Understanding the Difference between Arterial Blood Gas vs. SpO2

Upon admittance to a hospital, a patient typically undergoes a battery of tests, including an arterial blood gas (ABG) test to measure the levels of oxygen and carbon dioxide in their blood, as well as their body’s acid-base (pH) level. Their SpO2 test, administered using pulse oximetry, shows the percentage of oxygen in their blood, referred to as oxygen saturation values. If a patient experiences trouble breathing, such as shortness of breath, their doctor will typically order both tests. Let’s consider the details of both tests and what they indicate.


Arterial Blood Gas (ABG)

The term arterial blood gas test refers to a diagnostic test that involves taking a small blood sample from the patient and using a blood gas analyzer to determine blood pH levels and the partial pressure of gas in the blood. The blood sample, taken from a radial artery instead of a vein (venous blood gas analysis), provides the doctor with data used in diagnosing respiratory, circulatory, and metabolic conditions.

Why Run an Arterial Blood Gas Analysis?

When a patient presents breathing issues or experiences kidney failure, complications of diabetes, or severe infection, their doctor orders tests of the blood gases. ABG results help determine if the patient needs supplemental oxygen.

The ABG test also provides data that help diagnose the cause of the breathing issues. Doctors also use these test results to help determine the patient’s response to the medical treatments for diseases such as asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis, and obstructive sleep apnea.


Administering an ABG Test

Doctors may perform the ABG in a hospital or at their office. A doctor, nurse, or phlebotomist may conduct the test. Using a small needle, the medical staff withdraws some of the patient’s blood from an artery, typically the one at the wrist, but sometimes the groin, the inside of the arm, just above the elbow.

The medical practitioner frequently conducts the modified Allen test before drawing blood. In this test, the medical personnel applies pressure to the arteries of the patient’s wrist for a few seconds to check for normal blood flow in the hand. They will also ask for the patient to name the medications and over-the-counter (OTC) supplements and vitamins they take.

If an individual uses supplemental oxygen but can breathe without it, the medical practitioner will typically remove it for about twenty minutes before the ABG. Called the “room air” test, it ensures the most accurate results from the test.

Speedy Results for ABG

Practitioners rely on the ABG test because of its accuracy and quick results. In about fifteen minutes, the doctor receives the results. The test shows if the patient:

  • receives enough oxygen through partial pressure of oxygen (PaO2) levels,
  • exhales enough carbon dioxide,
  • has properly working kidneys, or
  • developed a serious infection affecting organ blood delivery.

Unlike the next test we’ll consider, the ABG doesn’t have a scale that defines a normal result. The right ABG results for each patient vary, depending on their current condition and medical history. The results of an ABG test can indicate the need for an SpO2 test.


Measuring Oxygen Saturation with Pulse Oximetry

Pulse oximetry is a noninvasive test administered using a pulse oximeter, which provides a numerical measure of the percentage of oxygen in a patient’s blood. Unlike the ABG, pulse oximetry uses a scale that provides normal oxygen saturation levels. A second scale exists for individuals diagnosed with any of the groups of diseases within COPD. For a healthy individual, the normal arterial oxygen saturation level falls between 95 percent and 100 percent, but for an individual with COPD, the oxygen saturation should fall between 88 percent and 92 percent.

In a healthy individual, arterial oxygen saturation measured to be less than 90 percent requires medical attention because it means their body lacks oxygen. In an individual diagnosed with COPD, a low oxygen saturation level of less than 88 percent requires medical attention.


Why Run a SpO2 Test?

When a doctor considers why an individual experiences breathing problems, the percentage of oxygen in the patient’s blood tells the medical staff some important things. Blood oxygen levels depend on the following:

  • amount of oxygen breathed in
  • effectivity of alveoli in swapping carbon dioxide for oxygen
  • amount of hemoglobin concentrated in the patient’s red blood cells
  • how well hemoglobin attracts oxygen
  • how well tissue oxygen delivery is in the body  

Under normal conditions, enough oxygen enters the hemoglobin to meet the patient’s bodily needs. A lack of oxygen in the blood indicates certain diseases that cause such a reduction.


Administering a Pulse Oximeter Oxygen Saturation Test

The low cost and ubiquity of pulse oximeters make them simple for inpatients and outpatients to take their own measurements. Athletes use pulse oximetry measurement to monitor their SpO2 to improve their workouts and response to training. In a medical or athletic setting, a low SpO2 result tells the individual that they need oxygen.

In a medical setting, a nurse or a doctor takes the blood oxygen level measurement by slipping a pulse oximeter onto the patient’s finger. If they cannot measure using the finger, the medical practitioner can place the oximeter on the individual’s foot. Other devices use a design that attaches to the head of the patient. These also include those that a doctor can attach to the top of the head/forehead of a fetus to monitor the unborn’s blood oxygen levels during the birthing process.

The oximeter uses a sensor that measures the oxygen percentage in the patient’s blood through their skin. Many smartwatches and sports devices now take SpO2 readings without medical equipment used in pulse oximetry. Individuals should not use these devices to obtain readings for health or medical uses, but these can offer helpful indicators during workouts. Because the test only takes about one minute to administer and it offers instant results, it provides up-to-date oxygen saturation data quickly.


Combining the Two Tests

Both pulse oximetry and ABG offer key indicators in diagnosing breathing disorders and blood disorders. Doctors frequently order both tests to obtain a full picture of how the patient’s body handles oxygen. The doctor uses these test results in the diagnosis and treatment of the following conditions:

  • anemia
  • asthma
  • chronic obstructive pulmonary disease (COPD)
  • diseases such as chronic bronchitis
  • congenital heart defects
  • heart disease
  • pneumothorax
  • pulmonary embolism
  • respiratory infections, including common diseases like cold or flu and advanced diseases like COVID-19

Combining these two common health tests typically happens in a hospital setting.


When Will a Patient Have Both ABG and Pulse Oximetry?

Since ABG tests require drawing blood, a patient typically has both tests only in a hospital setting. This may include a healthcare clinic or a nurse practitioner’s office. This means that a doctor usually orders these two tests together when admitting an individual to a medical facility for treatment.

Performing Pulse Oximetry at Home

Pulse oximeters cost less than $20, even those used in medical facilities. This makes it affordable for individuals to monitor their own SpO2 at home after release from the hospital. The percentage represents how effectively the body uses oxygen and circulates it via the blood, so it offers an important and easily measured indicator that the patient can obtain.

They can assist in their own treatment by phoning the doctor if the percentage drops below 90 percent. At this point, the doctor would, typically, run a new ABG.


A Review of the Big Differences between ABG and SpO2

Let’s quickly review the differences between the two tests. While they both test oxygen in the body, they do so differently.

The arterial blood gas test (ABG) uses an invasive testing method that requires the drawing of blood from an artery, while the SpO2 test uses a noninvasive test that measures via a sensor on the skin.

Typically, a medical practitioner administers the ABG test in a hospital setting, while the patient can test their SpO2 at home using a smartwatch, pulse oximeter, or fitness tracker.

The ABG test indicates if a medical treatment, such as oxygen therapy, for breathing-related issues works or how well it works for a specific patient, while the SpO2 test helps the medical staff monitor the oxygen in the blood during rest and treatment.

The ABG test uses no scale. No one normal result exists since each ABG result depends on the patient’s medical history and current condition. The SpO2 uses two scales to provide normal ranges for healthy individuals and those with diseases within the COPD group. The normal rate for a healthy individual during sleep ranges from 90 percent to 100 percent. During waking time, the normal rate ranges from 95 percent to 100 percent. A measurement of less than 90 percent during sleep indicates the possibility of sleep apnea.