Arterial Blood Gas (ABG) Test: What It Measures, Normal Values and How to Read the Result

The nurse asks you to open your palm, presses two fingers against the inside of your wrist, and waits. She is feeling for the radial artery, the same pulse you might check after a run. Then she tells you this one will feel different from a normal blood draw, and she is right. Ten seconds later the syringe is capped, packed in ice and hurrying toward a machine that will report numbers most people have never seen on a lab slip.

Those numbers describe something ordinary blood tests cannot: the actual conversation happening between your lungs, your kidneys and every cell that needs oxygen. Cholesterol tells you about years. An arterial blood gas tells you about the last few minutes.

That immediacy is why the test shows up in emergency departments and intensive care units far more than in routine checkups, and why the printout can look intimidating. It is more readable than it appears. Here is what each value means, what counts as normal, and how clinicians actually work through a result.

What does an arterial blood gas test actually measure?

An arterial blood gas test takes a small sample, usually one to two milliliters, from an artery rather than a vein and runs it through an analyzer that reports five or six core values within minutes. Each one answers a slightly different question about how the body is handling gas exchange and acid balance.

The partial pressure of oxygen, written PaO2, reflects how much oxygen has dissolved into arterial blood as it leaves the lungs. Oxygen saturation, or SaO2, describes what percentage of hemoglobin’s carrying seats are occupied. The partial pressure of carbon dioxide, PaCO2, shows how effectively the lungs are clearing the waste gas produced by metabolism; because CO2 dissolves into blood as a weak acid, this value doubles as a marker of breathing depth and rate. Then comes pH, the overall acidity, which the body defends within a remarkably narrow band. Bicarbonate, or HCO3, is the main chemical buffer, regulated largely by the kidneys, and base excess summarizes whether the total buffering capacity has drifted up or down.

Many analyzers add extras from the same sample: lactate, electrolytes such as sodium and potassium, hemoglobin, glucose and sometimes carboxyhemoglobin. Those additions explain why a single ABG can reshape a treatment plan in an emergency room in the time it takes to draw a routine metabolic panel.

What the test cannot do is tell anyone why a value is off. A low PaO2 looks identical whether the cause is pneumonia, a blood clot in the lung or a mountain cabin at 9,000 feet. The interpretation always depends on the person in the bed.

ABG normal values: the reference ranges on your report

Reference ranges vary slightly between laboratories, and your printout will list its own. The figures below are the ones most textbooks and hospital references use for a healthy adult breathing room air at or near sea level.

Value	Typical adult range	What it mainly reflects
pH	7.35–7.45	Overall acid-base balance
PaCO2	35–45 mmHg	Ventilation (how well CO2 is exhaled)
PaO2	75–100 mmHg	Oxygen uptake in the lungs
HCO3 (bicarbonate)	22–26 mEq/L	Kidney-controlled buffering
SaO2	95–100%	Hemoglobin loaded with oxygen
Base excess	−2 to +2 mEq/L	Net metabolic shift

Two caveats matter. PaO2 declines gently with age; a fit 80-year-old may sit in the low 80s without any lung disease, and clinicians mentally adjust for that. Altitude matters more. At roughly 5,000 feet the air holds less oxygen pressure, so a PaO2 in the high 60s or low 70s can be entirely normal for a resident, which is why laboratories in high cities publish different ranges.

Notice how tight the pH window is. A shift from 7.40 to 7.30 sounds trivial until you remember the scale is logarithmic; that change means roughly 25% more hydrogen ions in the blood. Enzymes that run every cell are built to work in that narrow band, which is why the body spends so much effort defending it, and why a pH outside 7.2–7.6 is treated as a serious finding regardless of the cause.

Why does a blood gas test use an artery instead of a vein?

Nearly every other blood test draws from a vein because veins are close to the surface, low-pressure and forgiving. Blood gases break the rule for a physiological reason. Venous blood has already visited the tissues, dropped off oxygen and picked up carbon dioxide. Its values describe what a particular arm or hand has been doing, not what the lungs have just delivered to the whole body.

Arterial blood, by contrast, is uniform. Whether drawn from the wrist, the elbow crease or the groin, it has left the left side of the heart moments earlier carrying exactly what the lungs loaded into it. That makes PaO2 and PaCO2 from an artery a direct readout of gas exchange. A venous PaO2, typically around 40 mmHg, tells you almost nothing about lung function, because it is supposed to be low.

