ABG Interpretation: A Nurse's Guide to Reading Blood Gases
Arterial blood gases — the ABG — are one of the fastest ways to see what's really happening inside a sick patient: their acid-base balance, their ventilation, and their oxygenation, all in one draw. Learn a systematic approach and they stop being intimidating.
This guide breaks down every value on the panel, walks through the four acid-base disturbances, and gives you a step-by-step method to interpret any gas with confidence — whether you're on a med-surg floor, in the ED, or at an ICU bedside.
What Are ABGs or a Blood Gas?
An ABG — Arterial Blood Gas — is a blood sample drawn from an artery, usually in the wrist (radial artery). That's different from routine labs, which come from a vein. The arterial sample is typically obtained by a respiratory therapist, a critical care nurse, or a provider.
Arterial blood gives a better picture of oxygen and carbon dioxide levels than venous blood, and the ABG also reports the blood's acidity and bicarbonate. In short, a blood gas is used to evaluate acid-base disturbances and the adequacy of oxygenation and ventilation. Four core values are reported:
When Are ABGs Ordered?
ABGs shine when you need to evaluate an acid-base problem or a ventilation/oxygenation problem. The patients who get them are usually sick — often ICU-bound. Common scenarios include:
Things to Keep in Mind with ABGs
A few principles make interpretation far less confusing.
Things Aren't Always Simple
Patients can have mixed acid-base disturbances at once, which muddies the picture. Final interpretation is ultimately the provider's call, especially in complex critical-care cases.
Compensation
The body is always chasing homeostasis. The respiratory system will try to compensate for a metabolic problem, and the kidneys (metabolic) will try to compensate for a respiratory one.
The ABG Measurements
Let's go value by value and unpack what each one means.
The pH
pH is the "potential of Hydrogen" — a measure of how acidic the blood is. The more hydrogen ions, the more acidic.
pH can be normal or near-normal in chronic disturbances because of compensation, or when a patient has more than one acid-base process happening at once.
The PaCO2
PaCO2 is the partial pressure of carbon dioxide in arterial blood — essentially how much CO2 is dissolved in the blood. The lungs deliver oxygen to the cells; the cells make energy (ATP) and produce CO2 as a byproduct, which is then exhaled.
CO2 isn't acidic on its own, but in blood it forms carbonic acid, which is. So retaining CO2 drives acidosis, and blowing off too much CO2 drives alkalosis. That makes PaCO2 the respiratory half of the equation.
Watch the Direction
In respiratory acidosis, PaCO2 is high and pH is low — they move in opposite directions. (CO2 is an acid, so as it rises, pH falls.) A high CO2 with a low pH is your classic respiratory acidosis.
"My Blood Is Boiling"
Hold your breath and eventually you feel like you have to breathe — like your blood is boiling. That's the CO2 building up and turning acidic. A handy way to remember: not breathing enough = CO2 climbs = acidosis.
The HCO3 (Bicarbonate)
HCO3 is the serum bicarbonate in the arterial blood. Bicarb is a buffer — think of it as the opposite of hydrogen ions. The less bicarb, the more acidic the blood. It's the metabolic half of the equation: bicarb reacts with H+ to form carbonic acid, which the body breaks into CO2 and water and breathes off.
The PaO2
PaO2 is the partial pressure of oxygen in arterial blood — the actual oxygen content of the gas.
A low PaO2 is associated with respiratory failure (though severe anemia can contribute). A very high PaO2 usually reflects over-oxygenation.
Oxygen Toxicity
Too much oxygen can harm, too. High levels can generate reactive oxygen species causing cellular injury and inflammation, and excess oxygen can worsen CO2 retention in some COPD patients.
The SaO2
SaO2 is the oxygen saturation, equivalent to the pulse ox reading.
The Four Acid-Base Disturbances
When you read an ABG, you're sorting it into one (or more) of four buckets:
Remember a patient can have more than one disturbance at once — medicine is rarely black and white — but these four categories point you toward the underlying cause.
