Finding the right tests…
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Finding the right tests…
Biochemistry
An arterial blood gas test draws blood from an artery rather than a vein, which is unusual among blood tests and is exactly why it can measure something a standard blood draw cannot: how efficiently your lungs are putting oxygen into your blood and clearing carbon dioxide back out, in real time. Arterial blood carries freshly oxygenated blood on its way from your lungs to the rest of your body, so testing it directly, rather than the used-up blood in a vein, is what makes the oxygen reading meaningful. Alongside oxygen and carbon dioxide, the test also reports blood pH and bicarbonate, since your lungs and kidneys work together to keep your blood's acid-base balance steady, and a lung problem serious enough to need this test often shows up in both readings at once, not just the gas levels.
PaO2 reports the pressure of oxygen dissolved in your blood, showing how well your lungs are moving oxygen from the air you breathe into your bloodstream. PaCO2 reports carbon dioxide, showing how well your lungs are clearing it back out with each breath. Blood pH shows whether your blood is running more acidic or more alkaline than usual, and bicarbonate, a separate figure calculated alongside it, shows whether your kidneys are compensating for a breathing problem or contributing to one of their own. Oxygen saturation, how much of the oxygen-carrying capacity in your red blood cells is actually filled, is usually reported too.
Because arterial blood is under higher pressure and typically not as easy to access as the veins used for a routine blood draw, this sample is usually taken from an artery at the wrist, and less often from the arm or groin, by a health care professional specifically trained for it. It generally involves more discomfort than a standard blood draw, which is part of why it's reserved for situations where the specific readings it gives, arterial oxygen and carbon dioxide levels, rather than an estimate of them, are actually needed.
Your lungs and kidneys both help keep your blood's acid-base balance within a narrow range, though they do it on different timescales. Lungs adjust within minutes by changing how fast and deep you breathe, while kidneys adjust more slowly by changing what they excrete. Reporting pH and bicarbonate alongside the oxygen and carbon dioxide readings lets a doctor see whether an abnormal pH is being driven by a breathing problem, a kidney problem, or some combination of the two, and whether the other organ is already compensating for it.
An ABG is ordered to diagnose and monitor serious lung conditions such as COPD, asthma flares, and respiratory failure, and to check whether someone on supplemental oxygen therapy is actually getting enough of it. It's a more direct, but more invasive, alternative to pulse oximetry, the clip-on sensor that estimates oxygen saturation without a blood draw. Pulse oximetry is easier and faster, but an ABG is what's needed when the actual gas levels, acid-base balance, or a more precise reading are required rather than an estimate.
The test itself is a single blood draw, though from an artery rather than a vein, and it's typically used in situations, such as sudden breathlessness or a known lung condition worsening, where the result genuinely changes what happens next.
Arterial blood is freshly oxygenated on its way from the lungs to the rest of the body, so testing it directly is what makes the oxygen reading meaningful. Venous blood has already given up much of its oxygen.
It generally involves more discomfort than a routine vein draw, which is part of why it's reserved for situations where the specific arterial readings are actually needed.
A pulse oximeter estimates oxygen saturation without a blood draw, which is faster and painless, but an ABG gives the actual gas levels and acid-base balance, which a clip-on sensor can't measure.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
They show whether your blood is running more acidic or alkaline than usual, and whether your kidneys are compensating for a breathing problem or contributing to one of their own.
It directly checks whether the oxygen you're receiving is actually reaching adequate levels in your blood, which a pulse oximeter can only estimate.
Yes, since kidneys help regulate blood pH over a slower timescale than the lungs do, a kidney problem can show up in the pH and bicarbonate readings on this test.
A health care professional specifically trained to draw blood from an artery, since it's a different technique from a standard vein draw.
This is worth asking your doctor or the centre directly, since it's typically ordered for an active symptom or condition rather than as routine screening.
Serious lung conditions such as COPD, asthma flares, and respiratory failure, along with unexplained acid-base imbalances that may involve the kidneys as well as the lungs.