ABGs, or arterial blood gases, are a set of measurements taken from a blood sample drawn from an artery. The test reveals how well your lungs are moving oxygen in and carbon dioxide out, and whether your blood’s acid-base balance is within a healthy range. It’s one of the most informative single tests in emergency and critical care medicine, giving clinicians a real-time snapshot of how your body is handling two essential jobs: breathing and maintaining blood chemistry.
What an ABG Test Measures
An ABG report includes five core values, each telling a different part of the story:
- pH (7.35 to 7.45): The acidity or alkalinity of your blood. Even small shifts outside this narrow window can disrupt how your cells function. A pH below 7.35 is called acidosis; above 7.45 is alkalosis.
- PaO2 (80 to 100 mmHg): The partial pressure of oxygen dissolved in arterial blood. This shows how effectively your lungs transfer oxygen from the air you breathe into your bloodstream.
- PaCO2 (35 to 45 mmHg): The partial pressure of carbon dioxide. Carbon dioxide is a waste product your body constantly produces, and your lungs are responsible for clearing it. A high PaCO2 means your lungs aren’t ventilating well enough; a low one means you’re breathing off too much.
- Bicarbonate/HCO3 (22 to 26 mEq/L): A chemical buffer your kidneys regulate to keep blood pH stable. When bicarbonate is too low, it often signals a metabolic problem like a buildup of acid. When it’s too high, the body may be compensating for another imbalance or dealing with persistent vomiting or certain medications.
- Oxygen saturation/SaO2 (95% to 100%): The percentage of hemoglobin molecules in your red blood cells that are loaded with oxygen.
Some reports also include a value called base excess or deficit, which normally falls between negative 2 and positive 2. A negative number points toward excess acid in the blood; a positive number suggests excess base. It’s a quick way to gauge whether a metabolic imbalance is present and how severe it is.
PaO2 vs. Oxygen Saturation
These two numbers both describe oxygen in your blood, but they aren’t interchangeable. PaO2 measures the actual pressure of oxygen dissolved in plasma, while oxygen saturation measures what percentage of your hemoglobin is carrying oxygen. The relationship between them follows a curved pattern rather than a straight line. When PaO2 is above about 60 mmHg, saturation stays high (around 90% or above), and large increases in PaO2 barely move the saturation number. This acts as a built-in safety buffer.
Below 60 mmHg, though, the curve steepens dramatically. Small drops in PaO2 cause large drops in saturation, meaning oxygen delivery to tissues falls off quickly. This is why a PaO2 in the 50s is treated as an urgent finding even though the number itself might not sound alarmingly low. Temperature, pH, and carbon dioxide levels all shift this curve, which is one reason ABGs provide more detail than a fingertip pulse oximeter alone.
Why the Test Is Ordered
ABGs are most commonly drawn when someone is in respiratory distress, on a ventilator, or critically ill. The test helps answer specific questions: Is this person getting enough oxygen? Are they able to clear carbon dioxide? Is the blood becoming dangerously acidic or alkaline? Conditions that frequently call for an ABG include COPD flare-ups, pneumonia, asthma attacks, sepsis, kidney failure, diabetic emergencies, and drug overdoses that affect breathing.
The test is also used to monitor how well treatments are working. If someone is on supplemental oxygen or a breathing machine, repeated ABGs show whether settings need to be adjusted. In the operating room, ABGs help anesthesiologists keep tabs on a patient’s lung function and acid-base status during long or complex surgeries.
How the Blood Is Drawn
Unlike a standard blood draw from a vein in your arm, an ABG sample comes from an artery, most often the radial artery at the wrist. Arterial blood reflects what your lungs just did, while venous blood reflects what your tissues have already used up, so the two give very different oxygen and carbon dioxide readings. Venous blood naturally has less oxygen, more carbon dioxide, and a slightly lower pH.
Before the draw, a circulation test is sometimes performed to confirm that your hand has adequate blood supply from more than one artery. The practitioner compresses both arteries at your wrist while you make a fist, then releases one to watch color return to your palm. If blood flow looks good, the draw proceeds. A small needle is inserted into the artery, and because arteries have more pressure and more nerve endings than veins, the stick tends to be sharper than a typical blood draw. Firm pressure is held on the site afterward, usually for several minutes, to prevent bleeding or bruising. Soreness or a small bruise at the puncture site is common and typically resolves within a day or two.
Understanding Acid-Base Imbalances
One of the main reasons ABGs exist is to identify and classify acid-base problems. There are four primary imbalances, and the ABG values point directly to which one is occurring.
Respiratory acidosis happens when carbon dioxide builds up because the lungs can’t expel it fast enough. The PaCO2 rises above 42 mmHg and the pH drops below 7.38. This can occur with severe COPD, an overdose of sedatives that slows breathing, or any condition that weakens the muscles involved in breathing.
Respiratory alkalosis is the opposite: breathing too fast or too deeply blows off too much carbon dioxide, pushing PaCO2 below 38 mmHg and pH above 7.42. Anxiety-driven hyperventilation is a classic trigger, but it can also occur with high fevers, pain, or early stages of sepsis.
Metabolic acidosis shows up as low bicarbonate (below 22 mEq/L) and low pH. The problem originates outside the lungs, often from a buildup of acids the body can’t clear. Uncontrolled diabetes, kidney failure, severe dehydration, and lactic acid buildup from shock or prolonged seizures are common causes. Clinicians sometimes calculate something called the anion gap, a simple formula using sodium, chloride, and bicarbonate levels, to narrow down whether the acid is coming from an identifiable source like lactate or ketones.
Metabolic alkalosis presents as high bicarbonate (above 26 mEq/L) and elevated pH. Persistent vomiting, which removes stomach acid from the body, is one of the most frequent causes. Certain diuretic medications can also trigger it by causing the kidneys to waste too much chloride.
In many real-world situations, the body tries to compensate for one imbalance by adjusting the other system. If a metabolic problem drops your pH, your lungs may speed up breathing to blow off extra carbon dioxide and pull the pH back toward normal. If a lung problem raises your carbon dioxide, the kidneys may retain more bicarbonate over hours to days. ABG results often show these compensation patterns layered on top of the primary problem, which is why interpreting them requires looking at all the values together rather than any single number in isolation.
ABG vs. Venous Blood Gas
A venous blood gas (VBG) is drawn from a vein, the same way routine lab work is collected. It’s less painful, easier to obtain, and carries fewer risks. For assessing pH and carbon dioxide trends in stable patients, a VBG can sometimes substitute for an ABG, since venous pH typically runs only about 0.03 to 0.05 lower than arterial pH. The key limitation is oxygen measurement: venous blood has already delivered its oxygen to tissues, so PaO2 and oxygen saturation from a VBG don’t reflect lung function the way arterial values do. When the clinical question is specifically about oxygenation, an ABG remains the standard.