The radial artery at the wrist is the usual site. It sits just under the skin, is easy to compress afterward, and the hand has a second artery, the ulnar, that can supply blood if the radial is briefly bruised or narrowed. Before puncturing, many clinicians perform a quick Allen test: they press both wrist arteries until the palm blanches, release the ulnar side, and watch for color to return within a few seconds. Pink hand, safe site.

People who need repeated measurements, such as those on a ventilator, often have a thin arterial catheter placed so samples can be drawn painlessly from a port rather than by fresh needle sticks. That single tube can supply dozens of blood gas tests over a stay.

When should an arterial blood gas be drawn?

An ABG earns its discomfort when the answer will change what happens next and no gentler test can supply it. That usually means one of a handful of situations.

Sudden or severe breathing trouble is the classic trigger: an asthma flare that is not settling, a worsening flare of chronic obstructive pulmonary disease, suspected pneumonia in someone who looks unwell, or a possible blood clot in the lung. Here the value clinicians most want is PaCO2. A pulse oximeter can show oxygen holding steady while carbon dioxide quietly climbs because a person is tiring and breathing too shallowly. Only a blood gas test catches that.

Anyone on a mechanical ventilator gets regular ABGs to fine-tune how much air and oxygen the machine delivers. Critical illness of almost any kind, from sepsis to severe dehydration to diabetic emergencies, prompts a gas because acid-base disturbances are common and often silent. Suspected poisonings, carbon monoxide exposure and kidney failure round out the list.

Timing within those scenarios matters too. A sample drawn while someone is still receiving high-flow oxygen reflects the oxygen, not the lungs, so clinicians either note the setting or wait after a change, typically 20 to 30 minutes, for values to stabilize. An ABG is also drawn as a baseline before major surgery in people with known lung disease, and during evaluation for home oxygen, where insurers and guidelines generally rely on an arterial value rather than a fingertip reading.

What an ABG is not for: routine physicals, vague tiredness or screening healthy people. If you are reading this because one was ordered, something specific prompted it.

How painful is an arterial blood gas test?

Honest answer: more than a standard blood draw, less than most people fear. Arteries sit deeper than veins, they are wrapped in more nerve endings, and the needle passes through a muscular wall that veins lack. Most people describe a sharp, brief pinch followed by a dull ache, sometimes with a moment of tingling if the needle brushes a nerve. The active part of the procedure usually lasts under a minute.

Skill matters, and so does preparation. Clinicians often use a small amount of local numbing at the site, which studies in emergency and respiratory settings have found reduces pain scores without making the artery harder to find. Keeping the wrist extended over a rolled towel brings the artery closer to the surface and shortens the search. If you have had a difficult draw before, say so; another site or an ultrasound-guided approach may be offered.

Afterward comes the part people underestimate. Because arterial pressure is high, firm pressure is held over the site for at least five minutes, longer for anyone on blood thinners, to prevent a bruise the size of a plum. Once the bandage is on, the arm should feel normal. Numbness, coldness, pale fingers or a rapidly swelling wrist over the following hours are uncommon but should be reported right away, since they can signal bleeding or reduced blood flow to the hand.

Children and people who need many samples are generally spared repeated sticks through arterial lines or, where appropriate, capillary samples from a warmed earlobe or heel.

How to read an ABG result in three steps

Clinicians learn a fixed sequence for interpreting blood gases, and it works for patients too. Resist the urge to start with oxygen; the acid-base story comes first.

Step one: look at pH. Below 7.35 is acidemia, above 7.45 is alkalemia. This names the direction of the problem. A pH in range does not rule out disease, as the compensation section explains, but it tells you the body is at least holding the line.

Step two: look at PaCO2 and ask whether it explains the pH. Carbon dioxide is an acid in disguise. If pH is low and PaCO2 is high, above 45, the lungs are retaining CO2 and the disturbance is respiratory. If pH is high and PaCO2 is low, below 35, the person is breathing off too much CO2: respiratory alkalosis.

