How do you interpret an ABG step by step?
Use the same five steps every time, in the same order: check oxygenation against the FiO2, then decide whether the pH is acidemic, alkalemic or normal, then ask whether pCO2 (respiratory) or HCO3 (metabolic) explains it, and finally judge compensation. Most wrong answers come from skipping a step because you thought you already knew.
What are the five steps to interpret an ABG?
The five steps are oxygenation, pH, pCO2, HCO3 and compensation, worked in that order on every gas, out loud if necessary. Treat it as a repeatable procedure, not a memory test.
| Step | Ask | Typical published teaching reference (confirm against your facility's references) |
|---|---|---|
| 1. Oxygenation | Is the patient oxygenating? | PaO2 commonly 80–100 mmHg on room air; interpret against FiO2 |
| 2. pH | Acidemic, alkalemic, or normal? | 7.35–7.45 |
| 3. pCO2 | Is the respiratory system the cause? | 35–45 mmHg |
| 4. HCO3 | Is metabolism the cause? | 22–26 mEq/L |
| 5. Compensation | Is the other system trying to fix it, and how far has it got? | Uncompensated, partial, or full |
People who get blood gases wrong are almost always people who skipped a step because they thought they already knew the answer. The method should confirm a normal gas as confidently as it identifies an abnormal one.
Why check oxygenation first on an ABG?
Oxygenation comes first because a gas can have a normal acid-base picture while the patient is severely hypoxemic. If you stop reading at the pH, you miss it.
Always read the PaO2 against the FiO2 the patient is on. A PaO2 that looks acceptable on high-flow oxygen means something very different from the same number on room air.
One way to hold on to this: the gas can be normal while the patient is not. Assessing oxygenation against the FiO2 is its own step, not an afterthought once the acid-base question is answered.
How do you tell if the problem is respiratory or metabolic?
Match the abnormal value to the pH. If the pCO2 change explains the pH (high pCO2 with low pH, low pCO2 with high pH), the problem is respiratory. If the HCO3 change explains it (low HCO3 with low pH, high HCO3 with high pH), it is metabolic. The other system, if it has moved, is compensating.
- Low pH with high pCO2: respiratory acidosis.
- Low pH with low HCO3: metabolic acidosis. Bicarbonate is the metabolic side, and it is down, driving the acidemia.
- High pH with low pCO2: respiratory alkalosis.
- High pH with high HCO3: metabolic alkalosis.
The single rule above resolves most of the confusion new nurses have with blood gases. Say the rule out loud until it is automatic.
How do you know if an ABG is compensated?
Look at the system you did not name as the cause. If it is still normal, the disorder is uncompensated; if it has moved but the pH is still abnormal, it is partially compensated; if the pH has returned to normal, it is fully compensated.
- Uncompensated: for example, low pH, high pCO2, normal HCO3 is respiratory acidosis, uncompensated.
- Partially compensated: low pH, high pCO2 and high HCO3 is respiratory acidosis, partially compensated.
- Fully compensated: a normal pH with an abnormal pCO2 and an abnormal HCO3.
When the pH is normal but both other values are abnormal, use the side of 7.40 the pH sits on to identify the primary problem. A patient with known COPD who is comfortable and at baseline can show exactly this pattern: chronic CO2 retention with renal compensation. The comfortable patient is the clue that this is their normal.
What is the anion gap and when should you calculate it?
Calculate the anion gap whenever you find a metabolic acidosis, because it separates acidosis from accumulated unmeasured anions from acidosis caused by losing bicarbonate or gaining chloride. The formula is Na+ − (Cl- + HCO3-).
The commonly cited normal range is roughly 8–12 mEq/L, a typical published teaching figure to confirm against your facility's references, and institutions differ on whether potassium is included.
- High anion gap metabolic acidosis: think lactate, ketones, renal failure, toxic alcohols, salicylates.
- Normal anion gap metabolic acidosis: think diarrhea, renal tubular acidosis, large-volume normal saline.
A worked example from the guide: Na 138, Cl 104, HCO3 13 gives a gap of 138 − (104 + 13) = 21, a high anion gap acidosis in an anuric patient who missed dialysis.
On a cardiac unit, a rising lactate with a metabolic acidosis is a statement about oxygen delivery until proven otherwise, and it frequently changes before blood pressure does. Trend it, because the direction tells you far more than a single value. A metabolic acidosis with high lactate, cool extremities and low urine output is a perfusion problem: escalate it rather than charting it and moving on.
