An ABG, or arterial blood gas test, is a blood test that measures how well your lungs are moving oxygen into your blood and removing carbon dioxide from it. It also checks whether your blood’s acid-base balance (pH) is within a healthy range. Unlike most blood tests that draw from a vein, an ABG uses blood from an artery, because arterial blood carries oxygen directly from the lungs and gives a more accurate picture of how your respiratory system is performing.
What an ABG Measures
An ABG test reports four key values, each telling your medical team something different about your body:
- pH (normal: 7.35 to 7.45): This reflects how acidic or alkaline your blood is. Even small shifts outside this range can signal serious problems with your lungs, kidneys, or metabolism.
- PaO2 (normal: 80 to 100 mmHg): This is the oxygen level in your arterial blood. A low reading means your lungs aren’t delivering enough oxygen, which can happen with pneumonia, asthma attacks, or lung disease.
- PaCO2 (normal: 35 to 45 mmHg): This is the carbon dioxide level. Your lungs are supposed to exhale CO2 with every breath, so a high number suggests your breathing is too slow or shallow to keep up. A low number means you’re breathing too fast, blowing off more CO2 than normal.
- Bicarbonate/HCO3 (normal: 22 to 26 mEq/L): Bicarbonate is a chemical your kidneys produce to neutralize acid in the blood. Abnormal levels point to a kidney problem or a metabolic issue like uncontrolled diabetes.
Together, these four numbers let a medical team pinpoint whether a problem is coming from the lungs (respiratory), the kidneys or metabolism (metabolic), or both. Oxygen level is the one measurement that specifically requires arterial blood, since venous blood has already delivered most of its oxygen to the body’s tissues and won’t reflect what your lungs are actually doing.
Why Doctors Order an ABG
An ABG is the standard test for evaluating oxygenation, ventilation, and acid-base status. It’s commonly ordered in emergency rooms and intensive care units when someone is in acute distress, but it can also be part of managing a chronic condition.
Situations that often call for an ABG include severe asthma or COPD flare-ups, pneumonia, respiratory failure (when the lungs can’t keep up with the body’s needs), sepsis, diabetic ketoacidosis, and drug overdoses that affect breathing. The test is also used to monitor whether treatments like supplemental oxygen or a ventilator are working as intended. For people with kidney disease, an ABG helps track whether the kidneys are maintaining the body’s acid-base balance on their own.
How the Test Is Done
The blood is almost always drawn from the radial artery on the inside of your wrist, near the base of your thumb. This artery sits close to the surface and is easy for the person drawing the blood to feel. Before the puncture, your care team may perform a quick circulation check called a modified Allen test. You’ll be asked to make a tight fist while the provider presses on both arteries in your wrist. When you open your hand, your palm will look pale. The provider then releases pressure on one artery at a time and watches for color to return within 5 to 15 seconds. This confirms that both arteries supply enough blood to your hand, so it’s safe to use one for the draw.
Once the site is selected, the provider inserts a small needle into the artery and collects 2 to 3 milliliters of blood. The sample is sent for analysis right away, and results typically come back within minutes.
What It Feels Like
There’s no way around it: an ABG draw hurts more than a standard blood draw from a vein. Research has described arterial punctures as the most painful routine laboratory procedure, and they’re frequently done without any pain management at all. The artery sits deeper than the veins typically used for blood work, and arteries have more nerve endings surrounding them.
Studies have found that a small injection of lidocaine (a local anesthetic) at the puncture site before the draw significantly reduces pain. If you’ve been told you need an ABG and you’re concerned about discomfort, it’s reasonable to ask whether a local anesthetic will be used. In emergency situations, though, there may not be time to numb the area first. The sharp pain from the needle is brief, usually just a few seconds, and most people describe a deep aching or throbbing sensation that fades quickly afterward.
After the Draw
Immediately after the needle is removed, firm pressure is applied to the puncture site for at least five minutes. This is longer than a typical blood draw because arteries carry blood under higher pressure than veins, and without adequate compression the site can bleed under the skin. If you take blood thinners or have a clotting disorder, pressure may need to be held even longer. You’ll likely be told not to lift anything heavy with that arm for a short period afterward.
Some bruising at the site is common and not a cause for concern. A small, tender lump where the blood pooled under the skin can develop but typically resolves on its own within a few days.
Risks and Complications
Serious complications from an ABG are rare. A large study analyzing over 473,000 arterial punctures found a major complication rate of just 0.14%. The most common serious issues were blood clots or blockages in the artery (about 49% of those rare complications) and small aneurysms at the puncture site (about 15%). Nerve damage occurred in roughly 1.5% of the complication cases.
People taking blood-thinning medications had a slightly higher risk, with the odds of a major complication increasing by about 31% compared to those not on blood thinners. Male sex was also associated with a modestly higher risk. For the vast majority of patients, the test is straightforward and uneventful.
What Abnormal Results Mean
ABG results fall into four broad patterns, each pointing toward a different underlying issue:
- Respiratory acidosis: Blood pH drops below 7.35 and CO2 is elevated. This happens when the lungs can’t expel CO2 fast enough, as in severe COPD, a drug overdose that slows breathing, or neuromuscular diseases that weaken the chest muscles.
- Respiratory alkalosis: Blood pH rises above 7.45 and CO2 is low. The most common cause is hyperventilation, whether from anxiety, pain, fever, or being on a ventilator set too aggressively.
- Metabolic acidosis: Blood pH drops below 7.35 and bicarbonate is low. This pattern appears in diabetic ketoacidosis, kidney failure, severe dehydration, or poisoning.
- Metabolic alkalosis: Blood pH rises above 7.45 and bicarbonate is elevated. Prolonged vomiting, overuse of antacids, and certain diuretics can cause this.
In many real-world situations, results show a mix of these patterns, especially in critically ill patients. The body also tries to compensate for one type of imbalance by adjusting the other system. For example, if a lung problem causes CO2 to build up, the kidneys will gradually produce more bicarbonate to buffer the acidity. Your medical team reads these values together, not in isolation, to figure out what’s going wrong and whether your body is compensating effectively.
ABG vs. Pulse Oximetry
If you’ve been in a hospital, you’ve probably had a pulse oximeter clipped to your finger. That device estimates oxygen saturation (the percentage of your red blood cells carrying oxygen) and is completely painless. So why bother with an ABG?
A pulse oximeter tells you one thing: roughly how much oxygen is in your blood. An ABG tells you that plus CO2 levels, pH, and bicarbonate. It’s the difference between a snapshot and a full panel. Pulse oximetry can also be inaccurate in certain situations, such as poor circulation, dark nail polish, or carbon monoxide poisoning. When precision matters, or when the question goes beyond oxygen alone, an ABG is the more complete test.