Step three: look at bicarbonate. If the CO2 does not fit the pH, the kidneys and body chemistry are the culprit. Low pH with low bicarbonate, under 22, is metabolic acidosis. High pH with high bicarbonate, over 26, is metabolic alkalosis.

A quick example: pH 7.28, PaCO2 58, HCO3 25. Acidemic, CO2 high, bicarbonate normal. That pattern points to acute respiratory acidosis, the picture of someone who has stopped ventilating adequately in the last hours. Swap in HCO3 of 32 and the story changes: the kidneys have had days to respond, suggesting a chronic condition.

Only then turn to PaO2 and saturation, which describe a separate axis entirely. Someone can have a perfect acid-base panel and dangerously low oxygen, or the reverse.

Respiratory acidosis and alkalosis: when breathing sets the tone

Every minute the body produces roughly 200 milliliters of carbon dioxide, and the lungs are the only exit. Breathe less and it accumulates; breathe more and it drains away. Because dissolved CO2 behaves as an acid, the rate of breathing is, in effect, a real-time acid dial.

Respiratory acidosis means the dial is turned too far toward retention. PaCO2 rises above 45 and pH falls. The causes cluster into a few groups: airways that are narrowed or clogged, as in a severe asthma or COPD exacerbation; a brain that is not driving breathing normally, which can follow sedation, head injury or stroke; muscles too weak to move the chest, as in some neuromuscular diseases; or chest wall problems, including severe obesity hypoventilation. In an emergency the tell-tale sign is often drowsiness, because a high CO2 acts as a sedative on the brain, which is why a sleepy asthmatic worries clinicians far more than a frightened, wheezing one.

Respiratory alkalosis is the mirror image. PaCO2 falls below 35 as a person breathes faster or deeper than metabolism requires. Anxiety and panic are common causes, but so are pain, fever, early sepsis, liver disease, pregnancy and, importantly, low oxygen itself. The body responds to hypoxemia by breathing harder, so a low PaCO2 alongside a low PaO2 is a signal that the lungs are compensating for a problem, not that the patient is simply anxious. The tingling lips and lightheadedness many people feel when hyperventilating come from the way falling CO2 shifts calcium and constricts brain blood vessels.

Metabolic acidosis and alkalosis: when body chemistry takes over

If CO2 cannot explain the pH, the disturbance is metabolic, and bicarbonate is the number to watch. Bicarbonate is the body’s main chemical sponge for acid, manufactured and reabsorbed by the kidneys and consumed whenever excess acid enters the blood.

Metabolic acidosis, bicarbonate below 22 with a low pH, arises in two broad ways. Either the body is generating or absorbing too much acid, or it is losing bicarbonate. The first group includes uncontrolled diabetes, in which the body burns fat and produces ketone acids; lactic acid buildup when tissues are starved of oxygen or blood flow, as in shock or severe infection; kidney failure, which lets normal daily acid accumulate; and certain poisonings. The second group is dominated by prolonged diarrhea and some kidney tubule disorders. Clinicians often calculate an anion gap from the electrolytes to sort one from the other, because the treatments differ completely.

Metabolic alkalosis, bicarbonate above 26 with a high pH, is less dramatic but far from trivial. Repeated vomiting or stomach suctioning removes hydrochloric acid, leaving bicarbonate behind. Some water pills, low potassium, and excess intake of antacid-type compounds can do the same. Symptoms tend to be muscle cramps, twitching and irritability, all downstream of how alkalosis alters calcium availability.

A useful pattern to remember: metabolic problems develop over hours to days and shift bicarbonate first, while respiratory problems can change PaCO2 within minutes. That difference in tempo is what lets a reader distinguish an acute event from a chronic adaptation on a single printout.

What happens if your arterial blood gas oxygen is low?

A low PaO2, called hypoxemia, means the blood leaving the lungs is carrying less oxygen than it should. Most references flag values below 75 to 80 mmHg as reduced and below 60 mmHg as clinically significant, and the second threshold is not arbitrary.

Hemoglobin does not release oxygen in a straight line. Its saturation curve is flat at the top and steep below about 60 mmHg, where saturation sits near 90%. Above that knee, a drop of 20 mmHg barely dents saturation. Below it, the same drop can pull saturation from 90% to the low 70s, and every organ feels the difference. That is why 60 mmHg and 90% saturation function as warning lines in most clinical guidelines rather than the exact edge of normal.

