Pulse Oximeter: How to Measure Blood Oxygen Saturation

What Is a Pulse Oximeter and How Does It Work?

A pulse oximeter is a small, clip-like medical device that measures the percentage of oxygen-saturated hemoglobin in your blood (SpO2) by shining light through your finger, toe, or earlobe. It provides instant readings of both oxygen saturation and pulse rate without requiring a blood draw.

Pulse oximetry has become one of the most widely used monitoring tools in modern medicine, found in hospitals, clinics, ambulances, and increasingly in homes. The technology relies on a principle called spectrophotometry, which measures how much light is absorbed by oxygenated versus deoxygenated hemoglobin in your red blood cells. When hemoglobin carries oxygen, it absorbs different wavelengths of light compared to hemoglobin without oxygen, allowing the device to calculate the percentage of oxygen-saturated hemoglobin.

The device consists of a sensor probe that emits two types of light — red light (approximately 660 nanometers) and infrared light (approximately 940 nanometers). Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through, while deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through. A photodetector on the other side of the probe measures the light that passes through your tissue, and a microprocessor calculates the ratio of oxygenated to total hemoglobin, displaying this as a percentage called SpO2 (peripheral oxygen saturation).

Modern pulse oximeters also measure your pulse rate by detecting the rhythmic changes in light absorption that occur with each heartbeat. As your heart pumps blood through the arteries, the amount of blood in the tissue expands and contracts, creating measurable variations in light absorption. This pulsatile signal allows the device to distinguish arterial blood from venous blood and tissue, providing a more accurate oxygen saturation reading and simultaneously displaying your heart rate.

Types of Pulse Oximeters

Several types of pulse oximeters are available, each designed for different clinical situations and monitoring needs. Fingertip pulse oximeters are the most common type for home use — they're portable, affordable, and easy to operate. These compact devices clip onto your finger and display readings on a small screen built into the unit. Tabletop or bedside pulse oximeters are larger devices used in hospitals and clinics, offering continuous monitoring capabilities, alarms, and the ability to store data over time.

Handheld pulse oximeters provide a middle ground between portable and stationary devices, often used by healthcare professionals during home visits or in ambulatory settings. Wrist-mounted pulse oximeters with separate finger probes allow for continuous overnight monitoring, useful for diagnosing sleep apnea or monitoring oxygen levels during sleep. Pediatric and neonatal pulse oximeters use specialized sensors designed for smaller fingers, hands, or feet, with some using wrap-around sensors that can be secured to an infant's foot or palm.

Clinical Applications

Pulse oximetry serves numerous clinical purposes across virtually every medical specialty. In emergency medicine, it helps rapidly assess patients with respiratory distress, trauma, or cardiac emergencies. During surgery and anesthesia, continuous pulse oximetry is considered standard of care for monitoring patient safety. In pulmonology, the test helps diagnose and monitor conditions like chronic obstructive pulmonary disease (COPD), asthma, and pulmonary fibrosis, and guides decisions about supplemental oxygen therapy.

Cardiology uses pulse oximetry to assess patients with heart failure, where fluid accumulation in the lungs can impair oxygen exchange. Sleep medicine relies on overnight pulse oximetry to screen for sleep apnea and related disorders. In pediatrics, pulse oximetry screening of newborns has become a universal practice for detecting critical congenital heart defects that might otherwise go undiagnosed until symptoms appear. The COVID-19 pandemic significantly increased home pulse oximetry use, as monitoring oxygen levels became crucial for managing respiratory infections and detecting silent hypoxemia.

What Are Normal Blood Oxygen Levels?

Normal blood oxygen saturation (SpO2) in healthy individuals at sea level is between 95% and 100%. Readings between 90-94% may indicate mild hypoxemia, while readings below 90% are considered low and typically require medical attention. Individual baseline levels may vary based on underlying health conditions.

Understanding what constitutes normal oxygen saturation is essential for interpreting pulse oximeter readings correctly. In healthy adults and children breathing room air at sea level, SpO2 typically ranges from 95% to 100%. This means that 95% to 100% of the hemoglobin in arterial blood is carrying oxygen. A reading of 100% is not necessarily "better" than 97% — both are within the healthy range. The slight variations you may see from reading to reading (for example, fluctuating between 96% and 98%) are normal and reflect minor changes in breathing patterns, body position, and measurement factors.

