Let’s Talk Blood Oxygenation: Difference between SpO2, SaO2, PaO2

The main difference between SpO2, SaO2, and PaO2 is in what each represents. SpO2 is the oxygen saturation level per the pulse oximeter reading. SaO2 is the oxygen saturation reading per a blood gas analysis. Lastly, PaO2 is the partial pressure of the oxygen flowing from the alveoli to the capillaries and the vessels. These are the key differences:



  • SpO2 is oxygen saturation measured using a pulse oximeter.
  • Pulse oximetry, which is used to measure SpO2, is a noninvasive procedure.
  • SpO2 levels are important in determining whether a patient requires mechanical oxygen support.



  • SaO2 is the saturation of oxygen in arterial blood.
  • The SaO2 levels are determined by arterial blood gas (ABG) analysis.
  • The procedure is invasive and contraindicated in patients with blood disorders.



  • PaO2 is the pressure exerted by oxygen on the arterial wall.
  • Dissociation of oxygen affects the PaO2 levels.

PaO2, SpO2, and SaO2 help medics determine the amount of oxygen in the patient’s blood and if it is adequate. Let’s explain further the role each plays in oxygen adequacy in the peripheral blood. 


Oxygen Saturation (SpO2) and Pulse Oximetry Explained

SpO2 represents the peripheral blood oxygen saturation levels. The circulating blood contains hemoglobin, which also carries oxygen, and SpO2 measures the oxygen-carrying capacity of the hemoglobin in the blood. Oxygen is carried and utilized by the body’s organs to maintain normal body function.

Normal oxygen saturation levels are from 94 percent to 100 percent. Low SpO2 levels can result in hypoxemia, which can progress to hypoxia (low oxygen levels in the tissues). The body maintains normal SpO2 by oxygen intake through breathing. The inhaled oxygen goes to the lungs, binds to the hemoglobin, and circulates in the body.

A pulse oximeter measures SpO2 levels. The pulse oximeter is a small device a physician will place on your finger, and the oxygen saturation levels are displayed on the screen. If you take a pulse oximetry test yourself and your SpO2 levels are below 90 percent, you should see a physician.     


Arterial Oxygen Saturation (SaO2) Explained

To determine the SaO2, the physician will order an ABG analysis test. The test aims to ascertain the percentage of oxygen on the hemoglobin molecules in the arterial blood, which cannot be achieved through pulse oximetry. 

Standard SaO2 measurements are from 94 percent to 100 percent. Levels below 90 percent indicate hypoxia, which may be a result of underlying conditions like the following:

  • Anemia
  • Acute or chronic kidney failure
  • Uncontrolled diabetes
  • Hemorrhage
  • Shock
  • Heart failure
  • Drug overdose
  • Chemical poisoning
  • Inborn errors of metabolism
  • Asthma
  • Severe sepsis

Low SaO2 levels below 80 percent indicate the low oxygen-carrying capacity in the arteries. Low oxygen levels will compromise the functioning of major organs like the brain, kidneys, liver, and heart. If the oxygen levels drop further, the patient will get into shock with cardiac and respiratory arrest.

Unlike pulse oximetry, the measurement of SaO2 is an invasive procedure as the medic will need to draw blood from an artery. An ABG analysis gives blood oxygen levels, carbon dioxide, and pH.


What Is Measured in an ABG Test?

The blood drawn from an artery includes the following measurements: 

  • Oxygen content (O2CT) indicates the oxygen levels in the blood.
  • Hemoglobin (Hb) is the amount of hemoglobin available for carrying oxygen to your blood cells.
  • SaO2 is the arterial oxygen saturation level of your arterial blood, responsible for carrying oxygenated blood from the lungs to various cells and body organs. 
  • The partial pressure of oxygen (PaO2) measures the pressure at which the oxygen dissolves in your blood.
  • pH measures how acidic or alkaline your blood is. Normal pH ranges between 7.35 to 7.45. If it’s above that, it is considered alkaline or basic; if lower, it is acidic.
  • Partial pressure of carbon dioxide (PaCO2) is the level of CO2 in your blood. The measurement determines how well your body can get rid of CO2.  
  • Bicarbonate (HCO3) is derived from the pH, and PaCO2 values determine the levels of basic compounds made from the body’s CO2.   


Partial Pressure of Oxygen (PaO2) Explained

PaO2 measures the oxygen partial pressures in arterial blood. PaO2 is one of the measurements done under the ABG test. 