The body’s first response is to breathe faster, which is why hypoxemia so often travels with a low PaCO2. Heart rate rises to move the available oxygen more quickly. If the shortfall persists or deepens, confusion, bluish lips or fingertips, and eventually drowsiness follow. Over months, chronic hypoxemia prompts the bone marrow to make extra red cells and strains the right side of the heart, which is pumping into stiffened lung vessels.

Common causes include pneumonia, fluid in the lungs from heart failure, COPD, asthma, blood clots, interstitial lung diseases and simply being at high altitude. Whether supplemental oxygen is appropriate, how much and for how long depends on the underlying cause and the CO2 level, which is exactly why the two axes of an ABG are read together.

Signs and symptoms that point to an arterial blood gas problem

Abnormal blood gases rarely announce themselves with a single unmistakable symptom. They express themselves through whatever system is most sensitive to the shift, which is usually the brain and the breathing muscles.

Shortness of breath is the common thread, but its character offers clues. Rapid, deep breathing without an obvious reason can signal metabolic acidosis; the body is trying to blow off acid as CO2, and in diabetic ketoacidosis the breath may carry a fruity odor. Slow or shallow breathing paired with drowsiness suggests rising carbon dioxide. Rapid breathing with tingling fingers and lightheadedness fits respiratory alkalosis.

Changes in alertness deserve particular attention. High CO2 produces a fog that can progress from mild confusion to a morning headache that fades on waking, then to somnolence. Low oxygen produces restlessness and agitation first, often before any color change. A bluish tint to the lips, tongue or nail beds, called cyanosis, generally appears only once saturation has fallen well below 85%, so its absence is not reassurance.

Muscle symptoms lean toward alkalosis: cramps, twitching, spasms of the hands or feet. Nausea, vomiting and abdominal pain accompany many acidoses. A racing heartbeat or irregular pulse can come from either direction, because both hydrogen ion and potassium levels move with pH.

Several of these are also symptoms of anxiety, dehydration or a viral illness, which is precisely the problem. The blood gas test exists because the body’s warning signs overlap so heavily. Nobody can, or should, diagnose an acid-base disturbance by feel.

Compensation: why one normal number can hide a real problem

The body dislikes an abnormal pH so much that it will distort its other values to protect it. This process, called compensation, is the single most important idea for making sense of a confusing ABG.

Suppose diarrhea has drained bicarbonate and pH is sliding down. Within minutes the brainstem senses the extra acid and drives breathing harder, dumping CO2 and pulling pH back toward normal. The printout then shows low bicarbonate, low PaCO2 and a pH that may look only mildly off. The kidneys are the slower partner: when lungs retain CO2 for days, as in chronic lung disease, the kidneys hold onto more bicarbonate, and a person can live with a PaCO2 of 60 and a pH of 7.37.

The catch is that compensation is predictable. In metabolic acidosis, PaCO2 is expected to fall by roughly 1.2 mmHg for each 1 mEq/L drop in bicarbonate. If the measured CO2 is lower than that prediction, the person has a second, separate respiratory alkalosis. If it is higher, the lungs are failing to keep up, which can be an early sign of exhaustion. Clinicians use these rules of thumb to detect mixed disorders, which are common in critically ill patients and easy to miss by eye.

Two more principles: compensation almost never overshoots, so a pH on the far side of 7.40 from the primary problem points to a second disorder, and it almost never fully normalizes pH in acute disease. A truly normal pH alongside clearly abnormal CO2 and bicarbonate usually means either a long-standing condition or two opposing problems at once.

Why an ABG result can mislead: sampling, timing and temperature

Blood gas values are fragile in a way most lab tests are not, and several small technical errors can produce a convincingly abnormal report from a perfectly healthy artery.

Air bubbles are the classic culprit. Room air holds far more oxygen and almost no carbon dioxide compared with blood, so a bubble left in the syringe pushes PaO2 up and PaCO2 down within minutes. Careful clinicians expel bubbles and cap the syringe immediately. Delay is the second problem. Blood cells keep consuming oxygen and producing CO2 in the syringe; at room temperature, a sample left for 15 to 30 minutes can lose several mmHg of oxygen and gain acid. Chilling the sample slows this, which is why an ABG travels on ice or goes straight into a bedside analyzer.

