mixed.
- Check the compensation against the expected number, not just its direction. This is where the extra marks are.
- Calculate the anion gap on every metabolic acidosis. It is one subtraction and it changes the differential completely.
- Read the vignette again. Vomiting, diarrhoea, COPD, diabetes, overdose and sepsis each have a signature result, and the story usually confirms what the numbers said.
Speed comes from repetition, not from a new mnemonic. Ten gases a day for a week and the pattern stops needing conscious thought.
Frequently asked questions
What are the normal ABG values?
The ranges most nursing and medical courses use are pH 7.35 to 7.45, PaCO2 35 to 45 mmHg and HCO3 22 to 26 mEq/L. PaO2 is usually quoted as 80 to 100 mmHg on room air.
What does ROME mean in ABG interpretation?
Respiratory Opposite, Metabolic Equal. In a respiratory problem the pH and PaCO2 move in opposite directions. In a metabolic problem the pH and HCO3 move in the same direction.
How do you tell full compensation from partial compensation?
If the pH is still outside 7.35 to 7.45 but both PaCO2 and HCO3 are abnormal, it is partially compensated. If the pH has returned to the normal range while both are abnormal, it is fully compensated, and you use 7.40 as the midpoint to decide which side the primary problem is on.
What is the easiest way to interpret an ABG?
Use the same order every time: look at the pH, then decide whether the PaCO2 or the HCO3 explains that pH, then check whether the other value is moving to compensate. Many students also use the tic-tac-toe method to lay the three values out visually.
Is there a free app for ABG practice questions?
Yes. ECG Quiz is free to download on Android and includes an ABG Quiz that follows the four-step method with explanations, alongside EKG rhythm practice.
What is Winter's formula used for?
It checks whether the lungs have compensated enough for a metabolic acidosis. Expected PaCO2 = (1.5 x HCO3) + 8, plus or minus 2. If the measured PaCO2 is higher than that range there is an extra respiratory acidosis, and if it is lower there is an extra respiratory alkalosis.
How do you calculate the anion gap?
Anion gap = sodium minus (chloride plus bicarbonate). A normal result is roughly 8 to 12 mEq/L. A high gap means an acid has been added, such as lactate or ketones, and a normal gap means bicarbonate has been lost, usually through the gut or the kidneys.
What is a normal P/F ratio?
The P/F ratio is PaO2 divided by FiO2, and above 400 is normal. Under 300 is mild lung injury, under 200 is moderate and 100 or less is severe. Always read the PaO2 next to the oxygen the patient is receiving, never on its own.
ECG Quiz: EKG Rhythms & ABG
The ABG Quiz in ECG Quiz walks the same four steps on every blood gas, including compensation, and sits next to unlimited EKG rhythm strips. Free to download, works offline.
More guides from AK Apps Studio
For education and exam revision only. Real blood gases must be interpreted alongside the patient, the oxygen or ventilator settings and the clinical picture, and some patients have mixed disorders that need a fuller assessment than this four-step method.