To read an ABG, look at the pH first to decide acidosis or alkalosis, then check whether the PaCO2 (lungs) or the HCO3 (kidneys) matches that pH. The one that matches is the cause, and the other value tells you whether the body is compensating. Once you follow the same five steps every time, ABG interpretation becomes a quick routine instead of a guessing game.
Normal ABG values
Memorize these adult ranges first. Labs vary a little, so always use your facility's reference ranges in practice.
| Value | Normal range | What it tells you |
|---|---|---|
| pH | 7.35–7.45 | Below 7.35 = acidosis; above 7.45 = alkalosis |
| PaCO2 | 35–45 mm Hg | Controlled by the lungs. High = acid; low = base |
| HCO3 | 22–26 mEq/L | Controlled by the kidneys. Low = acid; high = base |
| PaO2 | 80–100 mm Hg | Oxygen in arterial blood; below 80 = hypoxemia |
| SaO2 | 95–100% | Percent of hemoglobin carrying oxygen |
The key idea: CO2 acts as an acid and HCO3 is a base. The lungs change CO2 within minutes by speeding up or slowing breathing. The kidneys change HCO3 slowly, over hours to days.
How to interpret ABGs in 5 steps
- Check the pH. Below 7.35 is acidosis and above 7.45 is alkalosis. If it is normal, note which side of 7.40 it falls on. That clue matters in step 5.
- Check the PaCO2. Is it above 45 (acid) or below 35 (base)? If it moves in the opposite direction of the pH, the problem is respiratory.
- Check the HCO3. Is it below 22 (acid) or above 26 (base)? If it moves in the same direction as the pH, the problem is metabolic.
- Match and name it. Whichever value matches the pH is the primary problem. Name it: respiratory or metabolic, acidosis or alkalosis.
- Decide compensation. Look at the value that did not match. If it is normal, the ABG 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 in the normal range, it is fully compensated.
After acid-base, look at the PaO2 and SaO2 as a separate question. A patient can have a perfectly balanced pH and still be hypoxemic, and low oxygen needs action first.
Memory trick: ROME. Respiratory Opposite: in respiratory problems, pH and PaCO2 move in opposite directions (pH down, CO2 up). Metabolic Equal: in metabolic problems, pH and HCO3 move the same way (both down or both up).
The tic-tac-toe method
If arrows confuse you, try the tic-tac-toe grid. Draw three columns labeled Acid, Normal and Base. Then place each value:
- pH below 7.35 goes under Acid; above 7.45 goes under Base.
- PaCO2 above 45 goes under Acid; below 35 goes under Base.
- HCO3 below 22 goes under Acid; above 26 goes under Base.
Whatever value lands in the same column as the pH is the cause. If the third value sits in the opposite column, the body is compensating. If it sits under Normal, there is no compensation yet.
Understanding compensation
| Compensation | pH | The other value |
|---|---|---|
| Uncompensated | Abnormal | Normal |
| Partially compensated | Abnormal | Abnormal, moving the opposite way to help |
| Fully compensated | Normal | Abnormal, moving the opposite way to help |
The body does not overcompensate. A fully compensated pH stays on the side of the original problem, so a pH of 7.37 points back to acidosis and 7.43 points back to alkalosis.
3 worked examples
Example 1: pH 7.28, PaCO2 55, HCO3 25
The pH is low, so this is acidosis. The PaCO2 is high (acid) and moves opposite to the pH, so it is respiratory. The HCO3 is normal, so the kidneys have not started helping. Answer: uncompensated respiratory acidosis. Think of a post-op patient who is oversedated from opioids and breathing slowly.
Example 2: pH 7.32, PaCO2 30, HCO3 15
The pH is low, so this is acidosis. The HCO3 is low (acid) and matches the pH, so it is metabolic. The PaCO2 is low, meaning the lungs are blowing off acid to help, but the pH is still abnormal. Answer: partially compensated metabolic acidosis. This pattern fits DKA with deep, rapid Kussmaul breathing.
Example 3: pH 7.42, PaCO2 30, HCO3 19
The pH is normal but above 7.40, so it leans alkaline. The PaCO2 is low (base) and matches that lean, so the cause is respiratory. The HCO3 is low, meaning the kidneys have dumped base to balance it out. Answer: fully compensated respiratory alkalosis.
Practice question
A client has had an NG tube to low continuous suction for three days. The ABG shows pH 7.50, PaCO2 43, HCO3 33. How should the nurse interpret these results?
- Uncompensated respiratory alkalosis
- Uncompensated metabolic alkalosis
- Partially compensated metabolic acidosis
- Fully compensated respiratory acidosis
Answer: B. Uncompensated metabolic alkalosis.
Rationale: The pH is high, so this is alkalosis. The HCO3 is high and moves the same way as the pH, which makes it metabolic. The PaCO2 is still normal, so the lungs are not compensating yet. Losing stomach acid through suction or vomiting is a classic cause of metabolic alkalosis.
FAQ
What is the first step in ABG interpretation?
Always start with the pH. It tells you whether the overall problem is acidosis or alkalosis, and every other step builds on that answer.
How do I know if an ABG is respiratory or metabolic?
Find the value that matches the pH. If the PaCO2 explains the pH, it is respiratory. If the HCO3 explains it, it is metabolic. ROME is a quick way to check.
Why does compensation take longer for respiratory problems?
Respiratory problems are fixed by the kidneys, and the kidneys need hours to days to adjust bicarbonate. Metabolic problems are fixed by the lungs, which can change breathing within minutes.
Can a normal pH still be an abnormal ABG?
Yes. If both the PaCO2 and HCO3 are abnormal while the pH is normal, the ABG is fully compensated. Use the 7.40 midpoint to find the original problem.
Keep practicing ABGs
ABGs get easier with repetition. Labs, Fluids & ABGs Cheat Sheets walks through the same 5-step method with ROME, a tic-tac-toe grid, a causes table for all four imbalances and 8 practice ABGs, alongside normal lab values and IV fluid tables. For more quick review pages, see our cheat sheets collection, or connect the numbers to the disease process with Pathophysiology Notes.
This article is a study aid, not medical advice. Always follow your instructor, facility policy and current clinical references.