Fever and hypothermia matter because gases dissolve differently at different temperatures; analyzers run at 37°C, so a reading in a person with a high fever slightly understates their true PaO2 unless corrected. Excess anticoagulant in the syringe dilutes the sample and can drag bicarbonate and PaCO2 downward. And if the needle catches a vein instead of an artery, which happens more often than anyone likes, the result shows low oxygen and high CO2 in someone who may be fine; a saturation that clashes with the fingertip oximeter is the giveaway.

Then there is context. Drawing a sample immediately after a change in oxygen flow, during a coughing fit or while a frightened patient hyperventilates captures a moment, not a baseline. Good interpretation always begins with asking how, when and under what conditions the blood was drawn.

ABG vs pulse oximeter vs venous blood gas: which test tells you what

Three tests measure overlapping things, and understanding their trade-offs explains why a clinician reaches for one over another.

A pulse oximeter, the clip on the fingertip, estimates oxygen saturation by shining light through the tissue. It is painless, continuous and remarkably useful for trends. Its limits are real: it says nothing about carbon dioxide or pH, becomes unreliable below about 70% saturation, and can be thrown off by cold hands, nail polish, poor circulation and some skin tones, with several studies showing it may overestimate saturation in people with darker skin. Carbon monoxide poisoning fools it entirely, reading normal while the blood is starved of usable oxygen.

A venous blood gas, drawn from an ordinary vein, has grown in popularity because it avoids arterial puncture. For pH and bicarbonate it tracks arterial values closely, typically within a few hundredths of a pH unit, so it is often good enough to identify and follow a metabolic acidosis. Venous PaCO2 runs about 5 to 6 mmHg higher than arterial and the correlation loosens in shock, and venous oxygen values are simply not interpretable for lung function.

The arterial blood gas remains the reference standard whenever precise CO2 or oxygen numbers will guide decisions: setting a ventilator, deciding on long-term home oxygen, or judging whether a tiring asthma patient needs escalation. Many clinicians now combine an oximeter for oxygen with a venous gas for acid-base and reserve the arterial stick for the cases where nothing else will do.

When to see a doctor about breathing changes or an abnormal ABG

Because an ABG is almost always ordered by a clinician who is already evaluating you, the more useful question is which symptoms should prompt care in the first place, and how urgently.

Call emergency services or go to an emergency department without delay for severe shortness of breath at rest, breathing that is very fast or very slow and shallow, blue or gray lips or face, new confusion or unusual drowsiness alongside breathing trouble, chest pain with breathlessness, or fainting. These are the presentations in which blood gases most often reveal a dangerous acid-base or oxygen problem, and minutes matter.

Seek same-day medical attention for breathlessness that is clearly worse than your usual, a known lung condition that is flaring despite your usual plan, persistent vomiting or diarrhea lasting more than a day or two, especially with weakness or dizziness, or unexplained rapid breathing with nausea in someone with diabetes.

If you already have a result in hand and the numbers fall outside the ranges on the report, the clinician who ordered it is the right person to interpret them. Ask what the primary disturbance was, whether the body was compensating, what the oxygen setting was at the time, and what should trigger a repeat test. For people with chronic lung disease, a resting saturation that has drifted below your usual range, more morning headaches, or increased daytime sleepiness are worth mentioning at your next visit even without an emergency.

After the puncture itself, contact your care team promptly for numbness, coldness, paleness or rapidly increasing swelling in the hand.

Frequently asked questions

What happens if your arterial blood gas is low?

A low PaO2, called hypoxemia, means the blood is carrying less oxygen than it should; values under 60 mmHg are considered clinically significant because hemoglobin saturation falls sharply below that point. The body responds by breathing faster and raising heart rate, and if the shortfall continues, confusion, bluish lips and drowsiness can follow. Causes range from pneumonia and COPD to blood clots and high altitude, so the treatment depends on the reason, not the number alone.

How painful is an arterial blood gas test?

Most people find it sharper than a routine blood draw because arteries sit deeper and have more nerve endings, but the needle is usually in for under a minute. A brief pinch followed by a dull ache is typical, and a small amount of local numbing at the site has been shown to reduce discomfort. Firm pressure is held for at least five minutes afterward to prevent bruising, and the arm should feel normal soon after.