Respiratory Acidosis
Respiratory acidosis comes from alveolar hypoventilation: the lungs can't clear CO2 fast enough, so CO2 (and hydrogen ions) build up. PaCO2 is high and pH is low. It can be acute or chronic, and it shows up classically in COPD, where blunted response to hypoxia, increased dead-space ventilation, and a fatigued, hyperinflated diaphragm all contribute.
COPD & Oxygen
A COPD patient on high-flow oxygen carries a real risk of hypoventilation and CO2 retention. This is the classic patient to keep picturing whenever you think "respiratory acidosis."
Treatment targets the underlying cause (for example, naloxone for an opioid overdose), but more often it comes down to BiPAP or intubation.
Respiratory Alkalosis
The mirror image: alveolar hyperventilation blows off too much CO2, producing hypocapnia (low PaCO2) and a high pH. It's common in the critically ill, but shows up in several settings:
Treatment again addresses the cause — adjusting ventilator settings, treating anxiety, and so on.
Metabolic Acidosis
A bicarb < 22 mEq/L with a pH < 7.35 is metabolic acidosis: increased plasma acidity from the metabolic side. It's split by whether the anion gap is normal or elevated. A severe HCO3 < 12 almost always means true metabolic acidosis rather than mere compensation.
Normal-Gap Metabolic Acidosis
Usually accompanied by high chloride. Bicarbonate is lost through the GI tract or kidneys. Common causes:
Elevated-Gap Metabolic Acidosis
The anion gap is the difference between the main measured cation (sodium) and the measured anions (chloride and bicarb). A high gap means unmeasured acids — lactate, ketones, toxins — are accumulating. Common causes:
Treatment corrects the underlying problem. A bicarb drip may be used in severe acidosis (roughly pH < 7.1).
Metabolic Alkalosis
Caused by increased serum bicarb and decreased acidity. A bicarb > 35 mEq/L almost always reflects true metabolic alkalosis rather than compensation. Hydrogen ions are usually lost through the GI tract or kidneys:
Steps to Interpret an ABG
A systematic approach keeps you from missing a step. Work through it in order every time:
Is the patient acidic (pH < 7.35) or alkalotic (pH > 7.45)? This tells you the primary direction.
Which values are abnormal, and are they leaning acidic or alkalotic?
Which value matches the pH? Example: pH 7.20 (acidic) with CO2 56 and HCO3 30 — the CO2 is acidic and matches, so this is respiratory.
Look at the value that didn't match. Is it abnormal in the opposite direction? That's compensation. If the pH is still abnormal, it's only partial compensation.
Optional, for metabolic acidosis. A high gap points to AKI, sepsis, or DKA; a normal gap points to causes like severe diarrhea.
Clinical Pearls
Related Guides
References
- Theodore, A. C. Arterial blood gases. UpToDate.
- Emmett, M., & Szerlip, H. Approach to the adult with metabolic acidosis. UpToDate.
- Emmett, M., & Szerlip, H. Causes of metabolic alkalosis. UpToDate.
- Hopkins, E., Sanvictores, T., & Sharma, S. Physiology, Acid Base Balance. StatPearls, National Library of Medicine.
- Sood, P., Paul, G., & Puri, S. Interpretation of arterial blood gas. Indian J Crit Care Med, 14(2), 57–64.
Simple and lucid explanations along with easy-to-remember pointers. Excellent job! Have recommended this site to many of my colleagues from allied health sciences.
Thank you so much for the wonderful compliment!
This site is fantastic. It is visually appealing and gives concise information on all the important topics.
Thank you, that’s the exact goal! 🙂
in the context below the heading “the paco2” you have mentioned that respiratory acidosis is when both the ph and paco2 are high,, but isnt it the opposite? ph is always below 7.35 and pac02 is always above 45.
Very good thanks
Your article helped me a lot, is there any more related content? Thanks!
This guide is really helpful! I always found ABGs confusing, but this article explains everything in a simple way. I like how it talks about pH, CO2, oxygen, and bicarbonate, and what their levels mean. The part about how the body tries to balance itself with compensation really made sense. I also like the tips about remembering the underlying cause and that patients can have mixed problems. I will definitely use this guide when I’m learning or in the hospital. Thanks for making a hard topic easier.