What is the difference between an ABG and a VBG?
An ABG is best for oxygenation and a precise pCO2; a VBG is best for pH, bicarbonate, lactate and trending. A VBG is not a substitute for arterial oxygenation assessment.
- pH: the ABG is the reference standard. Venous pH runs slightly lower than arterial, typically a small, fairly predictable offset.
- pCO2: venous pCO2 runs higher than arterial and is less reliable in shock states.
- Practical side: an ABG needs an arterial puncture or arterial line. A VBG is easier, less painful and adequate for many trending questions.
The practical question to ask is what you need the gas to answer. If it is oxygenation or an exact pCO2, you need arterial blood; if it is whether the acidosis or lactate is trending the right way, a venous sample may be enough.
The full reference, built for the pocket
The full ABG / VBG Interpretation Worksheet lays out the five-step method with a complete cause-to-pH matching table, a Causes and the Anion Gap section with delta ratio interpretation, and a side-by-side VBG versus ABG comparison. Its Practice Set gives twelve worked cardiac-unit problems, with an Answers section explaining what each gas means at the bedside.
Or buy the guide inside the CVICU Core bundle (5 guides, $49) or the Complete Library (all 20, $109). Precepting? The Nurse Preceptor Toolkit is free.
Frequently asked questions
What does it mean when the pH is normal but the pCO2 and HCO3 are both abnormal?
A normal pH with abnormal pCO2 and HCO3 usually means a fully compensated disorder. Use the side of 7.40 the pH sits on to identify the primary problem. A comfortable patient with known COPD at baseline may show chronic respiratory acidosis with renal compensation, which is their normal rather than a new emergency.
What is the delta ratio used for?
In a high anion gap metabolic acidosis, the delta ratio compares how much the gap rose against how much the bicarbonate fell. Roughly 1 to 2 suggests a pure high-gap acidosis. Below 1 suggests a coexisting normal-gap acidosis; above 2 suggests a coexisting metabolic alkalosis or a pre-existing high bicarbonate.
Can two acid-base disorders hide each other on a blood gas?
Yes. A post-operative cardiac patient can be lactic from low output and alkalotic from nasogastric suction at the same time. The two partly cancel, so the pH and bicarbonate may look reassuring. The delta ratio is what exposes the acidosis you would otherwise miss.
What causes a respiratory acidosis in a hospitalized patient?
Respiratory acidosis comes from hypoventilation. Common causes include oversedation, neuromuscular weakness, COPD exacerbation, airway obstruction, inadequate ventilator settings and chest wall problems. A very sleepy post-operative patient who recently received opioids is a classic example, and the bedside response is to stimulate the patient, assess airway and respiratory rate, consider reversal per order, and stop giving more sedation.
When is an acute respiratory acidosis a call-now finding?
In a ventilated patient with high peak pressures, sudden deterioration and absent breath sounds on one side, an acute respiratory acidosis points to pneumothorax until proven otherwise. The guide calls that combination a call-now finding, meaning you escalate to the provider right away.
More cardiac nursing answers
The ABG interpretation article is educational reference material for licensed clinicians and students, drawn from a Cardiac Nurse Playbooks guide. It is not medical advice, not a protocol, and not a substitute for your facility's policies, physician orders, device manufacturer instructions for use, or current drug references. Any values shown are typical published reference figures; verify every dose, setting and parameter against your facility's approved sources. When this and your unit policy disagree, your unit policy wins.
Sources
Official and professional sources on the topic. Guidelines and handbooks are updated, so confirm current details there before you rely on this page. This is an independent study aid, not a clinical protocol.
- Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility - StatPearls (NCBI Bookshelf (StatPearls)). Explains how arterial blood gases are collected, analyzed, interpreted and used clinically, including sample handling and quality control.
- Fundamentals of Arterial Blood Gas Interpretation (Kidney360 (via PubMed Central)). Peer-reviewed article on validating and interpreting ABG results and identifying mixed acid-base disorders, illustrated with a case.
- Blood gas analysis: Clinical applications, interpretation and future directions (Review) (Medicine International (via PubMed Central)). Peer-reviewed review of blood gas physiology, clinical uses, interpretation and venous versus arterial sampling in critical care.