When should an arterial blood gas be drawn?

An ABG is drawn when precise oxygen, carbon dioxide or acid-base values will change treatment and no gentler test can provide them: severe or worsening breathing difficulty, suspected carbon dioxide retention, mechanical ventilation, critical illness such as sepsis or diabetic emergencies, poisonings and evaluation for long-term home oxygen. Clinicians typically wait 20 to 30 minutes after any change in oxygen flow so the sample reflects a stable state.

What are the signs and symptoms of arterial blood gas problems?

Shortness of breath is the common thread, but the pattern varies with the disturbance: rapid deep breathing often accompanies metabolic acidosis, slow shallow breathing with drowsiness suggests high carbon dioxide, and fast breathing with tingling fingers fits respiratory alkalosis. Confusion, restlessness, muscle cramps or twitching, nausea, a racing or irregular heartbeat and, late on, bluish lips can all appear. These symptoms overlap with many other conditions, which is exactly why the test exists.

What are normal ABG values?

For a healthy adult breathing room air near sea level, typical ranges are pH 7.35–7.45, PaCO2 35–45 mmHg, PaO2 75–100 mmHg, bicarbonate 22–26 mEq/L, oxygen saturation 95–100% and base excess −2 to +2 mEq/L. Laboratories publish their own reference ranges, which appear on the report. PaO2 declines slightly with age and is lower at altitude, so a value in the low 70s can be normal for an older adult or a resident of a high city.

How do you read an ABG result?

Start with pH to decide whether the blood is acidic or alkaline, then check whether PaCO2 explains it; high CO2 with low pH is respiratory acidosis and low CO2 with high pH is respiratory alkalosis. If CO2 does not fit, look at bicarbonate: low with acidic pH is metabolic acidosis, high with alkaline pH is metabolic alkalosis. Then assess whether the body is compensating, and only afterward evaluate PaO2 and saturation as a separate question.

What is the difference between a blood gas test from an artery and from a vein?

Arterial blood has just left the lungs, so its oxygen and carbon dioxide values directly reflect gas exchange; venous blood has already delivered oxygen to tissues, so its oxygen level is normally low and uninformative about the lungs. Venous pH and bicarbonate track arterial values closely and are often sufficient for following a metabolic problem, but venous PaCO2 runs about 5 to 6 mmHg higher and the agreement weakens in shock.

Can an arterial blood gas test be wrong?

Yes, and the errors are usually technical rather than analytical. An air bubble in the syringe raises PaO2 and lowers PaCO2, a sample left at room temperature for 15 to 30 minutes loses oxygen as cells keep metabolizing, excess anticoagulant dilutes bicarbonate, and accidentally sampling a vein produces falsely low oxygen and high CO2. A saturation that conflicts with the fingertip oximeter is a common clue that something went wrong with the draw.

Why does a pulse oximeter not replace an ABG test?

A pulse oximeter estimates only oxygen saturation; it cannot measure carbon dioxide, pH or bicarbonate, which are often the values that matter most in breathing emergencies. It also becomes unreliable below about 70% saturation, can be affected by cold hands, poor circulation, nail polish and darker skin tones, and reads normal in carbon monoxide poisoning. It is excellent for trends, but the arterial sample remains the standard for exact numbers.

What does bicarbonate on an ABG report mean?

Bicarbonate is the blood’s main chemical buffer against acid and is regulated mostly by the kidneys, so it reflects the metabolic side of acid-base balance. A value below 22 mEq/L with a low pH points to metabolic acidosis from conditions such as uncontrolled diabetes, kidney failure, severe infection or prolonged diarrhea; a value above 26 with a high pH suggests metabolic alkalosis, often from vomiting or low potassium. Bicarbonate changes over hours to days, unlike CO2.

References

• MedlinePlus Medical Encyclopedia: Blood gases
• MedlinePlus Lab Tests: Blood Oxygen Level
• NIH National Library of Medicine (StatPearls): Arterial Blood Gas
• NHS: Blood tests

This article is for general information only and is not a substitute for professional medical advice. Please consult a qualified doctor about your individual situation.