Oxygen saturation is a measurement that tells how much oxygen is being carried through the blood at any given moment, helping doctors understand if the body’s organs and tissues are receiving enough oxygen to work properly.

Introduction: Who Should Undergo Diagnostics

Oxygen saturation testing is an important part of modern healthcare that can reveal whether the body is receiving adequate oxygen. Anyone experiencing breathing difficulties, shortness of breath, rapid heartbeat, confusion, or a bluish tint to the skin, fingernails, or lips should seek oxygen level testing immediately, as these symptoms may indicate dangerously low oxygen levels in the blood.^[1]

People with certain medical conditions should have their oxygen levels monitored regularly, even when they feel well. Those with lung diseases such as chronic obstructive pulmonary disease (COPD), which means the lungs have long-term damage that makes breathing difficult, asthma, pneumonia, or lung cancer need routine oxygen checks to ensure their treatment is working properly.^[2] Similarly, people with heart disease, heart failure, anemia (low red blood cells), or cystic fibrosis benefit from regular oxygen monitoring because these conditions can affect how well oxygen moves through the body.^[3]

During acute medical situations, oxygen testing becomes essential. If someone has recently injured their head or neck, has been exposed to carbon monoxide, experienced smoke inhalation, or suffered a drug overdose, medical providers will quickly check oxygen levels to guide emergency treatment decisions.^[2] People hospitalized with coronavirus disease (COVID-19) often require frequent oxygen monitoring, as this infection can significantly impact lung function and oxygen levels.^[2]

People living at high altitudes or those planning to travel to mountainous regions may also need oxygen testing. The air at higher elevations contains less oxygen, which can cause oxygen levels in the blood to drop and lead to altitude sickness.^[1] Mountain climbers and others visiting high-altitude areas sometimes need supplemental oxygen to maintain safe levels.

Diagnostic Methods

Medical professionals use several methods to measure oxygen levels in the blood. These tests help doctors identify whether a person has hypoxemia, which means the blood contains less oxygen than normal, or hypoxia, which means body tissues are not receiving enough oxygen.^[4] Understanding the difference between these two conditions helps guide treatment decisions.

Pulse Oximetry

The most common and simplest way to check oxygen levels is through a device called a pulse oximeter. This small, noninvasive device clips painlessly onto a fingertip, toe, or earlobe and provides results within seconds.^[1] The pulse oximeter works by shining two types of light—red light and infrared light—through the skin and blood vessels. Different amounts of these light types are absorbed depending on how much oxygen the blood is carrying.^[3]

The device measures the percentage of hemoglobin (the protein in red blood cells that carries oxygen) that is currently bound to oxygen molecules. This measurement is called SpO2, which stands for peripheral oxygen saturation.^[5] A healthy oxygen saturation reading for most adults ranges between 95% and 100%, meaning nearly all the hemoglobin in the blood is carrying oxygen to the body’s cells and tissues.^[4] Readings between 90% and 92% are considered low and may indicate that supplemental oxygen or further medical attention is needed. Any reading below 90% is considered concerning and requires immediate medical evaluation.^[4]

Pulse oximetry has become so important in healthcare that it is now often considered a “fifth vital sign,” alongside body temperature, heart rate, breathing rate, and blood pressure.^[1] Healthcare providers routinely use pulse oximeters in hospitals, clinics, and emergency settings. Many people also purchase their own pulse oximeters from pharmacies to monitor their oxygen levels at home, which can be particularly useful for those with chronic lung or heart conditions.^[2]

However, pulse oximeters have some limitations that users should understand. The readings can be less accurate if a person has very cold hands, poor blood circulation to the extremities, or conditions that restrict blood flow to the fingers such as Raynaud’s phenomenon.^[5] Dark nail polish, particularly deep blue or black colors, and artificial nails can interfere with the light sensors and produce inaccurate readings.^[5] Despite these limitations, pulse oximetry remains a quick, painless, and highly useful screening tool with no known risks or dangers.^[2]

Arterial Blood Gas Test

When doctors need more detailed and precise information about blood oxygen levels, they order an arterial blood gas (ABG) test. This is a more invasive procedure than pulse oximetry but provides much more comprehensive information.^[2] During an ABG test, a healthcare provider uses a needle and syringe to draw blood from an artery, usually in the wrist, rather than from a vein as in standard blood tests.^[4]

The blood sample is then analyzed in a laboratory to measure several important values. The test measures the amount of oxygen gas dissolved in the blood, called arterial oxygen pressure (paO2), which normally ranges from 75 to 100 millimeters of mercury (mm Hg).^[4] It also measures the amount of carbon dioxide in the blood and checks the pH balance, which indicates whether there is too much acid or base in the blood. Having proper acid-base balance is crucial for health, as imbalances can be harmful to the body.^[2]

A healthy arterial oxygen level is about 75 to 100 mm Hg. When this value drops below 60 mm Hg, it indicates hypoxemia and means supplemental oxygen is needed.^[4] The ABG test is considered the most accurate method for determining lung function and oxygen levels, and it provides information that a pulse oximeter cannot capture.^[2]

Oximetry Versus Blood Gas Testing

In clinical practice, doctors often start with pulse oximetry because it is quick, noninvasive, and provides immediate results. If the pulse oximeter reading suggests low oxygen levels or if the patient’s condition is complex, the healthcare provider may then order an arterial blood gas test for more detailed information.^[4] This two-step approach allows for rapid screening while reserving the more invasive test for situations where comprehensive information is essential for diagnosis and treatment planning.

Both methods measure oxygen saturation, but they do so in different ways and provide different types of information. The pulse oximeter estimates oxygen saturation by measuring light absorption through the skin, while the arterial blood gas test directly measures oxygen dissolved in arterial blood along with other important blood chemistry values.^[3]

Additional Diagnostic Testing

When low oxygen levels are detected, doctors may order additional tests to understand the underlying cause. Pulmonary function tests measure how well the lungs inhale and exhale air and how effectively they transfer oxygen into the bloodstream.^[1] These breathing tests can help identify conditions such as asthma, COPD, or other lung diseases that may be causing oxygen levels to drop.

For people with suspected sleep-related breathing problems, doctors may recommend an overnight oximetry study. This test monitors oxygen saturation levels throughout the night while the patient sleeps, helping to identify conditions like sleep apnea where oxygen levels drop during sleep even though the person feels fine during the day.^[8]

In some cases, healthcare providers measure oxygen saturation in different parts of the body. Venous oxygen saturation (SvO2) measures the percentage of oxygenated hemoglobin returning to the right side of the heart after delivering oxygen to the body’s tissues. This typically ranges between 60% and 80% and helps doctors determine whether oxygen delivery is meeting the body’s needs.^[6] Tissue oxygen saturation (StO2) can be measured using a technique called near infrared spectroscopy, which provides information about oxygen levels in specific tissues.^[6]

Diagnostics for Clinical Trial Qualification

When patients are being considered for enrollment in clinical trials, particularly those testing new treatments for lung diseases, heart conditions, or other disorders affecting oxygen levels, oxygen saturation measurements serve as important screening criteria. Clinical trial protocols often specify exact oxygen level requirements that participants must meet to qualify for the study.

For trials testing new medications or devices for conditions like COPD, asthma, pulmonary fibrosis, or heart failure, researchers use both pulse oximetry and arterial blood gas tests to establish baseline oxygen levels before treatment begins. These initial measurements help ensure that participants have the condition the study is designed to address and that they fall within specific oxygen level ranges defined in the research protocol.^[1]

Throughout the clinical trial, participants undergo regular oxygen monitoring to track how their levels change in response to the experimental treatment. This ongoing testing allows researchers to measure whether the new therapy improves oxygen saturation compared to standard treatments or placebo. The frequency of testing varies depending on the trial design but may include daily home monitoring with pulse oximeters, weekly clinic visits for more comprehensive assessments, or periodic arterial blood gas tests at specified intervals.^[1]

Some clinical trials also use specialized oxygen testing protocols. For example, trials for portable oxygen devices may include exercise testing where oxygen levels are measured while the patient walks or performs other activities. Trials for altitude-related treatments might use high altitude simulation tests that measure oxygen saturation at simulated elevations of 8,000 feet, which mimics the cabin pressure of commercial airliners.^[14]

Clinical trials examining medications that could potentially affect breathing or lung function include oxygen saturation as a safety monitoring parameter. Even if the primary purpose of the trial is to test a treatment for a non-respiratory condition, researchers regularly check oxygen levels to ensure the experimental therapy is not causing unexpected breathing problems or reducing oxygen delivery to tissues.^[1]

For studies involving supplemental oxygen therapy itself, precise oxygen measurement becomes even more critical. Researchers must carefully document current oxygen delivery methods, flow rates, and how oxygen levels respond when these parameters are adjusted. This detailed monitoring helps determine the optimal oxygen therapy approach for different patient populations and medical conditions.