It's important to understand that oxygen saturation represents a point on the oxygen-hemoglobin dissociation curve, which describes how readily hemoglobin binds to and releases oxygen. This curve is S-shaped, meaning that small changes in oxygen levels at lower saturations represent larger changes in actual oxygen content than the same percentage change at higher saturations. For instance, a drop from 98% to 95% represents a relatively small decrease in oxygen content, while a drop from 92% to 89% represents a much more significant decrease despite being the same three-percentage-point change.

Several factors can affect what's considered "normal" for an individual. People with chronic lung diseases such as COPD may have a lower baseline oxygen saturation, with levels between 88-92% sometimes being acceptable for these patients when prescribed by their physician. At high altitudes, lower atmospheric pressure means less oxygen is available, so healthy individuals may have SpO2 readings of 90-95% at elevations above 8,000 feet (2,400 meters). During sleep, oxygen levels naturally dip slightly, with reductions of 3-4% from awake baseline being common.

Factors That Affect Normal Ranges

Age can influence oxygen saturation levels, though healthy elderly individuals typically maintain similar SpO2 readings to younger adults. However, older adults are more likely to have underlying conditions that affect oxygen levels. Newborns and infants have slightly different normal ranges during the first few days of life, with readings above 95% being expected after the first 24 hours. During pregnancy, oxygen saturation remains normal or may even increase slightly due to increased blood volume, though this can be affected by the physical changes of late pregnancy.

Body position affects oxygen saturation as well. When lying flat, particularly in individuals with obesity, heart failure, or lung disease, oxygen levels may decrease compared to sitting or standing positions. This is why healthcare providers often ask patients to sit upright when taking pulse oximeter readings. Physical activity temporarily increases oxygen consumption, which may cause a slight decrease in SpO2 during or immediately after exercise, though this typically normalizes quickly in healthy individuals.

How Do You Use a Pulse Oximeter Correctly?

To use a pulse oximeter correctly, sit down and rest for 5 minutes, warm your hands if cold, remove nail polish, place your finger fully into the clip with nail facing up, keep still for 1-2 minutes until the reading stabilizes, and record both the SpO2 percentage and pulse rate.

Accurate pulse oximetry readings depend on proper technique. Even small deviations from best practices can significantly affect the accuracy of your measurements, leading to falsely low or high readings that could cause unnecessary worry or, more dangerously, mask true hypoxemia. Healthcare providers undergo training on proper pulse oximeter use, and following established guidelines helps ensure your home measurements are as reliable as possible.

Before taking a measurement, prepare yourself and the environment. Sit comfortably in a chair with your feet flat on the floor and rest for at least 5 minutes to allow your body to reach a stable baseline. Cold extremities are a common cause of inaccurate readings because poor peripheral blood flow reduces the signal strength. If your hands are cold, warm them by rubbing them together or holding them under warm water for a minute. Remove any nail polish, gel manicure, or artificial nails from the finger you'll use, as these can interfere with light transmission through the nail bed.

Position the pulse oximeter probe on your finger — the index or middle finger typically works best on adults. Insert your finger fully into the clip so that the sensor covers the nail bed and the fleshy pad of your fingertip. Your fingernail should face upward toward the top of the device. Ensure the probe fits snugly but not too tightly; excessive pressure can restrict blood flow and affect readings. Keep your hand relaxed and at approximately heart level — holding your hand significantly higher or lower than your heart can affect the reading.

During the Measurement

Once the probe is in place, remain still and avoid talking or moving. Movement is one of the most common causes of inaccurate readings, as it creates artifact in the signal that the device must filter out. Keep your hand steady and try to breathe normally — don't hold your breath or breathe deeply unless specifically instructed to do so. Bright ambient light, especially from fluorescent lights or direct sunlight, can interfere with the optical sensors, so try to position yourself away from intense light sources or cover the probe with your other hand if necessary.

Wait for the display to stabilize before recording your reading. Most pulse oximeters need 30-60 seconds to acquire a stable signal, but it's best to wait 1-2 minutes for the most accurate reading. The display should show a steady SpO2 value and pulse rate without excessive fluctuation. Many devices also display a signal strength indicator or plethysmographic waveform — a good reading typically shows a regular, smooth waveform pattern. If the readings continue to fluctuate significantly or the device shows an error message, reposition the probe and try again.

Recording and Tracking

When monitoring oxygen levels regularly, keep a log of your readings including the date, time, SpO2 value, and pulse rate. Note any relevant factors that might affect your readings, such as activity level, symptoms, or whether you're using supplemental oxygen. This information helps your healthcare provider understand your oxygen status over time and can identify trends or patterns that single measurements might miss. Many smartphone apps can help track and visualize pulse oximetry data, and some modern pulse oximeters can sync directly with health tracking apps.

How Accurate Are Pulse Oximeters?

FDA-cleared pulse oximeters are typically accurate within ±2-3% when SpO2 is between 70-100%. However, accuracy can be affected by poor circulation, cold extremities, nail polish, skin pigmentation, movement, and ambient light. Home devices may have wider accuracy ranges than medical-grade equipment.

Understanding the limitations of pulse oximetry is as important as knowing how to use the device. The FDA requires that prescription pulse oximeters demonstrate accuracy within 2-3% of arterial blood gas measurements (the gold standard) across a range of oxygen saturations. This means that when a pulse oximeter displays 96%, the actual arterial oxygen saturation is likely between 93-99%. Over-the-counter pulse oximeters intended for home use may have wider accuracy ranges, and the FDA recommends treating these readings as estimates rather than precise measurements.

The accuracy of pulse oximetry decreases as oxygen saturation falls. Most devices are tested and validated for saturations between 70-100%, with accuracy being best in the normal range (90-100%) and declining at lower saturations. Below 70% SpO2, pulse oximeter readings become increasingly unreliable. This is particularly relevant in clinical settings where patients with severe hypoxemia require the most accurate monitoring — in these situations, arterial blood gas analysis provides more reliable information.

In 2021, the FDA issued a safety communication highlighting that pulse oximeters may be less accurate in individuals with darker skin pigmentation. Research published in the New England Journal of Medicine found that pulse oximeters were more likely to miss hypoxemia in Black patients compared to white patients, with Black patients being nearly three times as likely to have occult hypoxemia (low oxygen levels not detected by pulse oximetry) as white patients. This finding has important implications for clinical practice and highlights the need for continued improvement in pulse oximetry technology.

Common Sources of Error

Poor peripheral perfusion is one of the most common causes of inaccurate pulse oximetry. Conditions that reduce blood flow to the extremities — including cold exposure, hypotension (low blood pressure), peripheral vascular disease, and Raynaud's phenomenon — can weaken the signal to the point where the device cannot obtain an accurate reading or produces falsely low values. During surgery or in critically ill patients, poor perfusion states often necessitate alternative monitoring methods.

Motion artifact occurs when movement disrupts the optical signal. Even small movements can create significant noise in the measurement, potentially causing the device to display inaccurate values or fail to obtain a reading at all. This is particularly problematic in pediatric patients, who may have difficulty staying still, and in patients with tremors or involuntary movements. Modern pulse oximeters incorporate motion-resistant technology, but no device is completely immune to motion artifact.

• Nail polish/artificial nails: Dark colors (especially blue, green, black) can block light transmission and cause falsely low readings
• Cold fingers: Reduced blood flow decreases signal strength and may cause inaccurate or unobtainable readings
• Movement: Even small movements during measurement can create signal artifact
• Ambient light: Bright sunlight or fluorescent lighting can interfere with optical sensors
• Skin pigmentation: Darker skin tones may result in overestimation of SpO2 levels
• Carbon monoxide: CO binds to hemoglobin and cannot be distinguished from oxygen, giving falsely normal readings
• Severe anemia: Very low hemoglobin levels can affect accuracy

What Medical Conditions Can Pulse Oximetry Help Detect?

Pulse oximetry helps detect and monitor conditions causing hypoxemia, including COPD, asthma, pneumonia, heart failure, sleep apnea, pulmonary embolism, and congenital heart defects in newborns. It is a screening tool that indicates when oxygen levels are abnormal, prompting further evaluation to determine the underlying cause.

Pulse oximetry serves as a vital sign measurement that can alert healthcare providers to respiratory compromise before visible symptoms like cyanosis (bluish discoloration of lips and skin) appear. Because the body can compensate for mild hypoxemia without obvious symptoms, pulse oximetry often detects oxygen problems earlier than physical examination alone. However, it's important to understand that pulse oximetry detects the result (low oxygen) rather than diagnosing the cause, which requires additional clinical evaluation and testing.

Chronic obstructive pulmonary disease (COPD) is one of the most common conditions managed with pulse oximetry monitoring. Patients with COPD often have chronically low oxygen levels and may require supplemental oxygen. Regular pulse oximetry helps assess disease stability, determine oxygen requirements, and detect exacerbations early. During acute COPD exacerbations, pulse oximetry guides treatment decisions and helps determine whether hospitalization is necessary. Long-term oxygen therapy decisions are typically based on arterial blood gas measurements but are often triggered by abnormal pulse oximetry findings.

Asthma attacks can cause significant hypoxemia, particularly during severe exacerbations. Pulse oximetry in emergency departments helps triage asthma patients and guides treatment intensity. An SpO2 below 92% in an asthma patient indicates a severe attack requiring aggressive treatment. Pneumonia and other respiratory infections affect oxygen exchange in the lungs, and pulse oximetry helps assess severity and monitor response to treatment. The COVID-19 pandemic demonstrated the value of home pulse oximetry for monitoring respiratory infections, particularly for detecting silent hypoxemia where patients have low oxygen levels without proportionate shortness of breath.

Newborn Screening

Since 2011, pulse oximetry screening of newborns has been recommended as a standard practice in the United States and many other countries. This screening, performed before hospital discharge, aims to detect critical congenital heart defects (CCHDs) that might not be apparent on physical examination. CCHDs are structural problems with the heart that are present from birth and require intervention within the first year of life. Without early detection, some of these conditions can cause rapid deterioration and death.

The screening involves measuring oxygen saturation in the right hand (which receives blood from before it passes through the ductus arteriosus) and either foot (which receives blood from after the ductus arteriosus). A significant difference between these readings or readings below 95% warrant additional evaluation, typically including an echocardiogram. Studies have shown that pulse oximetry screening detects approximately 75% of CCHDs when combined with physical examination, significantly improving outcomes through earlier diagnosis and treatment.

Sleep Disorders

Overnight pulse oximetry plays an important role in evaluating sleep-related breathing disorders, particularly obstructive sleep apnea (OSA). During apneic episodes, oxygen levels drop as breathing stops temporarily, then recover when breathing resumes. A pattern of repeated oxygen desaturations during sleep suggests OSA and may prompt further evaluation with polysomnography (sleep study). While overnight pulse oximetry alone cannot definitively diagnose sleep apnea, it serves as an effective screening tool, particularly for patients at high risk.

When Should You Monitor Oxygen Levels at Home?

Home pulse oximetry is recommended for individuals with chronic lung disease (COPD, pulmonary fibrosis), heart failure, those using supplemental oxygen, during recovery from respiratory infections, and for monitoring infants with breathing concerns. Always follow your healthcare provider's specific guidance on when and how often to measure.

Home pulse oximetry has become increasingly common and can provide valuable information for managing certain health conditions. However, it's important to understand when home monitoring is beneficial and when pulse oximeter readings should be interpreted with caution. The decision to monitor oxygen levels at home should typically be made in consultation with a healthcare provider, who can establish appropriate baseline values, target ranges, and action thresholds specific to your condition.

Patients with chronic lung diseases such as COPD, interstitial lung disease, or pulmonary fibrosis often benefit from home pulse oximetry. These conditions cause progressive impairment of oxygen exchange, and monitoring can help detect worsening of the underlying condition or development of exacerbations. For patients on long-term supplemental oxygen therapy, pulse oximetry helps ensure oxygen flow rates are appropriate and assists with adjusting oxygen during different activities. During exercise, for example, some patients need higher oxygen flow rates than at rest.

Heart failure can affect oxygen levels when fluid accumulates in the lungs (pulmonary edema), impairing gas exchange. Patients with heart failure may be asked to monitor their oxygen levels along with weight and symptoms to detect worsening of their condition early. A decline in oxygen saturation, particularly when combined with weight gain and increased shortness of breath, may indicate fluid overload requiring medication adjustment or medical evaluation.

During Illness

Respiratory infections, including influenza, pneumonia, and COVID-19, can cause significant hypoxemia. Home pulse oximetry during illness helps identify patients whose oxygen levels are dropping and who may need medical evaluation or hospitalization. This is particularly important for detecting silent hypoxemia, a phenomenon where patients have dangerously low oxygen levels without feeling proportionally short of breath. The COVID-19 pandemic brought widespread attention to this phenomenon and led to increased use of home pulse oximetry for monitoring infected individuals.

During respiratory illness, healthcare providers may recommend monitoring oxygen levels several times daily or whenever symptoms change. Specific thresholds for seeking care vary but commonly include SpO2 below 94% at rest, SpO2 below 92% on exertion, or any reading accompanied by severe symptoms such as difficulty breathing, chest pain, confusion, or bluish discoloration of lips or fingernails.

What Are the Limitations of Pulse Oximetry?

Pulse oximetry has important limitations including inability to detect carbon monoxide poisoning, reduced accuracy with dark nail polish and poor circulation, potential for racial bias in readings, and inability to assess oxygen delivery to tissues or carbon dioxide levels. It measures saturation, not the total amount of oxygen in blood.

While pulse oximetry is an invaluable monitoring tool, understanding its limitations is essential for appropriate use and interpretation. The technology measures the percentage of hemoglobin saturated with oxygen but does not directly measure how much oxygen is being delivered to tissues, how much hemoglobin is present, or other important aspects of respiratory function. A patient with severe anemia could have a normal SpO2 reading while still having inadequate oxygen delivery because they have too few red blood cells to carry oxygen effectively.

Carbon monoxide poisoning represents a particularly dangerous limitation of pulse oximetry. Carbon monoxide binds to hemoglobin with much greater affinity than oxygen, forming carboxyhemoglobin, which standard pulse oximeters cannot distinguish from oxyhemoglobin. As a result, patients with carbon monoxide poisoning may display normal or near-normal SpO2 readings despite having severely impaired oxygen-carrying capacity. In suspected carbon monoxide exposure, arterial blood gas analysis with co-oximetry is required to accurately assess oxygen status.

Pulse oximetry also cannot assess carbon dioxide levels in the blood. Many respiratory conditions cause both hypoxemia (low oxygen) and hypercapnia (high carbon dioxide), and monitoring only oxygen provides an incomplete picture. In conditions like COPD exacerbations, rising carbon dioxide levels can be as dangerous as falling oxygen levels, but pulse oximetry won't detect this. For comprehensive respiratory assessment, arterial blood gas analysis remains the gold standard.

Technical Limitations

Methemoglobinemia is another condition that can produce misleading pulse oximetry readings. Methemoglobin, formed when iron in hemoglobin is oxidized, cannot carry oxygen effectively. Standard pulse oximeters typically read SpO2 around 85% regardless of actual oxygen saturation when methemoglobin levels are significantly elevated. This condition can occur with certain medications, industrial chemical exposures, or genetic disorders.

Severe hypotension, hypothermia, and peripheral vasoconstriction can all reduce the pulsatile signal that pulse oximeters require to function. In critically ill patients, these conditions are common and may render pulse oximetry unreliable precisely when accurate monitoring is most needed. Healthcare providers in intensive care settings recognize these limitations and often use arterial blood gas analysis or alternative monitoring methods in hemodynamically unstable patients.

The technology also has inherent physical limitations. Very low oxygen saturations (below 70-80%) are less accurately measured because the devices are validated primarily in higher saturation ranges. Irregular heart rhythms (arrhythmias) can interfere with the algorithm that identifies the pulsatile signal. Intravenous dyes used in some medical procedures can temporarily affect readings. Understanding these limitations helps ensure that pulse oximetry is used appropriately as one component of patient assessment rather than as a standalone diagnostic tool.