Conditions that affect your breathing or cause breathing problems compromise the oxygen supply. The level of PaO2 assists doctors in determining whether a patient requires oxygen supplementation or mechanical breathing assistance.

PaO2 values help in the events of chronic medical conditions like the following:

  • Lung injury or chest trauma
  • Chronic obstructive pulmonary disease (COPD)
  • Cystic fibrosis
  • Asthma
  • Congestive heart failure
  • Sudden difficulty in breathing
  • Loss of consciousness
  • Heart attack

Unlike pulse oximetry, the test is invasive as it involves puncturing the arterial wall and drawing blood. That poses a risk of bruising or excessive bleeding. The test is contraindicated in patients on blood thinners like heparin and warfarin or those with bleeding disorders.


Interpreting PaO2 Results 

Normal PaO2 levels range from 75 to 100 mmHg (at sea level). The oxygen we inhale through the nose travels to the lungs and is stored in the alveoli. The deoxygenated blood from the body flows to the lungs through the pulmonary vein. The oxygen pressure in the alveoli is higher than that in the adjacent capillaries. Hence, it flows to the capillaries. The blood leaving the lungs is well saturated with O2.

If O2 flow from the alveoli to the blood is compromised, your PaO2 levels will be lower. Several factors can cause low PaO2 levels:

  • Neurological conditions like Guillain-Barre syndrome (GBS) or amyotrophic lateral sclerosis (ALS)
  • Lung damage due to trauma or cancer
  • Higher altitudes, like the mountains, where the atmospheric oxygen is lower, will reduce the oxygen pressure in your lungs.
  • Obesity
  • Low HB levels from iron-deficiency anemia


The Oxygen-Hemoglobin Dissociation Curve

The oxygen-hemoglobin dissociation curve is a curve showing the partial pressure of oxygen PaO2 in mmHg against the percentage of oxygen saturation. The reason is that body cells use the available O2, and the PaO2 begins to fall. For equilibrium, O2 should dissociate into the bloodstream at a speed equal to how the cells consume it.

In an average healthy adult, the oxygen saturation ranges from 94 to 100 percent with a PaO2 of 80 to 100 mmHg. The figures mean that the graph cannot be a straight line, but a curve. Below is a typical image of the oxyhemoglobin dissociation curve.


Important points to note on an oxyhaemoglobin dissociation curve are these:

  • Arterial blood, as the hemoglobin, is usually saturated at 100 percent with an oxygen pressure of 100 mmHg.
  • In venous blood, the hemoglobin is saturated at approximately 75 percent.
  • p50 is the point at which the oxygen is 50 percent saturated at an average pressure lower than 26.8 mmHg.


Conditions That Shift the Curve

The speed at which oxygen is utilized by the cells influences the partial pressure of oxygen. The curve shifts to either the right or the left.


Right Shift

A curve deviation to the right indicates a decrease in oxygen affinity as hemoglobin holds the oxygen less tightly and delivers more oxygen to the cells at a given arterial oxygen pressure. More significant amounts of oxygen dissociate from hemoglobin, which improves tissue oxygenation. A shift to the right side means the hemoglobin is in a tense state (T state). The CO2 and metabolic acid from the tissues shift the curve rightward.

The causes of a rightward shift are the following:

  • Low pH or more acidic
  • An increase in CO2
  • High temperatures
  • Low-affinity Hb variants
  • High 2,3-BPG


Left Shift

A leftward deviation indicates the cells have an increased affinity for oxygen, yet the hemoglobin holds more tightly onto it and delivers less oxygen to the tissues. The left-sided shift means the hemoglobin is in a relaxed state (R state). The left-sided curve is crucial for a fetus with Hb F as it allows oxygen transfer from the maternal to fetal circulation.

The left-shifted curve is caused by the following:

  • High pH or more basic
  • Fetal Hb, especially Hb F
  • Low temperatures
  • Low 2,3-BPG
  • High oxygen affinity Hb variants
  • Carboxyhemoglobinemia
  • Methemoglobinemia


Bottom Line

For the human cell to function normally, it requires a constant supply of oxygen. The oxygen from the air is inhaled into the lungs and stored by the alveoli. Low oxygen saturation leads to hypoxemia, which results in hypoxia and eventually, if not corrected, can cause irreversible damage to vital organs.

SpO2, SaO2, and PaO2 may differ, but they all depend upon the concentration of oxygen present in the circulating blood. An imbalance in oxygen saturation causes cardiac and respiratory deficiency. The SpO2, SaO2, and PaO2 levels should be within the normal range for the human cell to maintain homeostasis.