Arterial Blood Gas Test

What is Arterial Blood Gas Testing? An arterial blood gas (“ABG”) is a medical blood test taken from an artery, rather than a vein. The ABG is used to assess the lung’s function and gas exchange. It will measure your lung’s ability to move oxygen (O2) into the blood and remove carbon dioxide (CO2) from the blood. It will also test the acid/base status (pH).

What does the ABG test? The ABG measures the pH of the blood and the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2). It is used to determine the bicarbonate level, and can measure hemoglobin, carboxyhemoglobin, and methemoglobin levels.

Why order ABG testing? Your doctor might order ABG testing to assess for certain respiratory (lung), metabolic, or kidney diseases. Patients receiving supplemental oxygen, especially those on ventilators (breathing machines), and patients undergoing prolonged general anesthesia often undergo ABG testing. Patients with uncontrolled Diabetes Mellitus, heart failure, sleep apnea, kidney failure, or drug overdose might require ABG testing. Patients with lung diseases such as COPD, asthma, cystic fibrosis, pulmonary embolism, or pulmonary fibrosis might have ABG testing during the course of their illness.

How is ABG testing done? Drawing arterial blood for blood gas analysis is usually done by a doctor, nurse, respiratory therapist, or phlebotomist. Most commonly, the blood is taken from the Radial artery (in the crease of the wrist) using a thin needle and syringe. The Radial artery is easily accessible, can be compressed to control bleeding, and has less risk for occlusion. The Femoral artery (in the groin) or the Brachial artery (in the fold at the elbow)  is sometimes used, especially during emergency situations or with children. Blood can also be taken from an arterial catheter already placed in one of these arteries.

Arterial blood gases are more painful than having blood taken from a vein because the artery is located deeper from the surface.

To take your blood gas, the healthcare professional will:

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  • Palpate the area for your pulse and to check for alternative circulation
  • Look for signs of skin infection at the potential puncture site (if yes, look for alternate site)
  • Clean the site with alcohol (or sometimes betadine)
  • Inject a local anesthetic, like lidocaine (not always done)
  • Insert the needle into the artery (sometimes more than one attempt needed)
  • Allow the blood to fill the syringe, then remove the needle
  • Compress over the needle site to prevent bleeding
  • Check for complications

How are ABGs interpreted? The “normal” pH of the blood is 7.4. In most laboratories, the normal range is from about 7.35-7.45. Below 7.35 the blood is abnormally acidic (“acidotic”), and above 7.45 it is abnormally alkaline (“alkalotic”). The normal range (at sea level) for PaO2 is 80-100 mmHg and the normal range for PaCO2 is 35-45 mmHg. Hypoxia is a low PaO2 and indicates that the patient is not respiring properly. Under most circumstances, at a PaO2 of less than 60 mmHg, supplemental oxygen should be administered. An abnormal carbon dioxide level usually indicates a respiratory problem. For a constant metabolic rate, the PaCO2 is determined entirely by ventilation. A high PaCO2 indicates hypo- or underventilation (respiratory acidosis),and a low PaCO2 indicates hyper- or overventilation (respiratory alkalosis). PaCO2 levels can also become abnormal when the respiratory system is working to compensate for a metabolic issue so as to normalize the blood pH (metabolic acidosis or alkalosis with respiratory compensation). There are online guides available to help interpret the results.

What are the risks? The risks of having an ABG include bleeding, clotting, bruising, pain, fainting, hematoma (blood accumulating under the skin), arterial damage (rare), and infection.

How do I prepare? There is no special preparation needed. Be sure to tell the doctor or person drawing the blood if:

  • you have a bleeding disorder
  • you are taking aspirin or blood thinners
  • you have an allergy to local anesthetics, such as lidocaine

If you use home oxygen, check with your doctor if the test is to be done on or off your oxygen.

Where do I get ABG testing? Most doctor’s offices are not set up to perform this test. Usually, you will go to the hospital if this test is ordered by your doctor as an outpatient test. Most blood gases are performed on hospitalized patients.

Is there an alternative test? There is a non-invasive (no needle stick) test for oxygenation, called pulse oximetry. In this test, a small clip-like device is placed on the fingertip (or sometimes the ear lobe), and a sensor reads light that is transmitted through the skin. This test has the advantage of continuously making measurements of oxygen saturation (how much of your hemoglobin is carrying oxygen), which correlates with PaO2 levels, but there are factors that can affect its accuracy. These include abnormal forms of hemoglobin, poor detection of pulse, severe anemia, and certain colored nail polishes (especially reds and dark blues). The pulse oximeter does not measure the PCO2 levels.

Where can I read more?

  • Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins
  • Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby